Introduction

Since its introduction in 1944, aminoglycoside such as streptomycin, neomycin, gentamicin, and tobramycin has been a crucial class of antibiotics for treating a wide spectrum of human pathogenic Gram-positive and Gram-negative bacteria (e.g., Pseudomonas aeruginosa, Staphylococcus aureus, Escherichia coli, etc.) (Henry-Stanley et al. 2014; Krause et al. 2016; Labby and Garneau-Tsodikova 2013). The structure of aminoglycosides is composed of a 2-deoxystreptamine (2-DOS) ring linked to several amino-modified sugars, giving rise to their polycationic nature (Mingeot-Leclercq et al. 1999). The chemical structures of different types of aminoglycosides are given in Fig. 1. The polycationic nature causes aminoglycosides to target the negatively charged nucleic acid of bacterial cells as the major site of bactericidal action (Chittapragada et al. 2009). By interacting with the outer membrane and utilizing the energy-dependent phase I, aminoglycosides arrive at the protein synthesis system in the bacterial cellular cytosol (Taber et al. 1987). There, the antibiotic utilizes the energy-dependent phase II to irreversibly bind to the 16S ribosomal RNA (rRNA) at its A site of 30S subunit of bacterial ribosome, resulting in (1) misreading of codon, (2) terminating the peptide elongation by inhibiting tRNA translocation from the A to the P site, and (3) interfering the mobility of the ribosomal subunit and thus producing malfunctioning or nonfunctioning (immature) protein (Kotra et al. 2000; Walter et al. 1999). These proteins upon binding to the cell wall and membrane would disrupt the structure, thus allowing the rapid entry of more antibiotics into bacterial cells (Davis 1987).

Fig. 1
figure 1

Chemical structure of different aminoglycoside antibiotics

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Unfortunately, similar to several other antibiotics, the misuse and overuse of aminoglycosides over a long time have caused numerous bacteria to emerge resistant strains against the antibiotics (Li and Webster 2018; Mulani et al. 2019; Perez-Rodriguez and Mercanoglu Taban 2019). Up to the present, these traits are known to be genuinely similar across antibiotic classes, which include modifying enzymes, ribosomal mutation, cell wall permeability, and biofilm formation (Pang et al. 2019; Peterson and Kaur 2018) (Fig. 2). The production of enzymes that modify the antibiotics chemistry has been the most extensively studied mechanism of bacterial resistance to aminoglycosides (Ramirez and Tolmasky 2010; Zarate et al. 2018). These enzymes which are N-acetyltransferases, O-nucleotidyltransferases, and O-phosphotransferases target specific –NH2 and –OH groups of the aminoglycosides, causing the antibiotics to poorly bind to the ribosomes and thus reducing the antibacterial efficacy (Garneau-Tsodikova and Labby 2016; Shakil et al. 2008). In addition to aminoglycosides being modified themselves, their binding site which is the 16S rRNA of the 30S ribosomal subunit can be mutated or methylated for deactivation (Krause et al. 2016). Resistance can also be derived from modifications in the bacterial membrane to reduce the aminoglycosides’ permeability. Previous studies have shown that mutations of genes encoding for the components of the cell wall or cellular membranes such as lipopolysaccharides, porins, and efflux pumps disrupt the electrostatic interaction with aminoglycosides, thereby limiting their entry into bacterial cells (Garneau-Tsodikova and Labby 2016; Morita et al. 2012a, b). Similarly, the drug’s permeability across the bacterial membrane is also reduced by the formation of biofilm. Biofilm is defined as a mucoid matrix made of polysaccharides, proteins, and nucleic acid which is produced by a bacterial community adhering onto biotic (e.g., host organs, damaged tissues) or abiotic (e.g., medical devices) surfaces and displaying resistance to the extreme of the surrounding environment (e.g., host immune response, adverse conditions, and antimicrobial therapies). Such extreme capability of resistance is known to be attributed to (1) the increasing frequency of mutation and (2) horizontal transfer of resistant gene(s) among the bacterial population and between different species (i.e. intra- and interspecies biofilm) (Giaouris et al. 2015; Hoiby et al. 2010). Currently, most pathogenic bacteria in humans have employed biofilm formation as one of the resistance mechanisms against several antibiotics, causing numerous chronic infections (e.g., chronic wounds, urinary tract infections, tuberculosis, and dental caries) and hospital-acquired infections (e.g., catheter- and ventilator-associated infections) which could last up to a lifetime (Chen and Wen 2011; Cole et al. 2014; Di Domenico et al. 2017; Esteban and Garcia-Coca 2017; Hoiby et al. 2010). Furthermore, biofilm formation also causes a tremendous burden in the food industry through food spoilage and foodborne diseases, as the biofilm-forming pathogenic bacteria can also colonize on the surface of food processing and preservation facilities (Bai and Rai 2011; Bridier et al. 2015; Chmielewski and Frank 2003; Gopu et al. 2015). Resistance against aminoglycosides in biofilm-forming bacteria is majorly regulated by the nucleic acid (extracellular DNA, e-DNA) (Chiang et al. 2013). The e-DNA component of the biofilm matrix was proposed to hinder the penetration of aminoglycosides by (1) acidifying the biofilm environment, (2) chelating with positively charged drugs through electrostatic interaction, (3) modifying the outer membrane permeability, and (4) initiating surface protection (Das et al. 2010; Mulcahy et al. 2008; Wilton et al. 2016). Other barriers against aminoglycoside access include (1) the multicellular organization within the biofilm which mediates the metabolism rate and nutrient availability across the cell layers, (2) persister subpopulation which remains dormant throughout the antimicrobial treatment, (3) horizontal transfer of resistance gene(s), and (4) environmental stresses (Fraud and Poole 2011; Sato et al. 2018; Yu et al. 2018). Furthermore, the use of certain aminoglycosides at low concentrations (subinhibitory concentration) also contributed to bacterial resistance through diverse mechanisms (Aka and Haji 2015; Ranieri et al. 2018). Combining with other cellular-level resistance mechanisms mentioned earlier, biofilm formation poses a tremendous challenge for the use of aminoglycosides in current antimicrobial therapies. The present review paper firstly explains how biofilm formation and other mechanisms are involved in aminoglycoside resistance, and summarizes several alternative approaches that are currently conducted to improve the use of aminoglycosides in treating biofilm-forming pathogenic bacteria. Furthermore, some future perspectives are also proposed for extending the applications of aminoglycosides in the long term.

Fig. 2
figure 2

Different resistance mechanisms in bacteria against aminoglycosides [information obtained from the literature (Garneau-Tsodikova and Labby 2016; Lin et al. 2015; Olivares et al. 2013)]

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Emergence of the aminoglycoside antibiotic-resistant bacterial strain and possible mechanism of resistance

For biofilm-forming pathogenic bacteria, biofilm formation adds to their diverse mechanisms to resist aminoglycoside activity. Besides the efflux pumps, degrading enzymes, and membrane impermeability which are common resistance mechanisms against other antibiotics, the bacteria develop several mechanisms to specifically resist against aminoglycoside activities (Vestergaard et al. 2018; Westbrock-Wadman et al. 1999). Three major types of resistance mechanisms such as intrinsic, adaptive, and acquired have been explained in Fig. 2. These mechanisms which have been developed by both Gram-negative and Gram-positive (Garneau-Tsodikova and Labby 2016; Lin et al. 2015) are summarized as follows:

  1. 1.

    Presence of aminoglycoside-modifying enzymes causing O-adenylylation, O-phosphorylation, or N-acetylation of amine or hydroxyl groups by specific enzymes of aminoglycoside molecule at different locations (Ramirez et al. 2013).

  2. 2.

    Mutation of 16S rRNA-encoded gene or ribosomal proteins (Hobbie et al. 2006; Springer et al. 2001).

  3. 3.

    Methylation of 16S rRNA (Galimand et al. 2005).

  4. 4.

    Riboswitch, which is present on the leader DNAs encoded by acetyltransferase- and adenyltransferase-encoding genes, senses the binding of aminoglycoside and activates aminoglycoside resistance (Jia et al. 2013).

  5. 5.

    Reduction in aminoglycoside permeability through the outer membrane or their transport through the inner membrane (Vestergaard et al. 2018).

  6. 6.

    Export by efflux pumps (Westbrock-Wadman et al. 1999).

  7. 7.

    Magnet et al. (2003) showed that the aminoglycoside resistance also occurs as a result of tight binding with the altered aminoglycoside acetyltransferase.

  8. 8.

    Shielding of extracellular DNA present in the biofilms (Chiang et al. 2013; Wilton et al. 2016).

Biofilm-forming and virulence factors producing properties of aminoglycosides: contribution of aminoglycosides toward pathogenesis

The attempts to reduce the concentrations of antibiotics to below their minimum inhibitory concentrations (sub-MIC) as a solution for lowering selective pressure that resulted from overuse and misuse of these drugs have so far fallen behind (Wistrand-Yuen et al. 2018). Recent studies have reported multiple adverse effects of using aminoglycosides at sub-MIC in antimicrobial therapies. Firstly, the low concentration of drugs may face several difficulties upon penetration through the bacterial cell membrane and biofilm matrix such as (1) unexpected and uncontrollable loss of drug concentration which leads to low accumulation and limited drug activity and (2) instability against environmental changes and resistant responses during circulation in the bacterial system or biofilm matrix (Tseng et al. 1972). For instance, a study conducted by Bhattacharya et al. (2017) found that exposure to gentamicin at the sub-MIC level has triggered S. aureus to generate reactive oxygen species (ROS), thus becoming more resistant to antibiotics treatment. Secondly, the low dose of aminoglycosides is often associated with frequent administration route, which would trigger the bacteria to develop resistance over a long period. Finally, due to the complex organization of bacteria, it is exceedingly challenging for a small amount of conventional aminoglycosides to encounter multiple resistance mechanisms all at once. Taking the bacterial biofilm formation as an example, this multidrug resistance mechanism is the result of complicated signal sensing and regulatory networks and is essentially processed along with the production of numerous virulence factors. Thus, biofilm responds to aminoglycoside activity at sub-MIC through extremely various means (Kaplan 2011). For example, as it is widely known that elevation of the bis-(3′-5′)-cyclic dimeric guanosine monophosphate (c-di-GMP) second messenger level plays a determining role throughout the establishment and dispersal of biofilm, sub-MIC of aminoglycosides which elevates the c-di-GMP level would trigger biofilm formation. Tobramycin at sub-MIC has promoted the dense biofilm formation in P. aeruginosa by suppressing the expression of aminoglycoside response regulator gene (arr) that regulates the c-di-GMP production while elevating the expression of a multitude of biofilm-associated genes (Hoffman et al. 2005; Linares et al. 2006). Tobramycin at this level also upregulated the expression of other essential factors of biofilm formation such as e-DNA, quorum sensing, and iron uptake (Tahrioui et al. 2019). In contrast, biofilm establishment in Escherichia coli was resulted from sub-MIC of fluoroquinolone enhancing the c-di-GMP level and stresses (Boehm et al. 2009). In addition to c-di-GMP, bacterial biofilm formation is also linked with type VI and type III protein secretion systems as they are all under the regulation of Gac/Rsm regulatory pathway. Thus, by genuinely triggering the type VI protein secretion systems, the presence of kanamycin at the sub-MIC level has promoted P. aeruginosa biofilm formation (Jones et al. 2013).

Another target of sub-MIC aminoglycoside triggering bacterial biofilm formation is toward the components of extracellular polysaccharide substrates (EPS) such as exopolysaccharides (alginate and teichoic acids), extracellular proteins (protease, adhesins, lectins, and surface appendages), lipids (lipopolysaccharides and biosurfactants), and e-DNA. The important role of EPS in hindering the penetration of antimicrobial drugs which has been described in the previous section is also targeted by sub-MIC aminoglycosides. A previous study conducted by Szczuka et al. (2017) has shown that erythromycin, fluoroquinolone, and tigecycline upregulated several biofilm-related genes such as poly-N-acetylglucosamine gene (ica) and sigma factor (sigB) in Staphylococcus epidermidis, thereby promoting the attachment of biofilm-forming cells and production of teichoic acids, adhesins, and e-DNA. Expression of the Wyz membrane transporter system in E. coli was upregulated proportionally with colonic acid polysaccharide by the presence of sub-MIC of streptomycin, thereby enhancing biofilm formation (Kumar and Ting 2016). Although the curli fimbriae production and biofilm formation by E. coli were reduced by amikacin, this reduction appeared insignificant and highly dependent on the bacterial lifestyle (Wojnicz and Tichaczek-Goska 2013).

It is well-known that virulence properties play an equally important role as biofilm formation itself in biofilm-associated infections significantly contributes to bacterial pathogenesis and is closely linked to antibiotic resistance (Schroeder et al. 2017). In the establishment stage of biofilm, virulence properties function in sensing, translocating, and properly attaching to desirable surfaces to allow the switch from planktonic (free-floating) to sessile states (flagellar-mediated swimming and swarming, pili-mediated twitching motilities, and surface adhesins). During the development and maturation of biofilm, virulence properties are performed through initiating reactive oxygen damage, disrupting erythrocytes, sequestering the iron, releasing toxins, and damaging the host proteins (Schroeder et al. 2017). In general, virulence properties are regulated by “quorum sensing” (QS), which is a cell-to-cell communication network (Khan et al. 2019a). Recently, several reports have shown that by triggering either the bacterial virulence properties directly or indirectly through their QS regulation system, sub-MICs of aminoglycosides can also promote biofilm formation. For example, in the presence of kanamycin at the sub-MIC level, N-acyl-l-homoserine lactones signaling molecules of the QS system were upregulated, leading to the increasing level of biofilm formation, chitinase production, and flagellar activity (Liu et al. 2013). Previously, sub-MIC of tobramycin increased QS activity, e-DNA production, and iron acquisition, causing dense biofilm formation (Tahrioui et al. 2019). P. aeruginosa virulence factors, along with the surface charge, hydrophobicity, and adhesins, were also significantly affected by streptomycin activity, thus forming high biofilm biomass (Kumar and Ting 2016). Overall, the sub-MIC of several conventional aminoglycosides have been incapable of inhibiting the growth and biofilm formation of a wide range of Gram-negative and Gram-positive bacteria. In the attempts to reintroduce these treatments, several alternative strategies by combining multiple drugs (i.e., combination strategy) or using nonantibiotic potentiator/adjuvants/materials appear to be highly favorable at the present.

Alternative strategies for the application of aminoglycosides

Due to the downfalls in preventing the human pathogenic bacteria from forming biofilm and causing biofilm-associated infections, the application of conventional aminoglycoside monotherapy has been discontinued. To revitalize the aminoglycoside efficacy to catch up with such rapid rate of emerging resistance, several directions have been proposed: (1) widening the spectrum of aminoglycoside activities toward multiple targets, (2) shifting the aminoglycoside target from “killing” to “weakening,” and (3) potentiating the aminoglycoside activity by using additional materials (e.g., nonantibiotic compounds and nanocarriers) or by exploiting the active concentrations (e.g., sub-MIC) and modifying chemical structure. Up to the present, each direction has brought about promising results in biofilm inhibition by targeting various aspects of biofilm formation from development stages, signaling and regulation systems, biofilm architecture to the alongside virulence properties of a wide range of human pathogenic bacteria. Although the pharmacokinetics and interactions between combined drugs or compounds highly require further studies and discovery for new alternative strategies should remain ongoing, the significant achievements from modern alternative strategies toward biofilm formation of human pathogenic bacteria give rise to the future possibility of combating biofilm-associated infections. Table 1 presents a list of aminoglycoside antibiotics which showed antimicrobial activity at their active concentration.

Table 1 Aminoglycosides either alone or in combination with other antibiotic or antimicrobial agents used for treating the biofilm-forming pathogenic bacteria
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Optimization of subinhibitory concentration and environmental factors of aminoglycosides

Although conventional aminoglycoside drugs have been reported to induce resistance in various bacteria, several improvement approaches such as (1) combining the sub-MIC of multiple drugs, (2) shifting their application to antivirulence or anti-QS (quorum quenching) strategies, and (3) optimizing the bacterial culture environment in terms of temperature, pH, and media types have recently been introduced and set a promising future for aminoglycosides.

In the first approach, combination strategies between different aminoglycosides and between aminoglycoside and other antibiotics have shown effectiveness in maintaining the antimicrobial effect at a lower dose of individual drugs. For example, the combinations of kanamycin with ceftiofur, amoxicillin, colistin sulfate, lincomycin, clarithromycin, and berberine at their sub-MICs have effectively inhibited biofilm formation in S. aureus by suppressing several biofilm-related genes (Yang et al. 2017). The synergistic effects between tobramycin and clarithromycin or ceftazidime have exhibited high antibacterial and antibiofilm activities on P. aeruginosa and also eradicated the pre-existing mature biofilm (Ghorbani et al. 2017; Kapoor and Murphy 2018).

In the second approach, sub-MIC of aminoglycosides are used to target the virulence properties and QS of the biofilm-forming bacteria to disarm their biofilm formation as well as their pathogenesis (Fleitas Martinez et al. 2018; Fong et al. 2018). As the virulence properties and QS are nonessential to bacterial survival, their attenuation would reduce selection pressure for resistant strains as compared to conventional antimicrobial drugs, thus reducing the potential of resistance evolution (Dickey et al. 2017; Rasko and Sperandio 2010). The combination of curcumin and sub-MIC of gentamicin has synergistically downregulated the expression of QS-related genes, biofilm formation, and motility of P. aeruginosa (Bahari et al. 2017). Similarly, expression of the genes encoding for QS signaling molecules of Acinetobacter baumannii was also reduced by the presence of streptomycin at sub-MIC (Saroj and Rather 2013). Furthermore, the antivirulence activity of sub-MIC aminoglycosides can be further improved when they are incorporated with nonantibiotic compounds (e.g., bioactive compounds, small molecules, antimicrobial peptides) or delivered by nanocarriers, of which a detailed discussion shall be present in the following section of this review paper. Despite being highly potential to combat bacterial biofilm formation, antivirulence strategy yet requires further optimization work due to three main reasons (Fleitas Martinez et al. 2019). Firstly, the dynamics of their production/regulation are extremely varied throughout different stages of biofilm formation and different bacterial species (Dickey et al. 2017). Secondly, besides phenotypic tests and gene expression, the effective screening and diagnosis techniques that give out fast and precise results about virulence properties of various biofilm-forming bacteria remain under research (Ashrafudoulla et al. 2019; Tukenmez et al. 2019). Finally, the clinical application of sub-MICs of aminoglycoside as antivirulence agents onto animal models or infectious human patients requires long-term and complicated research and development and social approval (Fleitas Martinez et al. 2019; Maura et al. 2017).

In the third approach, the antibacterial and antibiofilm activities of aminoglycosides at sub-MIC are improved by optimizing the culture environment, including temperature, pH, and culture types. Due to its polycationic nature, the aminoglycoside activity was reported to be highly influenced by these environmental factors. For instance, changes in pH and the concentration of the nutrients of the culture media have affected the permeability of gentamicin through S. aureus biofilm as these conditions supported the electrostatic interaction between the positively charged aminoglycoside drug and the negatively charged bacterial biofilm components such as exopolysaccharides and e-DNA (Henry-Stanley et al. 2014). A similar observation was obtained when streptomycin at sub-MIC was applied to the biofilm of P. aeruginosa (Khan et al. 2019b). The drug performed optimal inhibitory activity to the bacterial biofilm formation and virulence factor production under the culture conditions of alkaline pH, 35 °C, and TSB/LB media (Khan et al. 2019b). In the current situation where in-depth studies about the underlying mechanisms of these effects remain lacking, the reported results provide insights for the future development of a new antibiofilm strategy.

Immobilization and combinatorial application of aminoglycosides

During the past few decades, frequent administration of a single antibiotic at high doses and frequency termed as monotherapy has resulted in resistance emergence in bacteria (Gonzalez 3rd and Spencer 1998; Traugott et al. 2011). Particularly, several human pathogenic bacteria have developed multiple mechanisms to resist a majority of commonly used antibiotics (multidrug resistance); thus, alternative therapeutic approaches are in urgent need. Under such circumstances, combinatory approaches which involve combining (1) two or more antibiotics and (2) conventional antibiotics with one or more nonantibiotic compounds have been used to improve the antimicrobial efficacy of monotherapies (Eom et al. 2016; Kim et al. 2017; Tyers and Wright 2019). Several combinations used in this approach are summarized in Table 2.

Table 2 Combinatorial application, immobilization, and nanoformulation of aminoglycosides
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Depending on the specific nature and function of individuals, combinations of antibiotics help to broaden the spectrum of antibiotic activity while lowering their doses and toxicity. For example, amikacin and isepamicin have been combined with fosfomycin to synergistically suppress the growth and biofilm formation of P. aeruginosa in the rat model (Cai et al. 2009). The mechanism was proposed that fosfomycin has performed the cell wall-degrading function, which improved the limited penetration of aminoglycosides through the bacterial cell wall and biofilm matrix (Cai et al. 2009). The combination of tobramycin and clarithromycin applied to biofilm-forming isolates of P. aeruginosa has exhibited antibiofilm effectiveness against a significant number of tested isolates (Ghorbani et al. 2017).

Recently, the combinations of antibiotics with the nonantibiotic compounds that are often referred to as “antibiotic adjuvants” have been extensively exploited (Douafer et al. 2019). These compounds are varied in sources (e.g., natural compounds, chemical compounds, plant phytochemicals, small molecules, and antimicrobial peptides) and functions (e.g., QS inhibitor, efflux pump inhibitor, drug uptake promoter, and antivirulent agent) (Wright 2016). Gentamicin upon mixing with chitosan polysaccharide has been improved in permeability through the biofilm architecture of L. monocytogenes, Listeria welshimeri, and Listeria innocua, thereby exhibiting inhibition and eradication activities toward the biofilm of the three Listeria bacteria (Mu et al. 2014). Combinations of a QS inhibitor called quercetin with tobramycin, levofloxacin, and amikacin exerted significant killing effects on P. aeruginosa biofilm cells, as the combination was added with anti-quorum-sensing activity of quercetin (Vipin et al. 2019). By combining with chitosan-A polycationic biopolymer, the antibiofilm activity of streptomycin toward Gram-positive bacteria has been significantly enhanced due to higher drug permeability and stronger ionic interaction with negatively charged biofilm constituents (Zhang et al. 2013). Similarly, conjugation with chitosan-oligosaccharide, which is low molecular weight chitosan and also well-known for its antimicrobial and antibiofilm activities, has supported the streptomycin efficacy in biofilm dispersal and inactivation of efflux pump and exopolysaccharide production (Li et al. 2019). On the other hand, the antimicrobial peptides (AMPs) such as G10KHc and GL13K targeted the P. aeruginosa cell membrane structure to synergistically elevate the penetration of tobramycin; thus, the drug activity against the bacterial biofilm was significantly improved (Dosler and Karaaslan 2014; Eckert et al. 2006; Hirt and Gorr 2013). Colistin is another AMP that has been combined with a wide range of antibiotics to treat biofilm-forming bacteria. The combinatory therapy using colistin combined with tobramycin has effectively killed the P. aeruginosa biofilm cells without causing adverse reactions when applied for mice models and cystic fibrosis patients (Herrmann et al. 2010). Besides, combinations of aminoglycosides (gentamicin, tobramycin, and streptomycin) with chemical compounds such as triclosan and nitric oxide have effectively eradicated the established P. aeruginosa biofilm and eliminated the persistent cells living within (Barraud et al. 2006; Maiden et al. 2018). Rhamnolipid as a membrane-acting agent induces the uptake of aminoglycosides inside the bacterial without the involvement of proton motive force, thereby it potentiates the bactericidal properties of aminoglycosides (Radlinski et al. 2019; Yarlagadda and Wright 2019). A similar eradicating effect was achieved when the combinations of aminoglycosides with plant phytochemicals such as (1) gentamicin with oleanolic acid or (2) tobramycin with tannic acid and gallic acid were applied to A. baumannii and S. aureus biofilms, respectively (Dong et al. 2018; Shin and Park 2015). Overall, with numerous significances in improving and potentiating the antibiofilm activity of aminoglycosides mentioned above, the combinatory strategies can be considered as highly helpful for drug use in the long run.

Nanoformulation of aminoglycosides

One of the major challenges of conventional antibiotic approaches is the undesirable loss of antibiotic concentration upon penetration through the bacterial cell membrane/cell wall/biofilm matrix. The insufficient amount of antimicrobial drugs, therefore, requires administering at a high frequency and high dose, which is most likely to result in in vivo toxicity, bacterial resistance, and tremendous economic burden (Allahverdiyev et al. 2011; Van Giau et al. 2019). The advanced development of nanotechnology during the past few years has revitalized this limitation of conventional antibiotic therapies. Numerous studies have recognized the tremendous benefits of encapsulating and grafting of antibiotics into nanomaterials, including (1) controlled release with minimized concentration leakage and (2) stability against the bacterial clearance responses (Baptista et al. 2018). Nanomaterials are extremely diverse in terms of source, production methods, compatibility, and functions, allowing them to carry different classes of antibiotics. Several types of nanomaterials such as liposomes, hydrogel, film, smart surface, and nanoparticles have been used for aminoglycoside encapsulation/immobilization and showed significant improvement in the drug’s activity (Jijie et al. 2017). Such diversity in nanocarrier types and antibacterial/antibiofilm actions is highly promising for the control of biofilm formation and the emergence of multidrug resistance in the future (Abed and Couvreur 2014).

A liposome is a universal lipid-based colloidal vesicle which has been used to deliver both hydrophilic and hydrophobic antibiotics for a long time (Langner and Kral 1999). Having similar physiochemical properties as a bacterial cell membrane, liposome nanoformulation easily fuses through this barrier to successfully deliver their containing drugs, providing beneficial pharmacokinetics, selectivity, and biodistribution which were expected to overcome drug resistance in bacteria, especially the human pathogenic ones (Alipour and Suntres 2014; Drulis-Kawa and Dorotkiewicz-Jach 2010). For example, tobramycin (Tob) to which P. aeruginosa has developed resistance was chemically encapsulated in polyethylene glycol (PEG)ylated-liposome to form Tob–PEG conjugated structure. The conjugate has shown an increase in stability and antibacterial and antibiofilm efficacy as compared to individual Tob (Du et al. 2015). Similarly, by encapsulating aminoglycosides (amikacin, gentamicin, and tobramycin) into a liposome, the permeability of the drug through the bacterial cell membrane was significantly increased, thereby performing more active inhibitory effect to P. aeruginosa growth (Alipour and Suntres 2014). On the other hand, gentamicin encapsulated in liposome was further stabilized with positively charged lysozyme enzyme to elevate the drug delivery and interaction with negatively charged biofilm constituents of Gram-positive (S. aureus) and Gram-negative (P. aeruginosa), thus significantly increasing the inhibition and disruption efficacy toward the biofilm formed by both bacteria (Hou et al. 2017). Despite these achievements, the use of liposome has currently become less favorable due to their instability against physical conditions (e.g., heat, storage temperature, and oxidation) and high-cost production. Further optimization work is demanded to improve and extend the liposome activities for future clinical applications (Drulis-Kawa and Dorotkiewicz-Jach 2010).

“Smart” surfaces that have been adopted in combating biofilm formation recently are varied in design and antibiofilm functions (Li et al. 2018). Although the antibiofilm activity of “smart” aminoglycoside surfaces has remained unexploited, their significances in physiochemical properties and antibacterial activity against S. epidermidis, E. coli, P. aeruginosa, and S. aureus recorded in vitro and in vivo has provided an insight to the potential applications of these surfaces (Hu et al. 2017).

Nanoparticles (NPs) which are defined as having at least one dimension less than 100 nm and synthesized from metal (i.e., metal-based/metallic/inorganic NPs) or polysaccharides (i.e., polysaccharide-based/polymeric/organic NPs) are extensively applied in drug delivery systems (Jeevanandam et al. 2018; Khan et al. 2017, 2018). Due to their large surface-area-to-volume ratio, small size, controlled release, diverse biological activities, stability, and minimized toxicity, NPs easily pass through the cell membrane and the biofilm matrix to effectively deliver their carry-on drug to the targeted infectious site with minimal concentration loss (Aderibigbe 2017; Andonova 2017; Javaid et al. 2018; Liu et al. 2008; Salouti and Ahangari 2014). Up to the present, aminoglycosides have been loaded onto these delivery systems either externally (as a coating or a stabilizing/capping/reducing agent) or internally (encapsulation). In the former case, it was proposed that the capping and reducing properties of the drugs allowed the drug adsorption onto the NPs’ surface (Shah et al. 2014). In return, the NPs which are capped/reduced by antibiotics are less likely to form aggregates and had increasing antibacterial activity (Gad El-Rab et al. 2018; Shedbalkar et al. 2014). For instance, by conjugation to gold NPs in the form of reducing/capping agents, kanamycin was rapidly delivered into the cytosol and effectively exerted a bactericidal effect on S. epidermidis and E. aerogenes (Payne et al. 2016). Streptomycin and kanamycin were employed as reducing agents along with sodium borohydride to synthesize antibiotic-conjugated gold NPs, which exhibited antibacterial activity against S. aureus, Micrococcus luteus, and E. coli and had high stability against heat, UV light, and long-term storage at room temperature (Bhattacharya et al. 2012). Hybrid nanoformulation of silica oxide (SiO2) and gentamicin exhibited (1) antibacterial and antibiofilm activities to methicillin-resistant S. aureus (MRSA) and (2) eradication and destructive activities to E. coli established biofilm structure (Mosselhy et al. 2018). Silica NPs which were synthesized using aminoglycosides (gentamicin, kanamycin, and neomycin) actively inhibited the growth of resistant bacterial strains without causing cytotoxicity (Agnihotri et al. 2015). Due to the synergism with aminoglycoside capping agent, the synthesized silver NPs exhibited higher antibacterial activity against E. coli and S. aureus than those which were capped with citrate or SDS (Kora and Rastogi 2013). In the latter case where aminoglycosides are encapsulated into the NPs, the permeability of the drug through the bacterial cell membrane or biofilm matrix, their control release, and their stability are improved, thus remaining active inside the living systems for a longer period (Deacon et al. 2015). Gentamicin loaded to gold NPs was able to inhibit the growth and biofilm formation as well as eradicated the preformed mature biofilm of P. aeruginosa, L. monocytogenes, and E. coli without causing cytotoxicity to macrophages (Mu et al. 2016c). By loading tobramycin onto small-sized citrate-capped silver NPs to treat P. aeruginosa biofilm formation, the NPs further potentiated the disruption effect of tobramycin toward the bacterial biofilm matrix and cell membrane (Habash et al. 2017). Likewise, the S. aureus cell membrane and biofilm were also targeted by the chitosan/Fe3O4@poly (ethylene glycol) (PEG)-gentamicin NPs, where the electrostatic interaction between gentamicin, protonated chitosan, and PEG aided the drug entry through the bacterial membrane, while the magnetic force of Fe3O4 NPs allowed the drug penetration through the bacterial preformed biofilm (Wang et al. 2018a). Besides, loading into nanocarriers was also found to affect the rate of drug release, as, in the case of gentamicin being loaded onto cysteine and glutathione-capped gold NPs, the addition of gentamicin enhanced the antibacterial efficacy of the synthesized NPs against S. aureus. Furthermore, the drug remained actively releasing for two more days, which was attributed to the neutral environmental pH supporting the binding of nanocarrier and biofilm polysaccharides (Perni and Prokopovich 2014). As the combination strategies of aminoglycosides are becoming highly favorable, the combinations were proposed to enhance their “killing” and biofilm inhibitory effectiveness upon co-delivery by NPs, as the drug conjugates can penetrate more rapidly through the cell membrane and biofilm matrix. For instance, the chitosan–streptomycin conjugates which had previously shown improved antibacterial and antibiofilm activities were used to synthesize gold NPs (Mu et al. 2016c). With the aid of gold NPs, the conjugates readily crossed the biofilm and cell membrane barriers, thus (1) actively inhibiting biofilm formation and eradicating preformed biofilm of P. aeruginosa and (2) exerting a bactericidal effect to both Gram-positive and Gram-negative bacteria (L. monocytogenes, S. aureus, S. Typhimurium, and E. coli) (Mu et al. 2016a).

In addition to metal-based NPs, polymeric NPs have also been used as a nanocarrier for aminoglycosides. In comparison to metal-based NPs, the polymeric NPs provide several different advantages in terms of minimal toxicity, biocompatibility, biodegradability, and environmental friendliness (El-Say and El-Sawy 2017). Tobramycin binding to alginate, which has been functionalized with DNase I and then encapsulated into chitosan NPs, was stably and effectively delivered and exhibited antibacterial activity against P. aeruginosa in the lungs of cystic fibrosis-infected patients (Deacon et al. 2015). Amikacin was loaded into poly-d,l-lactide-co-glycolide (PLGA)-based NPs and was readily delivered through a biofilm matrix to perform both antibiofilm and antibacterial activities to P. aeruginosa planktonic and biofilm cells without causing cytotoxicity (Sabaeifard et al. 2017). As tobramycin was reported as forming a weak bonding with the PLGA NPs, the aminoglycoside was firstly combined with dioctyl sulfosuccinate and then loaded onto PLGA NPs, which resulted in the sustainable antibacterial activity against P. aeruginosa (Hill et al. 2019). Polymeric NPs were also capable of co-delivering the combination of nitric oxide (NO) and gentamicin across the biofilm matrix of P. aeruginosa (Nguyen et al. 2016). The release of NO and gentamicin has effectively eradicated the bacterial mature biofilm and killed the dispersed biofilm cells (Nguyen et al. 2016). The chemistry used for the conjugation or nanoformulation of aminoglycosides has been explained in several literature (Agnihotri et al. 2015; Kondaveeti et al. 2018; Liu et al. 2017; Mugabe et al. 2006b; Rukholm et al. 2006; Yan et al. 2019). In most of the cases, the conjugation of aminoglycoside with other molecules/agents involved carbodiimide chemistry (Kondaveeti et al. 2018; Liu et al. 2017; Perni and Prokopovich 2014). For example, the synthesis of aminoglycoside–metal NPs comprises two steps: the first step involves the synthesis of metal NPs by using a reducing agent and the second step involves the conjugation of aminoglycoside via condensation reaction in the presence of 1-ethyl-3-(3-dimethyl aminopropyl) carbodiimide (EDC) and N-hydroxysuccinimide (NHS) (Perni and Prokopovich 2014). Another example for conjugation of aminoglycoside with chitosan involved the following step reactions: in the first step, chitosan gets oxidized by periodate which results in the C2–C3 bond cleavage and formation of the aldehyde group. In the second step, aminoglycoside conjugates with the aldehyde group of oxide chitosan via the Schiff base reaction (Yan et al. 2019). Similarly, for the encapsulation of aminoglycoside into liposome, the methodology included the dehydration–rehydration vesicle method (Alhariri et al. 2017; Kirby and Gregoriadis 1984; Mugabe et al. 2006a), where liposome was prepared by mixing 1,2-dipalmitoylsn-glycero-3-phosphocholine (DPPC) and cholesterol in the molar ratio of 2:1. The prepared vesicles were mixed with aminoglycosides and freeze dried followed by the rehydration of the mixture. The rehydrated mixture was ready to use after washing with phosphate buffer. A representative example of the chemical reaction (chemistry of conjugation) used for the covalent conjugation of gentamicin with different materials such as chitosan/alginate or nanoformulation with metallic or polymeric nanoparticles is explained in Fig. 3.

Fig. 3
figure 3

Chemical reaction used for the conjugating of gentamicin with other active agents and nanoformulation with metallic/polymeric nanomaterials [information obtained from the literature (Agnihotri et al. 2015; Kondaveeti et al. 2018; Liu et al. 2017; Mugabe et al. 2006b; Rukholm et al. 2006; Yan et al. 2019)]. a Conjugation of gentamicin to chitosan followed by two steps. In the first step, chitosan gets oxidized by periodate which results in the C2–C3 bond cleavage and formation of the aldehyde group. In the second step, gentamicin conjugated with the aldehyde group of oxide chitosan via the Schiff base reaction (Yan et al. 2019). b Formation of a chitosan–gentamicin film by carbodiimide chemistry (Liu et al. 2017). Firstly, chitosan film formed by air drying of chitosan solution; secondly, generation of amide and carboxyl group by citric acid on the surface of chitosan; and thirdly, covalent grafting of gentamicin to the available carboxyl group of chitosan via the help of 1-ethyl-3-(3-dimethyl aminopropyl) carbodiimide (EDC) and N-hydroxysuccinimide (NHS) as cross-linker. c Synthesis of the alginate–gentamicin conjugate by carbodiimide chemistry (Kondaveeti et al. 2018). d Encapsulation of gentamicin in a liposome (Rukholm et al. 2006). e Functionalized gentamicin-conjugated silica nanoparticles. First, synthesis of silica nanoparticles; second, generation of epoxy groups with the help of 3-glycidyloxypropyltrimethoxysilane; and third, functionalization with gentamicin (Agnihotri et al. 2015). f Synthesis of gentamicin-conjugated gold nanoparticles. In the first step, glutathione-capped gold nanoparticles are synthesized, and in the second step, the gentamicin is conjugated via condensation reaction upon EDC and NHS presence (which are involved in the activation of carboxyl group on the glutathione capping agent) (Perni and Prokopovich 2014)

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Application of chemically modified form of the aminoglycosides

Although their activity toward the bacterial biofilm structure has remained limitedly reported, the large diversity of forms and synthesis methods of chemically modified aminoglycosides has been extremely significant (Thamban Chandrika and Garneau-Tsodikova 2018). Modifying the currently available aminoglycosides provides the advantage of improving certain characteristics of the drugs within a shorter time period as compared to searching and developing a new drug (Bera et al. 2016). A few typical examples could be aminoglycoside derivatives, antibacterial amphiphilic aminoglycoside (AAG), and aminoglycoside-derived cationic amphiphilic drug. Firstly, synthetic derivatives of aminoglycosides such as plazomicin and netilmicin are some new generation of aminoglycosides that have been discovered in recent years. Since the –NH2 group majorly determines the aminoglycoside activity, modifications in their position and number which have given rise to plazomicin, netilmicin, and numerous modified aminoglycosides have been performed (Zarate et al. 2018). In most cases, they exhibited antibacterial activity to various human pathogens, including those which are referred to as “multidrug resistant” and have biofilm-forming ability (Cox et al. 2018; Landman et al. 2011; Noone 1984; Reyes et al. 2011). Secondly, AAG such as naphthylalkyl amine is a structurally modified form of neamine that has shifted to a new target site—the bacterial outer membrane and/or lipopolysaccharide and exhibited electrostatic interaction to destabilize the bacterial cells (Sautrey et al. 2014). Thirdly, various aminoglycoside-derived cationic amphiphilic drugs have also shown effective antibacterial activity against a wide range of biofilm-forming Gram-positive and Gram-negative bacteria (Benhamou et al. 2015). Overall, based on their active antibacterial potentials to a wide spectrum of biofilm-forming bacteria, the chemically modified aminoglycosides can be considered as a promising alternative for further application on biofilm inhibition approaches.

Conclusion and future perspectives

The major challenge to combination strategies in developing a new drug relies on complex antibiotic pharmacology. Searching for the precise treatment concentrations and duration for a single antibiotic is already difficult. In order to achieve the target for two compounds that are synergistically conjugative in dynamics and pharmacokinetics to maintain and encourage drug development, single agents may also be required for clinical trials. The toxicology of each agent, as well as the combination, must also be carefully investigated, whether unexpected drug–drug interactions take place. Developing combination therapy is more complex than monotherapy. Nevertheless, under the situation where monotherapy with single-target drugs has led to rapid resistance, the new single agents have shown efficacy during the past years despite that all antibiotics are at risk of being compromised by increasing resistance level.

Throughout the past few decades, knowledge about biofilm formation and other associated virulence properties has been extensively advanced. With the current situation where human pathogenic bacteria have vastly developed biofilm formation and with the production of virulence factors to resist a majority of conventional aminoglycosides, the discovery of an alternative control strategy is now highly urgent. Some of the up-to-date strategies to improve and develop aminoglycoside antibiofilm activity have been reviewed in the present paper, including (1) exploitation of the antivirulence potentials of aminoglycosides at the sub-MIC level, (2) immobilization/encapsulation of aminoglycosides to various types of nanocarriers, and (3) modifications in the chemical structure of aminoglycosides. The detailed mechanisms of antibiofilm and antibacterial activity of different forms of aminoglycosides (either free forms or in conjugation/nanoformulation forms) have been explained in detail in Fig. 4. Although these strategies have actively controlled bacterial biofilm in various means such as preventing biofilm formation, disrupting the pre-existing mature biofilm, or attenuating the expression and regulation of virulence properties, extensive studies are demanded to take place in the future in order to achieve higher control over bacterial biofilm in the long term. Some suggestions are presented as follows:

  1. 1.

    The antibiofilm activity of some aminoglycosides should be studied at the molecular level.

  2. 2.

    The use of aminoglycoside combinations should be carefully examined and changed if necessary to prevent new resistance emergence.

  3. 3.

    The adjuvants that are used to potentiate aminoglycoside activity should also be studied for their profiles and activities.

  4. 4.

    With the vast number of new antibiofilm and antivirulence strategies being developed, other aminoglycosides should also be exploited for their potentials using all the summarized alternative strategies.

  5. 5.

    Optimization work toward the culture environment and storage conditions should be paid more attention.

  6. 6.

    The options of aminoglycoside adjuvants and nanocarriers should continuously be extended and advanced.

  7. 7.

    Co-delivery between aminoglycosides with other antibiotics or bioactive compounds is recommended to improve the drug activity, especially when the drug and its nanocarriers are weakly bonded.

  8. 8.

    The antibiotic adjuvants/enhancers which are either naturally or chemically synthesized must positively interact with its conjugated antibiotics without causing side effects or antagonistic effects to the antibiotics.

  9. 9.

    For clinical trials, an appropriate schedule of applications should be constructed carefully and specifically to the aminoglycosides used.

  10. 10.

    Studies of resistant genes must be conducted for more specific understandings about the internal driving force of resistant responses.

  11. 11.

    The antibiofilm activity of newly synthesized aminoglycosides demands more exploitation.

  12. 12.

    The applications of aminoglycosides in inhibiting human pathogenic bacteria must be carefully maintained and regulated by responsible authorities.

Fig. 4
figure 4

Different strategies employed for treating biofilm-forming pathogenic bacteria by free form or conjugate forms of aminoglycoside antibiotics

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