Introduction

Increasing resistance of Gram-negative bacteria to β-lactam antibiotics currently represents one of the main concerns worldwide. The primary mechanism of resistance is the production of β-lactamase enzymes, which have the ability to hydrolyze β-lactams. In the last three decades, members of the order Enterobacterales producing enzymes capable of hydrolyzing also the expanded-spectrum cephalosporins have emerged as one of the main threats for human health, becoming endemic in many countries [1]. Extended spectrum β-lactamases (ESBLs) and AmpC-type β-lactamases (from now on abbreviated as AmpCs) represent the two groups of β-lactamases mainly involved in expanded-spectrum cephalosporins resistance, but display several peculiarities between each other [2].

The mechanisms that underlie the AmpC-mediated resistance are not easy to understand for clinicians not familiar with clinical microbiology, since many peculiarities are related to the effective and variable expression of the enzyme by the different bacterial strains carrying the gene on the chromosome or having acquired it by plasmids. Moreover, the literature on infections due to AmpC-producing Enterobacterales is sparse and very heterogeneous, pending to date the results of adequate randomized controlled clinical trials.

In this review, mostly aimed at clinicians such as Infectious Diseases specialists, Internists and General Practitioners, we summarize the most important points about the epidemiological impact, dynamics, recognition and treatment of AmpC-producing Enterobacterales.

Methods

A literature search was performed using PubMed, through October 2018. The following terms were searched in combination: AmpC, β-lactamases, Enterobacteriaceae, Enterobacterales, Enterobacter, Gram-negative, resistance, cephalosporins, and treatment. References of retrieved articles, guidelines, and review articles were manually searched to ensure identification of studies not found in the initial literature search. The selection was limited to publications written in English. After de-duplication, all authors independently screened titles and abstracts, and finally full texts, to identify all potentially relevant studies, resolving discrepancies through discussion and consultation between them.

AmpC β-lactamases: an overview

AmpCs are enzymes encoded by the chromosomes of several bacterial species. Their evolutionary history indicates that they are very ancient enzymes, originated over two billion years ago, so preceding antibiotic introduction for clinical use and reflecting the evolution of resistance mechanisms to natural β-lactams produced by microorganisms for biological competition [3]. Despite having been differently called in the 1940, the first enzyme reported inactivating penicillin was indeed an AmpC, in Escherichia coli, before penicillin had been introduced in clinical use [4, 5].

The term AmpC defines a class of enzymes that belong to the molecular class C according to the Ambler’s structural classification of β-lactamases (whereas the ESBLs found in Enterobacterales typically belong to Class A). A serine residue is contained within the active site of both AmpCs and ESBLs, but the protein sequences of each class are remarkably different, leading to structural and mechanistic differences in β-lactam hydrolysis [2, 5]. In the functional classification scheme of β-lactamases (Bush–Jacoby [6]), based on the hydrolysis and inhibition profiles of the enzyme, AmpCs are assigned to group 1, which is characterized by an overall greater hydrolysis of cephalosporins (including cephamycins) vs. penicillin G (hence the name “cephalosporinases” that has also been used for these enzymes), and resistance to inhibition by β-lactam-based β-lactamase inhibitors, such as clavulanate, sulbactam and tazobactam.

AmpCs are either found as resident enzymes, encoded by chromosomal genes (cAmpCs), in some species of Enterobacterales, but can also be found as acquired plasmid-mediated enzymes (pAmpCs); these represent two distinct situations, by both a microbiological and clinical points of view. pAmpCs have spread widely among Enterobacterales, although their overall prevalence has remained far lower than that of ESBLs. In Italy, a country endemic for third-generation cephalosporin (3GC)-resistant Enterobacterales, the ESBL/pAmpC ratio was found to be approximately 12:1 [1].

As ESBL-producing strains, also pAmpC producers may exhibit multidrug-resistant phenotypes, due to co-expression of multiple plasmid-mediated resistance determinants to non-β-lactams (including quinolones, cotrimoxazole and/or aminoglycosides), limiting the number of treatment options. Otherwise, the majority of Enterobacterales producing cAmpCs often retain high levels of susceptibility to fluoroquinolones and aminoglycosides [7, 8].

Enzymatic activity and inhibitors of AmpCs

In general, AmpCs exhibit a broad substrate specificity including penicillins (e.g., penicillin G; aminopenicillins such as amoxicillin and ampicillin; carboxypenicillins such as carbenicillin and ticarcillin; ureidopenicillin such as piperacillin), narrow-spectrum cephalosporins (e.g., cefazolin, cephalothin, cefamandole and cefuroxime), oxyimino-cephalosporins (e.g., cefotaxime, cefpodoxime, ceftazidime and ceftriaxone), cephamycins (e.g., cefoxitin and cefotetan) and aztreonam (variable), and their expression can confer resistance to all these compounds. The hydrolysis rate for fourth-generation cephalosporins (e.g., cefepime and cefpirome) is usually low, and that for carbapenems is very low, so that susceptibility to these drugs is usually maintained. Temocillin, a semi-synthetic 6-α-methoxy derivative of ticarcillin, is highly stable against most β-lactamases, and retains in vitro activity against ESBL- and AmpC-producing Enterobacterales [9].

Amino acid insertions, deletions, and substitutions have been described for both plasmidic and chromosomal AmpCs enhancing catalytic efficiency toward ceftazidime and other oxyimino-β-lactams: these variants have been termed extended-spectrum AmpC cephalosporinases (ESAC) [6]. As with other β-lactamases, the resistance mediated by AmpCs is enhanced by the presence of porin alterations which impair antibiotic entry across the outer membrane. In this case, the very weak carbapenemase activity exhibited by some AmpCs can contribute a phenotype of reduced susceptibility or resistance to carbapenems, with ertapenem being usually more affected [6, 10].

Concerning inhibitors, AmpCs are usually resistant to β-lactam-based inhibitors (e.g., clavulanate, sulbactam and tazobactam), while being inhibited by the new non-β-lactam-based inhibitors (e.g., diazabicyclo-octanes, such as avibactam and relebactam, and boronates, such as vaborbactam). However, these general functional features may exhibit some variability in different AmpCs, in terms of substrate specificity and susceptibility to inhibitors (e.g., tazobactam inhibits cAmpC in Morganella morganii) [2, 5].

The lack of inhibition by β-lactam-based inhibitors represents a characteristic that sharply differentiates AmpCs from ESBLs. Other main differences are represented by the high susceptibility to cefepime and the resistance to cephamycins that AmpC producers usually show. AmpCs are located in the periplasmic space, where they can intercept and destroy β-lactams before interaction with the PBP (penicillin-binding protein) targets. In addition to the relative stability of the fourth-generation cephalosporins to AmpC hydrolysis, also the rapid penetration of these molecules across the outer membrane due to their zwitterionic structure account for their preserved activity against most AmpC-producers.

Chromosomal AmpCs (cAmpCs)

Genes encoding AmpCs are located on the chromosome of some clinically relevant Gram-negative pathogens, including several members of the order Enterobacterales. In particular, we must remember those belonging to the so-named ESCPM group, acronym indicating the following species: Enterobacter (Enterobacter cloacae complex, Enterobacter aerogenes), Serratia marcescens, Citrobacter freundii, Providencia stuartii, and Morganella morganii. Enterobacter spp. represent the prototype of this group.

The main feature of cAmpCs is represented by the variable level of expression of the ampC gene by the different species: expression can be constitutive or inducible. When expression is inducible, several β-lactams can act as inducers making clinically relevant the mechanism of resistance. By definition, inducible expression is reversible, but strains producing inducible cAmpCs can easily segregate mutants in which expression is stably de-repressed eventually resulting in constitutive expression (also named “de-repressed mutants”).

Based on the propensity of the different antimicrobials of inducing cAmpC expression and of being hydrolyzable by the induced enzyme, we can distinguish the following situations:

  1. 1.

    Inducer/labile β-lactam compounds: aminopenicillins, first-generation cephalosporins, cefoxitin, cefotetan. These drugs induce ampC expression and are inactivated by the enzyme. Strains producing an AmpC enzyme either inducibly or constitutively are typically resistant to these drugs. Also clavulanate strongly induces cAmpC production (and does not have an inhibitory activity on it).

  2. 2.

    Inducer/stable β-lactam compounds: the typical example are carbapenems, which are strong inducers of ampC expression, but are overall stable. Strains producing inducibly or constitutively an AmpC enzyme usually remain susceptible to carbapenems, unless in the presence of porin alterations that reduce outer membrane permeability.

  3. 3.

    Weak inducer/labile β-lactam compounds: ureidopenicillins (e.g., piperacillin), third-generation cephalosporins and aztreonam. In this case, strains with an inducible AmpC usually appear susceptible to these compounds, while those constitutively producing the enzyme are resistant. Since mutants constitutively producing the enzyme are easily selected from strains with inducible production, the use of these antibiotics should be considered with caution with isolates of cAmpC-producing species, despite apparent in vitro susceptibility. A similar consideration could apply to piperacillin–tazobactam, for which in vitro susceptibility is often retained because piperacillin and tazobactam are only weak inducers, since AmpCs hydrolyze piperacillin and are usually not inhibited by tazobactam. Therefore, if the clinician suspects that the pathogen could produce a cAmpC, in our opinion the use of this antibiotic for severe infections should be considered with caution, regardless of susceptibility reports.

  4. 4.

    Weak inducer/stable β-lactam compounds: cefpirome and cefepime (fourth-generation cephalosporins). They usually retain activity against AmpC producers, unless in the presence of AmpC variants that exhibit increased activity against fourth-generation cephalosporins [5, 6].

The regulation mechanism of cAmpC expression is very complex (see Fig. 1).

Fig. 1
figure 1

AmpC regulation: induction and stable de-repression

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In the presence of β-lactams that inhibit the synthesis of the bacterial cell wall, an increased quantity of 1,6-anhydro-N-acetylmuramic acid oligopeptides (muropeptides) are released in the periplasmic space. Muropeptides can enter the cytoplasm via the AmpG transporter (an inner membrane permease) and compete with uridine diphosphate (UDP)-N-acetylmuramic acid peptides for binding with AmpR, a transcriptional regulator that at a baseline state represses expression of the ampC gene. With increasing 1,6-anhydro-N-acetylmuramic acid peptide binding and decreased UDP-N-acetylmuramic acid peptide binding, AmpR undergoes a conformational change leading to increased transcription of ampC. AmpD (N-acetyl-muramyl-L-alanine-amidase) is a regulatory protein responsible for cleavage of stem peptides from 1,6-anhydro-N-acetylmuramic acid peptides, which can be recycled for peptidoglycan biosynthesis. So, induction occurs when AmpD enzyme is unable to cleave all of the 1,6-anhydro-N-acetylmuramic acid peptides. Stable de-repression most commonly occurs due to ampD mutations reducing its cleaving activity, whereas less frequent causes are mutations in the ampR gene [5, 11].

De-repressed mutants may be present at a frequency of 10− 5–10− 7 of the total bacterial population, and may be selected by antibiotic therapy, especially by weak inducer/labile β-lactams. This situation can be detected as early as 24 h after starting therapy or can occasionally be delayed for up to 2–3 weeks. In the landmark study of Chow et al. [12], de-repressed mutants of Enterobacter spp. constitutively producing cAmpC, resistant to extended spectrum penicillins (i.e., ticarcillin, ticarcillin–clavulanate, piperacillin and mezlocillin) and to 3GCs, were mainly obtained following 3GCs therapy after a mean of 9 days (range 4–18 days). Similarly, Choi et al. [13] reported the emergence of mutants of Enterobacter spp. and C. freundii resistant to 3GCs after treatment with these agents after a median of 7 days (range 3–28 days).

Also E. coli carries a chromosomal ampC gene. However, it is almost always expressed at negligible levels and is not inducible; therefore, usually it does not represent a clinically relevant problem in this species. Nevertheless, E. coli strains can occasionally exhibit a higher level of production of the cAmpC enzyme by gene duplication or mutations in the ampC promoter or attenuator regions [14]. In a recent multicenter Spanish study [15], of 841 bloodstream infections (BSI) due to 3GC-resistant E. coli, only 17 cases (2%) were caused by AmpC-producing isolates, according to the relative rarity of the phenomenon in this species, but it is noteworthy to observe that 41.2% of these were cAmpC overproducers, being the remaining carriers of a pAmpC. More recently, a Dutch study [16] on the prevalence among hospitalized patients of rectal carriage of plasmid- and chromosome-encoded AmpC-producing E. coli showed a prevalence of 2.4%, contributed by 0.9% of pAmpC-producing strains and 1.4% of strains with cAmpC overproduction due to promoter/attenuator alterations.

Table 1 summarizes a practical classification of the main Enterobacterales carrying cAmpC and the importance of induction/de-repression phenomena on the emergence of resistant strains of clinical interest.

Table 1 Main species of Enterobacterales carrying chromosomal AmpCs: a practical classification based on clinical relevance of expression and induction/de-repression phenomena
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Plasmid-mediated AmpCs (pAmpCs)

More than 20 different AmpCs have been found to be mediated by plasmids: the first was described in 1989 [19], and since then they have been observed globally as a result of horizontal transfer of AmpC-encoding plasmids and clonal expansion. There are several lineages of pampC genes, originating from chromosomal ampC genes carried by several Gram-negative species and falling into at least five phylogenetic groups, namely the Enterobacter group (MIR, ACT), the Citrobacter freundii group (CMY-2-like, LAT, CFE), the Morganella morganii group (DHA), the Hafnia alvei group (ACC), and the Aeromonas group (CMY-1-like, FOX, MOX) [20]. The most prevalent and widely disseminated are the CMY-2-like enzymes, although the inducible DHA-like β-lactamases and some others have also extensively spread [20].

pAmpCs are usually constitutively expressed, conferring resistance patterns similar to that of de-repressed cAmpCs. For this reason, pAmpC-carrying bacteria should always be considered of significant clinical relevance. As an exception to this rule, some pampC genes, such as the blaDHA−1gene, are inducible by β-lactams, with expression regulated similar to that of campC genes [5].

The most important species of the Enterobacterales order that have acquired pAmpCs include Escherichia coli, Klebsiella pneumoniae and Proteus mirabilis, but other important species are also Klebsiella oxytoca, Salmonella enterica and Shigella spp. [3, 20].

In a recent Italian survey [1], the overall prevalence of pAmpC producers among E. coli, K. pneumoniae and P. mirabilis was 1.2%, and the overall prevalence of pAmpC-producing E. coli was 0.6%. This finding was similar to the 0.4% prevalence reported by Drinkovic et al. [21] in the Auckland community, where most had a CMY-2-like enzyme and 51% of cefoxitin-resistant E. coli were pAmpC producers (37% were instead assumed as hyper-producers of cAmpC) and only a few strains (4%) co-produced pAmpC and an ESBL. In that study, most pAmpC-producing E. coli were from community-acquired urinary tract infections, mainly in women, especially if they had previously received β-lactams, but it is noteworthy that a large proportion (43%) of patients were neither hospitalized nor had received any antimicrobial treatment in the previous six months. Furthermore, the isolates exhibited high resistance rates also to non-β-lactam antimicrobials (e.g., norfloxacin, trimethoprim and nitrofurantoin), leaving few treatment options. Harris et al. [22] characterized 70 3GC-resistant E. coli isolated from blood in patients enrolled in the MERINO trial from Australia, New Zealand and Singapore: the majority (61.4%) were ST131 isolates, 95% of which carrying blaCTX−M ESBLs; pampC genes (mainly blaCMY−2) were also frequent (17.1%), and more common among non-ST131 isolates. Only two strains carried both blaCMY−2 and blaCTX−M. The co-existence of pAmpCs and ESBLs is, therefore, quite rare, even if in a recent Swiss study an overall percentage of 13% of Enterobacterales producing both types of enzymes was reported [23].

A large outbreak caused by K. pneumoniae producing a FOX-7 AmpC was observed in a neonatal intensive care unit in central Italy from February 2008 to April 2010, with a mortality rate at 14 days in case of sepsis of 28.5%. All isolates were resistant to cefotaxime, ceftazidime and piperacillin–tazobactam, while 76% were susceptible to cefepime and 98–100% to carbapenems [24]. This experience clearly demonstrates that pAmpC-producing Enterobacterales can cause large outbreaks with significant morbidity and mortality, underscoring the role of laboratory-based surveillance and infection control measures to contain similar episodes.

The importance of recognizing these situations is underscored by the case of P. mirabilis circulating in Italy, where, for over a decade, a multifocal spreading of a clone producing an acquired AmpC β-lactamase (CMY-16) has been detected [25, 26]. In a northern Italian hospital, its prevalence rapidly increased from 0.3% in 2004 to 4.6% in 2006, due to a rapid clonal expansion of the AmpC-positive strain first isolated in 2003 from a geriatric ward. In this case, about 50% of isolates were obtained from hospitalized patients, most frequently in medical wards [26]. The isolates carrying the CMY-16 determinant showed multidrug-resistance and the majority were associated with urinary tract infections; treatment with amikacin or carbapenems was consistently effective, but also piperacillin–tazobactam produced a clinical response in 78% of the cases since tazobactam appeared to be effective at antagonizing the enzyme activity [25, 26]. This pathogen is still today often isolated in Italy, representing the 9% of all 275 isolates of P. mirabilis collected in the recent survey mentioned above [1]. For these reasons, every time a P. mirabilis is isolated, it is very important a correct phenotypic interpretation of the antibiogram to suspect it (cefoxitin R, cefotaxime and ceftazidime R, cefepime S) and, if possible, to confirm the hypothesis for the most appropriate antibiotic choice.

How to recognize AmpC-producing organisms by antibiogram reading, and when to do further phenotypic and genotypic tests

By reading of the antibiogram it is possible to hypothesize if an isolate is an AmpC-producer only if the gene is significantly expressed leading to relevant β-lactamase activity against target substrates.

Bacterial strains having chromosomal inducible AmpCs are particularly challenging for antibiotic susceptibility reporting since in vitro susceptibility may not correlate with clinical efficacy, as resistance can emerge by selection of mutants overproducing the enzyme during treatment [12, 27]. Indeed, the majority of Enterobacterales (such as Enterobacter spp. and the other members of ESCPM group) with inducible cAmpCs retain in vitro susceptibility to the oxyimino-cephalosporins (weak inducers). For strains which constitutively produce cAmpC and for most pAmpC-carrying strains (which constitutively produce the enzyme) the situation is quite different, and the classical resistance pattern is represented by the combination of resistance to oxyimino-cephalosporins (such as cefotaxime and ceftazidime), susceptibility to cefepime and resistance to cephamycins (such as cefoxitin; this is why the clinicians should expect and require this molecule to be routinely tested in antibiograms). A cefoxitin MIC > 8 mg/L combined with resistance to ceftazidime and/or cefotaxime may be used as a phenotypic criterion for investigation of acquired pAmpC production in species that are normally lacking these enzymes. This strategy, however, will not detect ACC-1, a pAmpC that does not hydrolyze cefoxitin [20]. For laboratories not testing cefoxitin, susceptibility to cefepime together with resistance to cefotaxime and/or ceftazidime is another phenotypic indicator of potential AmpC production, although less specific [20].

The hypothesis of AmpC production must be confirmed by additional phenotypic and genotypic assays. Phenotypic confirmation tests are generally based on inhibition by either cloxacillin or boronic acid derivatives that are good inhibitors of AmpCs. For E. coli, however, these confirmation tests cannot discriminate between acquired pAmpC and constitutive hyper-production of the cAmpC. The presence of acquired pAmpCs can be confirmed using PCR-based methods or with DNA microarray-based methods [20].

The low prevalence of pAmpC-producing Enterobacterales precludes routine universal microbiological screening, which is time-consuming and expensive. Unfortunately, it is not possible to identify a clinical profile that would allow targeted screening for pAmpCs, when compared to ESBL producers, because the risk factors and patients’ comorbidities are virtually the same [23].

It is important to underline that, in some strains, the co-existence of AmpCs and ESBLs makes the interpretation of the antibiograms even more challenging [20, 23]. In fact, for Enterobacterales with inducible cAmpCs, ESBL screening should also be performed with cefepime (stable to AmpC) in phenotypic testing with clavulanic acid [20].

Furthermore, AmpC overproduction in addition to decreased outer membrane permeability due to porin mutations can reduce susceptibility to carbapenems, in particular in plasmid-mediated AmpC producers, conferring resistance patterns similar to that of carbapenemase-producers, and these situations must be distinguished. Anyway, only genotypic methods can clearly discriminate these complex situations.

Clinical evidences

Clinical studies evaluating treatments of infections caused by AmpC-producing Enterobacterales are currently scarce, and most evidence is based on retrospective data. Carbapenems are usually considered first-choice options, but alternatives are needed because the rate of carbapenem resistance is rising and alarming [28]. The interpretation of available evidences is complicated by the heterogeneity of the different studies, including infections by different species: either all ESCPM [13] or only ESC organisms actively and stably producing cAmpC [29], or only Enterobacter spp. [12, 27, 30], or only members of the E. cloacae complex [8, 31]. The isolates were from blood in some studies [8, 12, 30, 31] or from various specimens in others [13, 27, 29, 32], reflecting very different clinical situations.

For Enterobacter spp., emergence of resistance to 3GCs during therapy with these agents was 19% in two important studies, being significantly more frequent when the initial site of isolation is blood [12, 27]. Therefore, when an Enterobacter spp. is isolated from blood it may be prudent to avoid 3GCs regardless of in vitro susceptibility, but also a biliary tract source seems to be significantly associated with the emergence of resistance [13]. On the other hand, concomitant exposure to quinolones seems to be associated with a decreased risk for emergence of resistance [27].

For infections due to isolates of the ESC group constitutively producing cAmpC, cefepime and meropenem showed no difference in 30-day mortality or length of hospital stay [29], and for BSI due to E. cloacae, cefepime represented a reasonable alternative to carbapenems irrespective of the inducible or de-repressed phenotypes, when the prevalence of ESBL producers was low [8]. Cefepime is particularly suggested for isolates with MIC ≤ 2 mg/L, since approximately 97% of patients with such isolates cleared bacteremia within 1 day [30]. Also other authors have shown that cefepime represents an effective option for E. cloacae bacteremia caused by strains with MIC ≤ 2 mg/L, resulting inferior to carbapenems on mortality only for higher MIC values [31]. In a systematic review and meta-analysis of studies on BSI due to cAmpC-producing Enterobacterales, no strong evidence was found to suggest that β-lactam/β-lactamase inhibitor (BLBLI) agents (specifically piperacillin/tazobactam or ticarcillin/clavulanate), quinolones or cefepime were inferior to carbapenems concerning mortality [33].

The recent report of the British Society for Antimicrobial Chemotherapy/Healthcare Infection Society/British Infection Association Joint Working Party on treatment of infections caused by multidrug-resistant Gram-negative bacteria [34] recommends that cefepime could be used to treat infections caused by AmpC-producing bacteria if susceptible at the EUCAST breakpoint of MIC ≤ 1 mg/L (conditional recommendation for), but not, even at increased dose, for isolates with MICs > 1 mg/L or those producing both AmpC and ESBL (strong recommendation against). Use of meropenem or imipenem or ertapenem is strongly recommended to treat serious infections with AmpC-producing Enterobacterales. This document states that, for infections due to AmpC-producing Enterobacterales, temocillin could be used for urinary tract infections and associated bacteraemia, and ceftazidime/avibactam could be used as an alternative to carbapenems but alternatives may be cheaper (grading: conditional recommendations for). Furthermore, since the activity of ceftolozane/tazobactam against Enterobacterales with copious AmpC enzyme is variable, and many Enterobacter spp. with de-repressed AmpC are resistant [35], this antibiotic should not be used in infections due to AmpC-producing Enterobacterales (grading: strong recommendation against use).

Table 2 shows a selection of the most relevant clinical studies enrolling patients with infections due to cAmpC-carrying Enterobacterales.

To our knowledge no comparative studies for pAmpC producers are available.

Table 2 Selection of relevant clinical or review studies dealing with patients with infections due to chromosomal AmpC-carrying Enterobacterales
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Discussion

Multidrug-resistant Enterobacterales are a major concern worldwide. In most cases, the resistance to 3GCs is mediated by ESBL production, but expression of AmpCs may be relevant; nevertheless, little attention to these β-lactamases is currently paid by many clinicians that every day face these complex situations.

We know that pAmpC-producing Enterobacterales are often involved in community-acquired infections, while cAmpC-carrying species are mainly involved in nosocomial infections. Therefore, not only the Infectious Diseases specialists, but also Internists and General Practitioners can frequently intercept these pathogens; hence, they must be aware of their epidemiological impact, dynamics, recognition and treatment. Also patients undergoing chronic hemodialysis are at high risk of infections due to cAmpC-producing Enterobacterales [37]; therefore, additional specialists, such as Nephrologists, should know this problem.

The clinicians should know that the different species may or may not express the resistance gene, and that some antimicrobials can have different efficacy depending on the possibility of in vivo selection of resistant mutants which constitutively produce the enzyme. In particular, for members of the ESCPM group and mainly for Enterobacter spp. and for BSI, it seems prudent to avoid 3GCs regardless of in vitro susceptibility, due to the risk of emergence of resistance during treatment in a significant percentage [12, 27].

To date no RCTs directly comparing different treatment options for AmpC-producing bacteria have yet been completed, and few and very heterogeneous observational studies are the sole source of evidence; thus, there is no conclusive evidence about the best treatment of these problematic situations, and carbapenems are often considered the preferred option. In our opinion, if the infection is severe, if an adequate source control is not achieved, and especially if a monotherapy is chosen, a carbapenem should be the first choice, but there are enough data from the literature to consider cefepime as a reasonable carbapenem-sparing option, especially when the MIC values are in the EUCAST susceptibility range (≤ 1 mg/L), and optimizing the way of administration (extended infusion).

Also the associations of β-lactams with the new β-lactamases inhibitors active on AmpCs (such as avibactam) can represent an interesting option, while we underline that data on ceftolozane mainly refer to Pseudomonas aeruginosa.

A phenotype of reduced susceptibility to carbapenems can be co-mediated by AmpC overproduction and outer membrane protein loss in some Enterobacterales; to our knowledge, there is no clinical study about the most appropriate treatment of these situations, and in our opinion we should rely on antibiogram results and on the molecules with the most favorable MIC values, taking into account, when appropriate, also non β-lactams antibiotics, such as fluoroquinolones and aminoglycosides.

In summary, we hope that well-conducted trials will be performed to clarify uncertainties about the most appropriate treatment, but pending definitive data, in our opinion, it seems reasonable to rely on the options with the most evidence from the literature (carbapenems and cefepime), taking into account the severity and site of infection, the previous use of antimicrobials known to be inducers, and the level of expression and effective production of AmpC β-lactamase.