Introduction

Pseudomonas is a genus of non-fermentative gram-negative Proteobacteria that have been isolated from soil and fresh and salt water. This genus consists of species that are diverse genetically [5, 41, 50]. Based on their rRNA–DNA similarity, members of the genus Pseudomonas have been divided into five groups. Group I includes the γ subclass of the Proteobacteria, i.e., the genus Pseudomonas (sensu stricto) [53]. The species in the other four groups have been classified according to the new taxonomy [41]. Anzai et al. [5] subdivided Pseudomonas (sensu stricto) into two clusters based on an analysis of their 16S rDNA sequences. The first cluster has six groups: P. syringae (12 species), P. chlororaphis (5 species), P. fluorescens (18 species), P. putida (six species), P. aeruginosa (11 species), and P. stutzeri (3 species). The second cluster has only one group, the P. pertucinogena group (two species).

As many as 3,800 biologically active microbial metabolites have been isolated from bacteria. Pseudomonas produces 795 bioactive substances, including 610 antibiotics and 185 substances with bioactive properties other than antibiotic activity [18]. More bioactive substances have been isolated from terrestrial Pseudomonas than from marine Pseudomonas, and there are a vast number of terrestrial Pseudomonas species that are well known as producers of bioactive substances. Some of these play very important roles in the biological control of pathogenic plant bacteria and fungi and in bioremediation. In contrast, there are many fewer isolates of marine Pseudomonas, and their bioactive substances are not as well studied.

Some reports indicate that there are marine and terrestrial Pseudomonas isolates that produce the same secondary metabolites, but novel secondary metabolites from marine isolates of this genus also have been described. The marine habitat has enormous ecological diversity, making it likely that marine Pseudomonas sp. represent an untapped reservoir of novel bioactive substances. This review provides a comprehensive picture of what is currently known about bioactive substances produced by marine isolates of Pseudomonas, their biological activities and modes of action, the ecological diversity of marine Pseudomonas, and the phylogenetic relationship among marine Pseudomonas isolates.

Bioactivities of Pseudomonas metabolites

Antimicrobial, antiviral, and cytotoxic agents

The first marine isolate of Pseudomonas known to produce a bioactive substance was isolated from Thalassia (turtle grass) near La Parguera, Puerto Rico [17]. The bacterium, P. bromoutilis, produces a pyrrole antibiotic, 2,3,4-tribromo-5(1′hydroxy,2′,4′-dibromophenyl) pyrrole. This pyrrole antibiotic inhibits gram-positive bacteria such as Staphylococcus aureus, Diplococcus pneumoniae, and Streptococcus pyogenes at concentrations of 0.0063 μg/ml, and inhibits Mycobacterium tuberculosis at 0.2 μg/ml. It is inactive against gram-negative bacteria and the fungus Candida albicans. This substance was not toxic in mice at 25 or 250 mg/kg by intravenous and subcutaneous injection, respectively. However, subcutaneous injection of 200 mg/kg of this pyrrole antibiotic did not protect mice infected with S. aureus. This was the first effort to explore the bioactivity of a substance from a marine isolate of Pseudomonas.

Wratten et al. [78] isolated the antibiotic-producing Pseudomonas sp. 102-3 from a seawater sample from a La Jolla, California tide pool. The bacterium inhibits the growth of Vibrio anguillarum, V. harveyi, S. aureus, and C. albicans, and produces three antibacterial compounds: 4-hydroxybenzaldehyde, 2-n-heptyl-4-quinolinol, and 2-n-pentyl-4-quinolinol (Fig. 1). 4-hydroxybenzaldehyde exhibits low antimicrobial activity, inhibiting C. albicans, S. aureus, and V. harveyi at 5 mg/disk, but not at 50 μg/disk. The other two compounds, 2-n-heptyl-4-quinolinol and 2-n-pentyl-4-quinolinol, both show antimicrobial activity against S. aureus, V. harveyi, and V. anguillarum at 50 μg/disk. As 2-n-heptyl-4-quinolinol is a known antibiotic that has been isolated from terrestrial P. aeruginosa, Wratten et al. [78] suggested that Pseudomonas sp. 102-3 may thus have the same biosynthetic capabilities as P. aeruginosa.

Fig. 1
figure 1

The chemical structures of secondary metabolites produced by marine isolates of Pseudomonas. Please see the text for references

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The marine isolates P. fluorescens, isolated from the surface of tunicates, have been reported to produce three unique substances, moiramides A, B, and C, along with a known compound, andrimid (Fig. 1); the latter is a member of a newer class of antibiotics, the pseudopeptide pyrrolidinedione antibiotics [51]. In contrast to moiramides A and C, moiramide B and andrimid both have antibacterial activity. This suggests that the intact succinimide moiety is required for their activity. Moiramide B and andrimid have broad-spectrum antibacterial activity because of their inhibition of bacterial acetyl-CoA carboxylase [26]. Moiramide B is the first potent antibacterial compound from marine bacteria with acetyl-CoA carboxylase inhibiting activity as its mode of action. Freiberg et al. [27] evaluated the in vivo efficacy of moiramide B and some of its synthetic derivatives using a S. aureus sepsis model in mice. These pyrrolidinedione derivatives exhibit antibacterial activity with minimum inhibitory concentrations (MICs) of 0.01–8, 0.25–32, and 16–64 μg/ml against S. aureus 133, S. pneumoniae G9A, and Escherichia coli Neumann, respectively. When evaluated in a murine model of S. aureus sepsis, two of the moiramide B derivatives also showed in vivo activity comparable to linezolid, an antibiotic that is used currently. These reports indicate that antibiotics produced by marine isolates of P. fluorescence may be potential lead compounds in the search for new classes of antibiotics to treat bacterial infections.

Pseudomonas sp. 1531-E7 isolated from a sponge, Homophymia sp., produces quinolones (2-undecyl-4-quinolone, 2-undecen-18-yl-4-quinolone, 2-nonyl-4-quinolone, and 2-nonyl-4-hydroxyquinoline N-oxide) (Fig. 1) [16]. Anti-Plasmodium falsifarum activity is exhibited by 2-undecyl-4-quinolone, 2-undecen-18-yl-4-quinolone, and 2-nonyl-4-quinolone. Cytotoxicity to KB cells is noticed for 2-undecen-18-yl-4-quinolone and 2-nonyl-4-hydroxyquinoline N-oxide. In addition, 2-undecyl-4-quinolone and 2-nonyl-4-hydroxyquinoline N-oxide are active against HIV-1 and S. aureus.

Isnansetyo et al. [35] purified the antibiotic 2,4-diacetylphloroglucinol (DAPG) from the culture supernatant of Pseudomonas sp. AMSN, which was isolated from a marine alga. The antibiotic exhibits potent activity against ten clinical isolates of methicillin-resistant S. aureus (MRSA), with an MIC range of 0.25–1 μg/ml, and has bactericidal activity against MRSA at 4 μg/ml, comparable to vancomycin. This antibiotic causes lysis of MRSA and V. parahaemolyticus at 1 and 24 μg/ml, respectively [38]. DAPG was also active against 23 vancomycin-resistant S. aureus (VRSA) strains isolated in Asia, Europe, Brazil, South Africa, and the USA, and against vancomycin-resistant Enterococcus spp. (VRE) genotypes A and B [34]. DAPG is active against a wide range of VRSA isolates as well as against a vancomycin-heteroresistant S. aureus (h-VRSA) at MIC 4 μg/ml. This substance also has moderate activity against both VRE-A and -B at MIC 8 μg/ml, but is inactive against VRE-C at up to 16 μg/ml. DAPG is a well-known antibiotic that is also produced by terrestrial P. fluorescens [52, 66, 79]; this suggests that AMSN has the same antibiotic biosynthesis capabilities as P. fluorescens.

P. aeruginosa isolated from an antarctic sponge, Isodictya setifera, produces six diketopiperazines and two phenazine alkaloids (Fig. 1) [37]. Phenazines are pigments with antibiotic properties. These two phenazine alkaloids are active against gram-positive bacteria, Bacillus cereus, Micrococcus luteus, and S. aureus, but diketopiperazines do not exhibit antibiotic and cytotoxic properties. P. aeruginosa, isolated from a mangrove environment, also produces phenazines [63]. The authors identified these pigments as pyocyanin and pyorubrin. These compounds exhibit antibacterial activity against Citrobacter sp. as well as hemolytic activity in a chick blood assay. Phenazine is commonly produced by terrestrial fluorescent Pseudomonas isolates [46], including terrestrial P. aeruginosa. These findings further support the idea that some marine Pseudomonas isolates have the same biosynthetic capabilities as their terrestrial counterparts.

A sulfated polysaccharide produced by Pseudomonas sp. WAK-1 isolated from the brown seaweed Undaria pinnatifida is active against herpes simplex virus 1 (HSV-1) (EC50 = 1.4 μg/ml) [47]. The polysaccharide has a repeating unit: -2)-b-d-Galp(4SO4)(1-4)[b-d-Glcp(1-6)]-b-d-Galp(3SO4)(1- [48]. Furthermore, anti-cancer activity of this substance is also determined by the panel of 39 cell lines with 63.2 μg/ml of average IC50. Apoptosis is responsible for its anti-cancer activity.

Of the marine isolates of Pseudomonas, P. stutzeri have been described in the greatest detail. However, few studies have focused on bioactive substances produced by P. stutzeri. Uzair et al. [77] reported that the marine Pseudomonas sp. CMG1030 has antimicrobial activity. This organism was originally identified as P. aeruginosa, but the strain identification was revised to P. stutzeri CMG1030, which produces the novel antibacterial compound zafrin [76]. Subsequently, the authors elucidated the chemical structure of zafrin and studied its antibacterial mechanism. Zafrin, [4b-methyl-5,6,7,8 tetrahydro-1 (4b-H)-phenanthrenone] (Fig. 1) is active against the human pathogenic bacteria, S. aureus and S. typhi, but is inactive against C. albicans and Schizosaccharomyces pombe. The MICs for gram-positive and gram-negative bacteria range from 50 to 75 and 75 to 125 μg/ml, respectively. Further study of the mechanism of zafrin revealed that the substance exerts its bactericidal effects by bacterial cell lysis.

Another novel antibiotic, bushrin (7-(3-furyl)-3,7-dimethyl-7,8-dihydro-1-naphthalenol), is also produced by P. stutzeri CMG1030 [1]. This antibiotic is active against V. harveyi, E. coli, Salmonella typhi, Proteus mirabilis, P. morgani, Shewanella putrifaciens, Hafnia alvei, Seratia marcescens, B. subtillis, B. steriothermophilus, B. cereus, Clostridium perfringens, C. sporogenes, S. aureus (MSSA and MRSA), S. epidermidis, E. faecalis, and E. faecium with MIC ranging from 50 to 125 μg/ml. However, this antibiotic is inactive against C. albicans, Klebsiella aerogenes, and Streptococcus group G. Bacterial cell lysis activity is responsible for the antibacterial activity of this substance.

There are several marine Pseudomonas sp. with antimicrobial activity that is due to substances that have not yet been isolated or characterized. For example, marine Pseudomonas strain I-2, isolated from estuary water, has anti-Vibrio activity [20]. The antibacterial activity was evaluated against the following shrimp pathogenic vibrios: V. harveyi, V. fluvialis, V. parahaemolyticus, V. damsela, and V. vulnificus. The active substance is a non-proteinaceous substance that is soluble in chloroform. The chloroform extract from the bacterium is active against V. harveyi at 20 μg/ml, but there is no toxic effect on shrimp larvae up to 50 μg/ml. This suggests that the substance can be used to control pathogenic marine Vibrio. As Pseudomonas sp. I-2 is non-pathogenic to shrimp larvae, the bacterium may be used as a biocontrol agent against vibriosis in marine aquaculture.

The extract of Pseudomonas sp. PB2 associated with a sponge, Suberites domuncula, exhibits anti-angiogenic, hemolytic, antimicrobial, and cytotoxic activities [71]. Its anti-angiogenic activity in the chick chorio-allantoic membrane (CAM) assay is 50% at 5 μg/ml and 100% at 10 μg/ml. Furthermore, the extract is also cytotoxic to PC12 and HeLa cells. Antimicrobial activity of the extract is noticeable against S. aureus, S. epidermidis, S. lentus, E. coli, and Candida albicans.

Kim et al. [43] reported P. fluorescens HAK-13, which has algal-lytic activity against Heterosigma akashiwo (Raphidophyceae), Alexandrium tamarense, and Cochlodinium polykrikoides, but is inactive against Gymnodinium catenatum. The substance responsible for the activity is a proteinaceous compound that localizes to the cytoplasmic membrane of the bacterium. P. fluorescens HAK-13 thus has potential for selectively controlling harmful algal blooms in the marine environment.

Radjasa et al. [55] isolated Pseudomonas sp. associated with the soft coral Sinularia polydactyla. This bacterium has antibacterial activity against Streptococcus equi subsp. zooepidemics. Although the nonribosomal peptide synthetase (NRPS) gene from this bacterium can be amplified, the substance responsible for the antibacterial activity remains unknown. The authors have not determined the relationship between the NRPS gene and the antibacterial activity of the Pseudomonas sp.

Cathepsin B inhibitors

Hoang et al. [32] isolated two cathepsin B inhibitors from the culture supernatant of marine Pseudomonas sp. PB01. The inhibitors were identified as dibutyl phthalate and di-(2-ethylhexyl) phthalate, which exhibit dose-dependent cathepsin B with IC50s of 0.42 and 0.38 mM, respectively. The substances inactivate the pericellular cathepsin B of murine melanoma cells. Cathepsin B (EC3.4.22.1), which belongs to the papain superfamily, is a cysteine proteinase with a cysteine residue in its active site. This enzyme promotes the growth, invasion, and metastasis of cancer cells by catalyzing the degradation of the interstitial matrix and basement membranes; this allows cancer cells to invade locally and to metastasize [24, 68]. Cathepsin B also plays an important role in a variety of pathologies, including inflammation, pancreatitis, osteoarthritis, tumor angiogenesis, apoptosis, and neuronal diseases [6, 15, 19, 29, 31, 33, 49, 74]. In addition, this enzyme markedly enhances infection by the Ebola virus by converting the 130-kDa viral glycoprotein GP1 to a 19-kDa species [65]. Because of the role of cathepsin B in disease development, including cancer cell proliferation and virus infection, studies of cathepsin B inhibitors from marine isolates of Pseudomonas should be intensified.

Diversity of marine Pseudomonas

Ecological diversity of marine Pseudomonas

Marine isolates of Pseudomonas are found in diverse ecosystems, including coastal regions, the deep sea, and more extreme environments. Marine Pseudomonas also can be found as bacterioplankton in seawater, in association with other marine organisms, and in sea sediment. The production of marine secondary metabolites can be viewed in an ecological context [25]. Thus, the diversity of Pseudomonas isolated from a wide range of marine ecosystems suggests that these organisms may produce novel and diverse bioactive substances.

P. fluorescence is usually found as bacterioplankton in seawater [22, 54], but Borges et al. [10] isolated the bacterium with denitrifying activity from a biofilter installed at a marine recirculation aquaculture system. Marine Pseudomonas bacterioplankton have great biodiversity as revealed by Hagström et al. [30] when they isolated several bacterial strains closely related to P. slutzeri, P. putida, P. megulae, P. anguilliseptica, P. gessardi, P. azotoformans, and P. veronii from the Baltic and Weddell Seas. Danovaro et al. [23] used automated ribosomal intergenic spacer analysis (ARISA) to determine bacterial diversity in the aquatic environment. Specifically, seven Pseudomonas spp. strains were isolated from five different environmental samples. Analysis of 16S rDNA indicated that isolate 1 was closely related to P. veronii and P. marginalis, with 98% sequence similarity. Isolates 2 and 3 were closely related to P. aurantiaca (97%). Isolate 5 was close to P. chloritidismutans (99%) and P. stutzeri (99%). Two other isolates, strains 6 and 7, were closely related to P. aeruginosa (99%). Notably, isolate 4 was not closely related to any specific species of Pseudomonas, but relatively close to Pseudomonas sp. strain MFY152 (98%).

In the marine ecosystem, Pseudomonas also associates with other micro- and macroorganisms. Alavi et al. [2] analyzed the bacterial community associated with Pfiesteria-like dinoflagellate cultures and identified two 16S rDNA clones closely related to P. hibiscicola and P. oleovorans with 99% sequence similarities. Thakur et al. [72] isolated two marine antibacterial-producing Pseudomonas sp. (strains PB1 and PB2) from a Suberites domuncula sponge primmorph; the n-butanol extract of these Pseudomonas strains has both antibacterial activity and autoinhibitory activity. Berg et al. [9] isolated Pseudomonas sp. 13BT from an axenic culture of Aureococcus anophagefferens (Pelagophyceae), a eukaryotic picoplankton. This isolate is closely related to P. luteola (formerly Chryseomonas luteola), with 99% 16S rDNA sequence similarity. This bacterium is able to hydrolyze urea and acetamide, but its capability is very limited for aminopeptides and citobiose. Anand et al. [4] screened for antibiotic-producing marine bacteria associated with sponges from the waters off the coast of southeast India and isolated a bacterium, strain SC11, which is closely related to Pseudomonas based on the 16S rDNA sequences.

Diverse Pseudomonas spp. are also found in association with corals. Brück et al. [14] isolated bacterial strains from Azooxanthellate deep-water octocorals Leptogorgia minimata, Iciligorgia schrammi, and Swiftia exertia, which grow at moderate depth (40–100 m). Based on 16S rDNA sequence analysis, the isolates are thought to be related to P. citronellolis (for strain LM1305), P. oryzihabitans (for strain LM1405), P. putida (for strain LM1505), P. aeruginosa (for strains LM1605, LM2705, DQ517271, and SE1005), P. pachastrellae (for strain LM2805), and P. pseudoalcaligenes (for strain LM2905). The other isolates, IS1005, LM1605, LM3005, LM3105, LM3205, and LM3305, are not related to any specific species of Pseudomonas, but can be included in this genus.

Marine Pseudomonas has been found to be a member of the microorganism community in marine extreme environments. Barotolerant marine Pseudomonas sp. BT1 was isolated in deep water (4,418 m) in a Japanese ocean trench [39]. This bacterium was closely related to P. stutzeri, based on the 98% sequence similarity of the 16S rDNA sequences. Zeng et al. [81] isolated a deep-sea psychrophilic bacterium identified as Pseudomonas sp. DY-A that produces a cold-active serine alkaline protease. Radjasa et al. [56] failed to isolate any Pseudomonas strains from deep-sea (1,000-9,671 m) samples in the Northwestern Pacific Ocean. Although two bacterial strains that are closest to P. beijirinckii based on 16S rDNA sequence analysis were recovered from surface samples at the same location, these strains are distantly related to and likely distinct from the genus Pseudomonas due to the low 16S rDNA similarity (93.6%).

P. stutzeri has diverse ecological origins in both terrestrial and marine ecosystems. This phenotypically and genetically heterogeneous group is composed of several genomovars [8, 21, 28, 45]. Marine isolates of P. stutzeri have been reported in marine sediment in Barcelona, Spain [11–13, 60, 61], in sea sediment in Dangast, Germany [67], and in marine sediment at 11,000-m depth in the Mariana Trench [70]. Marine isolates of P. stutzeri have also been reported to live in the Black Sea (southwest) at 120-m depth [69], in a deep-sea hydrothermal vent in the Galapagos Rift [62] and in the Ariake Sea tideland, Japan [40]. Strains of this species can degrade naphthalene [11–13, 60, 61], oxidize thiosulfate [69], and sulfur [62], denitrify [40], and produce urea [73] and antibiotics [1, 75–77]. Amachi et al. [3] also have isolated Pseudomonas strain SCT, a dissimilatory iodate-reducing bacterium closely related to P. stutzeri. Two strains of marine Pseudomonas closely related to P. stutzeri have been isolated from the chemocline of a hypersaline deep-sea basin (Urania Basin, Mediterranean Sea) [64].

P. aeruginosa is a well-known pathogenic bacterium. It is generally regarded as a freshwater or terrestrial bacterium, as it is frequently isolated from these environments as well as from diseased organisms. However, Kimata et al. [44] showed that this Pseudomonas sp. also lives in a marine environment. The detection by two outer membrane lipoprotein genes specific to P. aeruginosa, oprI and oprL, using API 20 NE kit and 16S rDNA sequence analysis, revealed that P. aeruginosa inhabits Tokyo Bay. Subsequent work indicated that this species can be isolated from open ocean as well [42]. Jamil et al. [36] isolated a bacterium strain CMG607w from sediment of the Karachi coast, Pakistan. The strain that produces polyhydroxyalkanoate (PHA) is identified to be P. aeruginosa with 98% 16S rDNA sequence similarity.

In summary, marine Pseudomonas can be isolated from diverse ecological environments. The unique ecological characteristics of their environment may influence the production of secondary metabolites by the bacteria. However, studies of Pseudomonas isolated from diverse marine environments, as well as studies of the bioactive substances produced by these bacteria, are still rare.

Phylogenetic relationship of marine isolates of Pseudomonas

A search of 16S rDNA sequences in the GenBank, DDBJ, and EMBL databases shows that hundreds of marine isolates of Pseudomonas have been described. However, the number of reported marine Pseudomonas sp. is relatively small compared to terrestrial Pseudomonas sp. Within Pseudomonas (sensu stricto), the marine isolates include P. stutzeri, formerly P. perfectomarina [7, 61], P. alcaligenes, P. pseudoalcaligenes [53], P. fluorescens [51], P. alcaliphila [80], P. aeruginosa [44], P. xanthomarina [59], P. pachastrellae [58], and P. marincola [57].

P. alcaliphila, a facultative psychrophilic alkaliphile, was isolated from seawater [80]. P. pachastrellae was isolated from a deep-sea sponge specimen from the Philippine Sea at a water depth of 750 m [58]. Romanenko et al. [59] also isolated P. xanthomarina from ascidians specimens in the Sea of Japan and P. marincola from a deep-sea brittle star in the Fiji Sea [57].

In bacterial taxonomy, the use of 16S rDNA as a phylogenetic marker has been developed for determining intergeneric relationships because the gene evolves at an extremely slow rate. The 16S rDNA sequences of Pseudomonas were used by Anzai et al. [5] to evaluate the affiliation, group, and reclassification of Pseudomonas sp. We constructed the phylogenetic tree of marine Pseudomonas sp. based on 16S rDNA sequences retrieved from the GenBank, DDBJ, and EMBL databases, and it shows that marine isolates of Pseudomonas can be divided into two main clusters. The first cluster consists of P. alcaliphila, P. alcaligenes, P. aeruginosa, P. xanthomarina, and the group of P. stutzeri. The second cluster consists of P. marincola, P. pachastrellae, P. fluorescens, P. fulva, and P. veronii (Fig. 2). This result largely confirmed an earlier report that the phylogenetic tree based on the sequences of 16S rDNA of species in the genus Pseudomonas (sensu stricto) has two main clusters [5]. The phylogenetic tree presented here (Fig. 2) includes several recently reported novel marine Pseudomonas sp.: P. alcaliphila [80], P. xanthomarina [59], P. pachastrellae [58], and P. marincola [57].

Fig. 2
figure 2

Phylogenetic tree based on the 16S rDNA of marine isolates of Pseudomonas. The branching pattern was generated by the neighbor-joining method. Bootstrap values of 700 or more (from 1,000 replicates) are indicated at the nodes. Sequence accession numbers are in parentheses. Bar indicates 0.01 Knuc unit. Bacteria that produce bioactive substances are shown in bold

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Marine isolates of Pseudomonas that produce bioactive substances are likely to be dominated by members in the second cluster. However, the relationship between the ability to produce bioactive substances and position on the phylogenetic tree is still unclear, probably because so few marine Pseudomonas sp. have been studied extensively. In the second cluster, bacteria reported to produce bioactive substances include Pseudomonas sp. AMSN [34, 35, 38], Pseudomonas sp. MP-6 (GenBank/DDBJ/EMBL databases), P. fulva An10 (GenBank/DDBJ/EMBL databases), P. fluorescens PS7 (GenBank/DDBJ/EMBL databases), Pseudomonas sp. DG01 (GenBank/DDBJ/EMBL databases), Pseudomonas sp. PB01 [32], P. veronii C9-4zhy (GenBank/DDBJ/EMBL databases), and Pseudomonas sp. 283 49zx (GenBank/DDBJ/EMBL databases). Two strains of Pseudomonas in the first cluster, namely P. aeruginosa CMG 24360 (GenBank/DDBJ/EMBL databases) and P. aeruginosa PS3 (GenBank/DDBJ/EMBL databases), are known to produce bioactive substances.

Conclusions

There are few reports of bioactive substance-producing marine Pseudomonas sp. compared to those describing terrestrial species that produce bioactive metabolites. However, some bioactive substances with novel biological activities and mechanisms have been extracted from marine isolates of Pseudomonas, and some of these metabolites have antimicrobial properties. The genetic and ecological diversity of Pseudomonas suggest that marine isolates are a potential source of bioactive metabolites that could form the basis of new medical therapies.