Analysis of the draft genome of Pseudomonas fluorescens ATCC17400 indicates a capacity to take up iron from a wide range of sources, including different exogenous pyoverdines.

All fluorescent pseudomonads (Pseudomonas aeruginosa, P. putida, P. fluorescens, P. syringae and others) are known to produce the high-affinity peptidic yellow-green fluorescent siderophore pyoverdine. These siderophores have peptide chains that are quite diverse and more than 50 pyoverdine structures have been elucidated. In the majority of the cases, a Pseudomonas species is also able to produce a second siderophore of lower affinity for iron. Pseudomonas fluorescens ATCC 17400 has been shown to produce a unique second siderophore, (thio)quinolobactin, which has an antimicrobial activity against the phytopathogenic Oomycete Pythium debaryanum. We show that this strain has the capacity to utilize 16 different pyoverdines, suggesting the presence of several ferripyoverdine receptors. Analysis of the draft genome of P. fluorescens ATCC 17400 confirmed the presence of 55 TonB-dependent receptors, the largest so far for Pseudomonas, among which 15 are predicted to be ferripyoverdine receptors (Fpv). Phylogenetic analysis revealed the presence of two different clades containing ferripyoverdine receptors, with sequences similar to the P. aeruginosa type II FpvA forming a separate cluster. Among the other receptors we confirmed the presence of the QbsI (thio)quinolobactin receptor, an ferri-achromobactin and an ornicorrugatin receptor, several catecholate and four putative heme receptors. Twenty five of the receptors genes were found to be associated with genes encoding extracytoplasmic sigma factors (ECF σ) and transmembrane anti-σ sensors.

A "love match" score to compare root exudate attraction and feeding of the plant growth-promoting rhizobacteria Bacillus subtilis, Pseudomonas fluorescens, and Azospirillum brasilense.

Introduction: The rhizosphere is the zone of soil surrounding plant roots that is directly influenced by root exudates released by the plant, which select soil microorganisms. The resulting rhizosphere microbiota plays a key role in plant health and development by enhancing its nutrition or immune response and protecting it from biotic or abiotic stresses. In particular, plant growth-promoting rhizobacteria (PGPR) are beneficial members of this microbiota that represent a great hope for agroecology, since they could be used as bioinoculants for sustainable crop production. Therefore, it is necessary to decipher the molecular dialog between roots and PGPR in order to promote the establishment of bioinoculants in the rhizosphere, which is required for their beneficial functions. Methods: Here, the ability of root exudates from rapeseed (Brassica napus), pea (Pisum sativum), and ryegrass (Lolium perenne) to attract and feed three PGPR (Bacillus subtilis, Pseudomonas fluorescens, and Azospirillum brasilense) was measured and compared, as these responses are directly involved in the establishment of the rhizosphere microbiota. Results: Our results showed that root exudates differentially attracted and fed the three PGPR. For all beneficial bacteria, rapeseed exudates were the most attractive and induced the fastest growth, while pea exudates allowed the highest biomass production. The performance of ryegrass exudates was generally lower, and variable responses were observed between bacteria. In addition, P. fluorescens and A. brasilense appeared to respond more efficiently to root exudates than B. subtilis. Finally, we proposed to evaluate the compatibility of each plant-PGPR couple by assigning them a "love match" score, which reflects the ability of root exudates to enhance bacterial rhizocompetence. Discussion: Taken together, our results provide new insights into the specific selection of PGPR by the plant through their root exudates and may help to select the most effective exudates to promote bioinoculant establishment in the rhizosphere.

HME, NFE, and HAE-1 efflux pumps in Gram-negative bacteria: a comprehensive phylogenetic and ecological approach.

The three primary resistance-nodulation-cell division (RND) efflux pump families (heavy metal efflux [HME], nodulation factor exporter [NFE], and hydrophobe/amphiphile efflux-1 [HAE-1]) are almost exclusively found in Gram-negative bacteria and play a major role in resistance against metals and bacterial biocides, including antibiotics. Despite their significant societal interest, their evolutionary history and environmental functions are poorly understood. Here, we conducted a comprehensive phylogenetic and ecological study of the RND permease, the subunit responsible for the substrate specificity of these efflux pumps. From 920 representative genomes of Gram-negative bacteria, we identified 6205 genes encoding RND permeases with an average of 6.7 genes per genome. The HME family, which is involved in metal resistance, corresponds to a single clade (21.8% of all RND pumps), but the HAE-1 and NFE families had overlapping distributions among clades. We propose to restrict the HAE-1 family to two phylogenetic sister clades, representing 41.8% of all RND pumps and grouping most of the RND pumps involved in multidrug resistance. Metadata associated with genomes, analyses of previously published metagenomes, and quantitative Polymerase Chain Reaction (qPCR) analyses confirmed a significant increase in genes encoding HME permeases in metal-contaminated environments. Interestingly, and possibly related to their role in root colonization, genes encoding HAE-1 permeases were particularly abundant in the rhizosphere. In addition, we found that the genes encoding these HAE-1 permeases are significantly less abundant in marine environments, whereas permeases of a new proposed HAE-4 family are predominant in the genomes of marine strains. These findings emphasize the critical role of the RND pumps in bacterial resistance and adaptation to diverse ecological niches.

A long-branch attraction artifact reveals an adaptive radiation in pseudomonas.

A significant proportion of protein-encoding gene phylogenies in bacteria is inconsistent with the species phylogeny. It was usually argued that such inconsistencies resulted from lateral transfers. Here, by further studying the phylogeny of the oprF gene encoding the major surface protein in the bacterial Pseudomonas genus, we found that the incongruent tree topology observed results from a long-branch attraction (LBA) artifact and not from lateral transfers. LBA in the oprF phylogeny could be explained by the faster evolution in a lineage adapted to the rhizosphere, highlighting an unexpected adaptive radiation. We argue that analysis of such artifacts in other inconsistent bacterial phylogenies could be a valuable tool in molecular ecology to highlight cryptic adaptive radiations in microorganisms.

Genome-wide analysis and literature-based survey of lipoproteins in Pseudomonas aeruginosa.

Pseudomonas aeruginosa is a Gram-negative opportunistic pathogen able to cause acute or chronic infections. Like all other Pseudomonas species, P. aeruginosa has a large genome, >6 Mb, encoding more than 5000 proteins. Many proteins are localized in membranes, among them lipoproteins, which can be found tethered to the inner or the outer membrane. Lipoproteins are translocated from the cytoplasm and their N-terminal signal peptide is cleaved by the signal peptidase II, which recognizes a specific sequence called the lipobox just before the first cysteine of the mature lipoprotein. A majority of lipoproteins are transported to the outer membrane via the LolCDEAB system, while those having an avoidance signal remain in the inner membrane. In Escherichia coli, the presence of an aspartate residue after the cysteine is sufficient to cause the lipoprotein to remain in the inner membrane, while in P. aeruginosa the situation is more complex and involves amino acids at position +3 and +4 after the cysteine. Previous studies indicated that there are 185 lipoproteins in P. aeruginosa, with a minority in the inner membrane. A reanalysis led to a reduction of this number to 175, while new retention signals could be predicted, increasing the percentage of inner-membrane lipoproteins to 20 %. About one-third (62 out of 175) of the lipoprotein genes are present in the 17 Pseudomonas genomes sequenced, meaning that these genes are part of the core genome of the genus. Lipoproteins can be classified into families, including those outer-membrane proteins having a structural role or involved in efflux of antibiotics. Comparison of various microarray data indicates that exposure to epithelial cells or some antibiotics, or conversion to mucoidy, has a major influence on the expression of lipoprotein genes in P. aeruginosa.

Antibiotic resistance patterns of Pseudomonas spp. isolated from faecal wastes in the environment and contaminated surface water.

The Pseudomonas genus, which includes environmental and pathogenic species, is known to present antibiotic resistances, and can receive resistance genes from multi-resistant enteric bacteria released into the environment via faecal rejects. This study was aimed to investigate the resistome of Pseudomonas populations that have been in contact with these faecal bacteria. Thus, faecal discharges originating from human or cattle were sampled (from 12 points and two sampling campaigns) and 41 Pseudomonas species identified (316 isolates studied). The resistance phenotype to 25 antibiotics was determined in all isolates, and we propose a specific antibiotic resistance pattern for 14 species (from 2 to 9 resistances). None showed resistance to aminoglycosides, tetracycline, or polymyxins. Four species carried a very low number of resistances, with none to ß-lactams. Interestingly, we observed the absence of the transcriptional activator soxR gene in these four species. No plasmid transfer was highlighted by conjugation assays, and a few class 1 but no class 2 integrons were detected in strains that may have received resistance genes from Enterobacteria. These results imply that the contribution of the Pseudomonas genus to the resistome of an ecosystem first depends on the structure of the Pseudomonas populations, as they may have very different resistance profiles.

Distribution and evolution of ferripyoverdine receptors in Pseudomonas aeruginosa.

Pseudomonas aeruginosa is a ubiquitous gram-negative bacterium, which is also able to cause severe opportunistic infections in humans. The colonization of the host is importantly affected by the production of the high-affinity iron (III) scavenging peptidic siderophore pyoverdine. The species P. aeruginosa can be divided into three subgroups ('siderovars'), each characterized by the production of a specific pyoverdine and receptor (FpvA). We used a multiplex PCR to determine the FpvA siderovar on 345 P. aeruginosa strains from environmental or clinical origin. We found about the same proportion of each type in clinical strains, while FpvA type I was slightly over-represented (49%) in environmental strains. Our multiplex PCR also detected the presence or absence of an additional receptor for type I pyoverdine (FpvB). The fpvB gene was in fact present in the vast majority of P. aeruginosa strains (93%), regardless of their siderovar or their origin. Finally, molecular analyses of fpvA and fpvB genes highlighted a complex evolutionary history, probably linked to the central role of iron acquisition in the ecology and virulence of P. aeruginosa.

Comparative Genomics of Environmental and Clinical Burkholderia cenocepacia Strains Closely Related to the Highly Transmissible Epidemic ET12 Lineage.

The Burkholderia cenocepacia epidemic ET12 lineage belongs to the genomovar IIIA including the reference strain J2315, a highly transmissible epidemic B. cenocepacia lineage. Members of this lineage are able to cause lung infections in immunocompromised and cystic fibrosis patients. In this study, we describe the genome of F01, an environmental B. cenocepacia strain isolated from soil in Burkina Faso that is, to our knowledge, the most closely related strain to this epidemic lineage. A comparative genomic analysis was performed on this new isolate, in association with five clinical and one environmental B. cenocepacia strains whose genomes were previously sequenced. Antibiotic resistances, virulence phenotype, and genomic contents were compared and discussed with an emphasis on virulent and antibiotic determinants. Surprisingly, no significant differences in antibiotic resistance and virulence were found between clinical and environmental strains, while the most important genomic differences were related to the number of prophages identified in their genomes. The ET12 lineage strains showed a noticeable greater number of prophages (partial or full-length), especially compared to the phylogenetically related environmental F01 strain (i.e., 5-6 and 3 prophages, respectively). Data obtained suggest possible involvements of prophages in the clinical success of opportunistic pathogens.

Structure, function and regulation of Pseudomonas aeruginosa porins.

Pseudomonas aeruginosa is a Gram-negative bacterium belonging to the γ-proteobacteria. Like other members of the Pseudomonas genus, it is known for its metabolic versatility and its ability to colonize a wide range of ecological niches, such as rhizosphere, water environments and animal hosts, including humans where it can cause severe infections. Another particularity of P. aeruginosa is its high intrinsic resistance to antiseptics and antibiotics, which is partly due to its low outer membrane permeability. In contrast to Enterobacteria, pseudomonads do not possess general diffusion porins in their outer membrane, but rather express specific channel proteins for the uptake of different nutrients. The major outer membrane 'porin', OprF, has been extensively investigated, and displays structural, adhesion and signaling functions while its role in the diffusion of nutrients is still under discussion. Other porins include OprB and OprB2 for the diffusion of glucose, the two small outer membrane proteins OprG and OprH, and the two porins involved in phosphate/pyrophosphate uptake, OprP and OprO. The remaining nineteen porins belong to the so-called OprD (Occ) family, which is further split into two subfamilies termed OccD (8 members) and OccK (11 members). In the past years, a large amount of information concerning the structure, function and regulation of these porins has been published, justifying why an updated review is timely.

Draft Genome Sequences of Stenotrophomonas maltophilia Strains Sm32COP, Sm41DVV, Sm46PAILV, SmF3, SmF22, SmSOFb1, and SmCVFa1, Isolated from Different Manures in France.

Stenotrophomonas maltophilia is a major opportunistic human pathogen responsible for nosocomial infections. Here, we report the draft genome sequences of Sm32COP, Sm41DVV, Sm46PAILV, SmF3, SmF22, SmSOFb1, and SmCVFa1, isolated from different manures in France, which provide insights into the genetic determinism of intrinsic or acquired antibiotic resistance in this species.

Comparative Genomics of Environmental and Clinical Stenotrophomonas maltophilia Strains with Different Antibiotic Resistance Profiles.

Stenotrophomonas maltophilia, a ubiquitous Gram-negative γ-proteobacterium, has emerged as an important opportunistic pathogen responsible for nosocomial infections. A major characteristic of clinical isolates is their high intrinsic or acquired antibiotic resistance level. The aim of this study was to decipher the genetic determinism of antibiotic resistance among strains from different origins (i.e., natural environment and clinical origin) showing various antibiotic resistance profiles. To this purpose, we selected three strains isolated from soil collected in France or Burkina Faso that showed contrasting antibiotic resistance profiles. After whole-genome sequencing, the phylogenetic relationships of these 3 strains and 11 strains with available genome sequences were determined. Results showed that a strain's phylogeny did not match their origin or antibiotic resistance profiles. Numerous antibiotic resistance coding genes and efflux pump operons were revealed by the genome analysis, with 57% of the identified genes not previously described. No major variation in the antibiotic resistance gene content was observed between strains irrespective of their origin and antibiotic resistance profiles. Although environmental strains generally carry as many multidrug resistant (MDR) efflux pumps as clinical strains, the absence of resistance-nodulation-division (RND) pumps (i.e., SmeABC) previously described to be specific to S. maltophilia was revealed in two environmental strains (BurA1 and PierC1). Furthermore the genome analysis of the environmental MDR strain BurA1 showed the absence of SmeABC but the presence of another putative MDR RND efflux pump, named EbyCAB on a genomic island probably acquired through horizontal gene transfer.

Phylogenetic relationships between environmental and clinical isolates of Pseudomonas fluorescens and related species deduced from 16S rRNA gene and OprF protein sequences.

The major surface protein of the genus Pseudomonas, OprF, is a non-specific porin that plays an important role in maintenance of cell shape, in growth in a low osmolarity environment, and in adhesion to various supports. The objectives of our study were (i) to carry out a comparative analysis of phylogenies obtained from the OprF protein and from the 16S rRNA gene in 41 isolates from various sources (water, soil, milk and the hospital) and (ii) to investigate the physiological characteristics correlated with the phylogeny of OprF. We report here an important incongruence between the phylogenies of the 16S rRNA gene and the OprF protein. Phylogenetic analysis of 16S rRNA genes grouped Pseudomonas fluorescens isolates into one cluster (termed fluorescens r-cluster) whilst the phylogeny of the OprF protein divided Pseudomonas fluorescens isolates into two quite distinct clusters (termed fluorescens 1 o-cluster and fluorescens 2 o-cluster) that may be related to the original habitat of the strain. The fluorescens 1 o-cluster contained the majority of non-rhizospheric soil isolates, while the fluorescens 2 o-cluster contained all our clinical isolates and most of the rhizospheric isolates (which are fixed to the roots). In order to check this correlation, we studied two physiological characteristics: the range of growth temperature and the capacity for non-specific adhesion to polystyrene. The temperature range study for strains did not explain the existence of the two o-clusters but it did confirm the capacity of certain P. fluorescens strains to grow at 37 degrees C. The adhesion capacities of the isolates in the two o-clusters seems to be correlated with ecological niche.

Low impact of phenanthrene dissipation on the bacterial community in grassland soil.

The effect of phenanthrene on the bacterial community was studied on permanent grassland soil historically presenting low contamination (i.e. less than 1 mg kg(-1)) by polycyclic aromatic hydrocarbons (PAHs). Microcosms of soil were spiked with phenanthrene at 300 mg kg(-1). After 30 days of incubation, the phenanthrene concentration decreased rapidly until its total dissipation within 90 days. During this incubation period, significant changes of the total bacterial community diversity were observed, as assessed by automated-ribosomal intergenic spacer analysis fingerprinting. In order to get a deeper view of the effect of phenanthrene on the bacterial community, the abundances of ten phyla and classes (Actinobacteria, Acidobacteria, Bacteroidetes, Alphaproteobacteria, Betaproteobacteria, Gammaproteobacteria, Firmicutes, Verrucomicrobiales, Gemmatimonadetes, and Planctomycetes) were monitored by quantitative polymerase chain reaction performed on soil DNA extracts. Interestingly, abundances of some bacterial taxa significantly changed as compared with controls. Moreover, among these bacterial groups impacted by phenanthrene spiking, some of them presented the potential of phenanthrene degradation, as assessed by PAH-ring hydroxylating dioxygenase (PAH-RHDα) gene detection. However, neither the abundance nor the diversity of the PAH-RHDα genes was significantly impacted by phenanthrene spiking, highlighting the low impact of this organic contaminant on the functional bacterial diversities in grassland soil.

Occurrence of multi-antibiotic resistant Pseudomonas spp. in drinking water produced from karstic hydrosystems.

Aquatic environments could play a role in the spread of antibiotic resistance genes by enabling antibiotic-resistant bacteria transferred through wastewater inputs to connect with autochthonous bacteria. Consequently, drinking water could be a potential pathway to humans and animals for antibiotic resistance genes. The aim of this study was to investigate occurrences of Escherichia coli and Pseudomonas spp. in drinking water produced from a karst, a vulnerable aquifer with frequent increases in water turbidity after rainfall events and run-offs. Water samples were collected throughout the system from the karstic springs to the drinking water tap during three non-turbid periods and two turbid events. E. coli densities in the springs were 10- to 1000-fold higher during the turbid events than during the non-turbid periods, indicating that, with increased turbidity, surface water had entered the karstic system and contaminated the spring water. However, no E. coli were isolated in the drinking water. In contrast, Pseudomonas spp. were isolated from the drinking water only during turbid events, while the densities in the springs were from 10- to 100-fold higher than in the non-turbid periods. All the 580 Pseudomonas spp. isolates obtained from the sampling periods were resistant (to between 1 and 10 antibiotics), with similar resistance patterns. Among all the Pseudomonas isolated throughout the drinking water production system, between 32% and 86% carried the major resistance pattern: ticarcillin, ticarcillin-clavulanic acid, cefsulodin, and/or aztreonam, and/or sulfamethoxazol-trimethoprim, and/or fosfomycin. Finally, 8 Pseudomonas spp. isolates, related to the Pseudomonas putida and Pseudomonas fluorescens species, were isolated from the drinking water. Thus, Pseudomonas could be involved in the dissemination of antibiotic resistance via drinking water during critical periods.

Fine-scale recombination and adaptive radiation could be linked.

The difficult reconstruction of the evolutionary history of the major surface protein gene oprF highlighted an adaptive radiation in the Pseudomonas fluorescens group. The recent work of Hao (2013) showed that partial recombination events in oprF gene occurred specifically in a P. fluorescens lineage under ecological niche segregation. So, I suggest that identification of lineage-specific fine-scale recombination may be a way to detect putative adaptive radiation in bacteria.

Identification of a metagenomic gene cluster containing a new class A beta-lactamase and toxin-antitoxin systems.

Several reports mention the presence of antibiotic resistance genes in natural and polluted environments, but many studies are based on their detection via polymerase chain reaction (PCR amplification of known genes and not on an activity screening. We constructed a metagenomic fosmid bank from DNA isolated from a polluted river in Brussels, Belgium, the Zenne. A total of 120,000 clones were pooled and plated directly on solid media containing different antibiotics. Several clones were isolated which could grow in the presence of ampicillin. The DNA from several clones was extracted and subjected to restriction analysis and, based on their restriction pattern, two different clones were found. One of the clones was selected for further study as it showed a higher level of resistance to different ß-lactams antibiotics (ticarcilline and ceftazidime). To find out which gene is responsible for the resistance, an in vitro transposon mutagenesis was performed and clones having lost the resistance phenotype were analyzed via inverse PCR amplification. Several clones had an insert in a gene encoding a new type of ß-lactamase. The amplified fosmid DNA was fully sequenced revealing an insert of 41 kb containing 39 open reading frames (ORFs). Transposon insertions inactivating the resistance to ß-lactams were also found in the ORF upstream of the blaA gene, encoding an aminotransferase, suggesting a polar effect on the transcription of the gene downstream. In addition, other genes were found such as histidine biosynthesis genes, which were found to be scattered on the insert, a relA/spoT gene, and genes belonging to type II toxin-antitoxin system. This predicted system was experimentally validated in Escherichia coli using an inducible expression system.

GammaProteobacteria as a potential bioindicator of a multiple contamination by polycyclic aromatic hydrocarbons (PAHs) in agricultural soils.

The impact of a multiple contamination by polycyclic aromatic hydrocarbons (PAHs) was studied on permanent grassland soil, historically presenting low contamination (i.e. less than 1 mg kg(-1)). Soil microcosms were spiked at 300 mg kg(-1) with either single or a mixture of seven PAHs. While total dissipation of the phenanthrene was reached in under 90 days, only 60% of the PAH mixture were dissipated after 90 days. Interestingly, after 30 days, the abundance of the GammaProteobacteria class (assessed by qPCR) become significantly higher in microcosms spiked with the PAH mixture. In addition, the specific abundance of the cultivable Pseudomonas spp., which belong to the GammaProteobacteria class, increased earlier and transiently (after 8 days) in the microcosms spiked with the PAH mixture. Consequently, we propose to use the GammaProteobacteria as a bioindicator to detect the impact on the bacterial community of a multiple contamination by PAHs in agricultural soils.

Abundance and diversity of copper resistance genes cusA and copA in microbial communities in relation to the impact of copper on Chilean marine sediments.

Microorganisms have developed copper-resistance mechanisms in order to survive in contaminated environments. The abundance of the copper-resistance genes cusA and copA, encoding respectively for a Resistance Cell Nodulation protein and for a P-type ATP-ase pump, was assessed in copper and non-copper-impacted Chilean marine sediment cores by the use of molecular tools. We demonstrated that number of copA and cusA genes per bacterial cell was higher in the contaminated sediment, and that copA gene was more abundant than cusA gene in the impacted sediment. The molecular phylogeny of the two copper-resistance genes was studied and reveals an impact of copper on the genetic composition of copA and cusA genes.

Variable copy number, intra-genomic heterogeneities and lateral transfers of the 16S rRNA gene in Pseudomonas.

Even though the 16S rRNA gene is the most commonly used taxonomic marker in microbial ecology, its poor resolution is still not fully understood at the intra-genus level. In this work, the number of rRNA gene operons, intra-genomic heterogeneities and lateral transfers were investigated at a fine-scale resolution, throughout the Pseudomonas genus. In addition to nineteen sequenced Pseudomonas strains, we determined the 16S rRNA copy number in four other Pseudomonas strains by Southern hybridization and Pulsed-Field Gel Electrophoresis, and studied the intra-genomic heterogeneities by Denaturing Gradient Gel Electrophoresis and sequencing. Although the variable copy number (from four to seven) seems to be correlated with the evolutionary distance, some close strains in the P. fluorescens lineage showed a different number of 16S rRNA genes, whereas all the strains in the P. aeruginosa lineage displayed the same number of genes (four copies). Further study of the intra-genomic heterogeneities revealed that most of the Pseudomonas strains (15 out of 19 strains) had at least two different 16S rRNA alleles. A great difference (5 or 19 nucleotides, essentially grouped near the V1 hypervariable region) was observed only in two sequenced strains. In one of our strains studied (MFY30 strain), we found a difference of 12 nucleotides (grouped in the V3 hypervariable region) between copies of the 16S rRNA gene. Finally, occurrence of partial lateral transfers of the 16S rRNA gene was further investigated in 1803 full-length sequences of Pseudomonas available in the databases. Remarkably, we found that the two most variable regions (the V1 and V3 hypervariable regions) had probably been laterally transferred from another evolutionary distant Pseudomonas strain for at least 48.3 and 41.6% of the 16S rRNA sequences, respectively. In conclusion, we strongly recommend removing these regions of the 16S rRNA gene during the intra-genus diversity studies.

Both Cycloclasticus spp. and Pseudomonas spp. as PAH-degrading bacteria in the Seine estuary (France).

Like other highly urbanized and industrialized estuaries, the Seine estuary (France) has, for decades, received high inputs of polycyclic aromatic hydrocarbons (PAHs). In order to estimate the bioremediation potentials and to identify the bacterial species involved in hydrocarbon degradation, we used microcosms containing seawater from the Seine estuary supplemented with either naphthalene, phenanthrene, fluorene or pyrene. In the microcosms enriched with naphthalene or phenanthrene, hydrocarbon biodegradation was significant within 9 weeks (43% or 46%, respectively), as shown by analyses in GC-MS. In similar microcosms incubated also with naphthalene or phenanthrene, analysis of the 16S rRNA gene sequences (DNA and cDNA) with denaturing gradient gel electrophoresis and clone libraries indicated that the PAH-degrading communities were dominated by Cycloclasticus spp., confirming their universal key role in degradation of low-molecular-weight PAHs in marine environments. However, in contrast to previous studies, we found that Pseudomonas spp. also degraded naphthalene and phenanthrene in seawater; this occurred only after 21 days, as was confirmed by real-time PCR. Although this genus has been abundantly described in the literature as a good PAH-degrading bacterial group in soil or in sediment, to our knowledge, this is the first evidence of a significant fitness in PAH degradation in seawater.