MicrobiologyOpen

Amino acid catabolism is a vital metabolic process in bacteria, providing energy, carbon and potentially nitrogen as resources, and affecting global cycles of these elements. The ability of a bacterium to catabolize an amino acid is often inferred from the presence of the relevant catabolic pathways in its genome, yet the "gene=function" inference is not straightforward. Here, we use growth assays in 96 well plates on individual amino acids and their combinations to directly measure the ability of a model marine bacterium, Alteromonas macleodii ATCC 27126, to utilize these resources for growth. With the exception of aspartate and glutamate, which did not support growth in any of our experiments, ATCC 27126 grew on all other amino acids. However, the probability of growth, together with growth yield and rate, differed depending on the entry point of the catabolic pathway to central carbon metabolism, with robust growth occurring only on amino acids catabolized into pyruvate or acetyl CoA. Growth on combinations of two amino acids revealed reproducible patterns, the clearest being inhibition of growth on other amino acids by asparagine, aspartate and their degradation product, oxaloacetate. Finally, growth was different in test tubes compared with 96 well plates. Our results reveal hidden complexity in amino acid utilization and suggest a "TCA-centric" viewpoint for amino acid utilization, perhaps reflecting the high metabolic flexibility of pyruvate and specific regulatory aspects of the TCA cycle in Alteromonas.

Plasmid host range (PHR) plays a key role in the spread of ecologically important genes, alongside applications in microbiome engineering, and environmental biotechnology. PHR is a complex trait arising from the combination of plasmid, donor and recipient properties. Most studies of PHR use a single donor strain, leaving the role of the donor unexplored, and often require genetically tagged recipient strains for counter selection, which limits use of non-genetically tractable strains. Here we developed a PHR screening method using auxotrophic donors that bypasses the need to genetically tag recipients, thus allowing the screening of culturable environmental bacterial strains. Specifically, we used two auxotrophic donors (P. fluorescens and P. putida), and the plasmid pQBR57-tphKAB, an environmental plasmid engineered for terephthalic acid bioremediation. We screened a library of 101 soil isolates, as potential recipients, including common soil genera of soil bacteria, Pseudomonas, Bacillus and Xanthomonas. We only observed conjugation into other Pseudomonas, but donor identity affected PHR, with P. fluorescens conjugating the plasmid into more recipient strains than P. putida. Phylogenomic analysis revealed that transconjugants clustered with P. citronellosis and P. putida lineages. In strains that were close relatives of transconjugants but who were unable to acquire the plasmid, we observed 5 defence systems not present in transconjugants that may act as barriers to plasmid acquisition. Our method provides a rapid, tag-free framework for screening PHR in environmental isolates and for investigating the influence of donor identity on plasmid conjugation.

Bacterial interactions-whether positive or negative - are crucial for the functioning of microbial communities. Though bacterial interactions are mainly expected to be negative, the sign and strength of interactions are predicted to be context dependent, with interactions typically being more positive in more stressful and nutrient-poor conditions. However, systematic studies investigating how the environment affects interactions between multiple taxa are lacking. Here, we determine if interactions between a panel of natural soil isolates change in response to the environment in which they are grown, with two different artificial media used (one simple and one complex) and a more ecologically relevant soil wash. To maximise natural variation in interactions, we collected multiple isolates from multiple sites: co-occurring (sympatric) isolates were predicted to show more negative interactions than allopatric isolates because of greater overlap in resource use. Pairwise interactions were in general negative, but more negative when grown in a complex lab-derived medium (Tryptic Soy Broth). Mutually beneficial interactions were most common in a simple resource medium (M9 minimal media) and exploitative interactions were most frequent in a soil broth. These patterns were independent of whether species originated from the same or a different site. The study supports the prediction that nutrient rich environments promote more negative interactions, and that measuring interactions of soil isolates in standard lab media is likely to misrepresent interactions occurring in natural environments.

The human gut microbiome is a complex community that plays an important role in health, where perturbations can result in dysbiosis and disease. Bacteriophages (phages) can provide treatment for bacterial gastrointestinal disease, and commercial preparations such as the Intesti bacteriophage cocktail can be taken orally to target bacterial pathogens. However, interactions between these phages and the native gut microbiota are understudied. To investigate the impact of phage treatment, we used simulated gut models seeded with healthy donor microbiota from three individuals, sequenced the DNA, and analysed the bacterial and viral portion from samples obtained over time. Each donor had a unique bacterial composition which diverged with time. When comparing phage treated to control samples, we observed that Escherichia coli abundance accounted for the largest portion of bacterial community variance and was more associated with the controls. The lower abundance in phage treated samples may have resulted from the lytic action of phages from the cocktail. Additionally, our analyses of the viral portion revealed a phage bloom exclusive to phage treated samples. A highly abundant phage in this bloom was matched with the Intesti bacteriophage cocktail, showed similarity to Enterobacteria phage phi92, and provided evidence of productive infection within the model. While we did observe fluctuations in relative abundance of additional viral sequences in the presence of the phage cocktail, these changes were often transient. Furthermore, we detected only slight differences to typical members of the virome, and low numbers of active prophages. Our experiments suggest that the phage cocktail had minimal interruption to the native gut microbiota within the model. Impact statementBacteriophages are increasingly investigated and tested for their efficacy in treating infections and are a key component in fight against antimicrobial resistant bacterial infections. Because of their specificity, it has become almost a dogma to state that they do not alter the gut microbiome. We have now tested this in an in vitro study using a commercially available cocktail and real human faecal microbiota. We show minimal effects on the composition of the healthy microbiota with an individual-specific effect on Escherichia coli caused by productive infection of one phage in the cocktail.

Auxotrophy, the inability of bacteria to synthesize one or multiple essential metabolites (e.g. amino acids, vitamins, metabolites) is thought to be common among bacteria. However, studies often rely either on bioinformatic tools to predict auxotrophies from genome data or on experiments with low numbers of strains. Here, we combine experimental and bioinformatic approaches to assess amino acid auxotrophy levels among 315 co-isolated natural Pseudomonas strains from pond and soil habitats. Both approaches revealed that Pseudomonas isolates are predominantly prototrophs. We identified one single histidine auxotroph and five non-specific auxotrophs featuring complex growth phenotypes incompatible with single amino acid auxotrophies. While different bioinformatic pipelines vary in the extent to which auxotrophy is over- or underestimated, none of the pipelines could resolve the basis of non-specific auxotrophies. Our analysis further revealed the existence of multiple alternative biosynthesis pathways for methionine, proline, and phenylalanine, with significant enrichments of specific pathways among soil or pond strains. We conclude that combining experiments with bioinformatics is a powerful approach to assess the metabolic potential of environmental bacteria. Moreover, taxa like Pseudomonas can be predominantly prototrophic possibly owing to their generalist lifestyle, thus calling for nuanced ecological concepts predicting auxotrophy levels based on lifestyle and habitat.

IntroductionAntimicrobial resistance has existed in the environment long before its rapid emergence and detection in clinically relevant pathogens. Studying the resistance of environmental bacterial strains may allow novel resistance mechanisms to be identified before they appear in pathogenic strains. Gap StatementSearching for antimicrobial resistance genes in environmental bacteria represents an understudied research area compared to resistance within clinically relevant pathogens. AimTo evaluate resistance genes present within environmental non-aeruginosa Pseudomonas spp. isolates. MethodologyWe screened a set of bacterial isolates from untreated wastewater from Liverpool, UK, for the presence of extended spectrum {beta}-lactamases and carbapenemases. A sub-set of three resistant Pseudomonas spp. isolates were selected for whole-genome sequencing. We performed minimum inhibitory concentration assays against several {beta}-lactams, and ectopic expression of four novel resistance genes within Escherichia coli. ResultsHere, we report the discovery of novel class C {beta}-lactamase genes blaPFL7, blaPFL8 and blaPFL9, as well as a novel subclass B2 metallo-{beta}-lactamase blaPFM5 present within these strains. The class C genes encoded proteins with between 61-71% amino acid identity to the closest known match, blaPFL-1. These novel {beta}-lactamases degraded the cephalosporin nitrocefin and confer piperacillin and ceftazidime resistance to susceptible Escherichia coli when ectopically expressed. The {beta}-lactamase inhibitor tazobactam was effective at inhibiting these enzymes. The sub-class B2 metallo-{beta}-lactamase had 88% amino acid identity to its closet match blaPFM-1 and conferred carbapenem resistance to susceptible E. coli. The {beta}-lactamase inhibitors relebactam, vaborbactam, xeruborbactam and captopril had no impact on the carbapenem resistance phenotype. Analogues of all these novel genes (>95% nucleotide sequence identity) were identified within publicly available whole-genome sequencing data, suggesting they are found sporadically. ConclusionOur analysis adds to the growing number of {beta}-lactamase genes found from environmental Pseudomonas spp. and suggests that continued surveillance of this environmental reservoir for novel, clinically relevant, {beta}-lactamase genes is warranted.

The Bacillus genus comprises physiologically versatile, endospore-forming bacteria widely distributed in natural environments. In this study, we report the isolation and genomic characterization of strain Bva_UNVM-123, recovered from agricultural soil in Pergamino, Argentina. Whole-genome sequencing using Illumina technology yielded a 5.1 Mbp draft genome assembled in 67 contigs with a GC content of 36%. Comparative genomic analyses using the TYGS server and digital DNADNA hybridization (dDDH) values supported its classification as a potentially novel species within the Bacillus sensu lato (s.l.) group. Genome annotation revealed 4,866 protein-coding genes, including multiple determinants conferring resistance to antibiotics (e.g., fosfomycin, tetracycline, beta-lactams) and toxic heavy metals (e.g., arsenic, cadmium, mercury), supporting its potential application in bioremediation. Additionally, PathogenFinder predicted a low probability of human pathogenicity (0.207), reinforcing its safety for environmental use. Functional classification based on Swiss-Prot further supported a metabolically versatile profile and revealed the presence of resistance-related categories associated with environmental adaptation. This study adds to the growing knowledge of environmental Bacillus species and their biotechnological potential

Prokaryotes, particularly those in extreme environments, are capable of diverse metabolic states resulting in altered cell envelope structure and function. However, these changes are difficult to assess as standard fluorescent probes are often incompatible with extreme conditions and/or extremophile cell physiology. Halophilic archaea present the challenge of near-saturated intra-/extra-cellular salts, high membrane potential, and extended survival in altered metabolic states including entrapped within salt crystal fluid inclusions. We evaluated the compatibility of six fluorescent markers of cell envelope stability and activity with two model species, Halobacterium salinarum and Haloferax volcanii. Redox activity markers alamarBlue and pure resazurin solutions, membrane potential probes MitoTracker Orange-CMTMRos and Rhodamine 123, and SYTO 9 and propidium iodide (LIVE/DEAD kit) to assess cell membrane integrity were evaluated for use in bulk (microplate reader) and cell-specific (microscopy) applications. Limitations of each probe were identified, clarifying the utilization of each based on cell physiology, growth phase, medium composition, and probe exposure time including extended timescales needed to simulate the environmental conditions of haloarchaea. Of particular note, propidium iodide behavior was unreliable leading to double-labeling of cells and false interpretation of cells as dead. These data provide important insights into the study of prokaryotes in non-standard conditions.

Horizontal gene transfer is an important evolutionary process by which DNA is exchanged between cells that are physically co-located but not direct evolutionary descendants. Horizontal transfer of highly divergent DNA is relatively easy to detect and can produce major phenotypic changes, exemplified by acquisition of antibiotic resistance determinants. However, transfer of high-identity DNA, for example between strains of the same species, is likely to be more frequent, harder to detect, and highly impactful in aggregate. In this work, we demonstrate that recombination between soil isolates of the alphaproteobacterium Novosphingobium aromaticivorans can exchange chromosomal DNA, leading to multiple unselected recombination events spanning approximately 10% of the chromosome. Chromosomal recombination was directional, more efficient near an integrative and conjugative element (ICE), and required a relaxase found in the ICE. Recombination could not be observed into strains from closely related Novosphingobium species. In combination, these results suggest that ICE-mediated recombination can efficiently recombine DNA within N. aromaticivorans, increasing the adaptive potential of the species while also enforcing species boundaries through preferential intraspecific recombination. ImportanceHorizontal gene transfer is a key process in bacterial evolution. Mechanisms for transfer of mobile genetic elements are well-characterized, but less is known about how chromosomal DNA is recombined. In this work, we demonstrate that integrative and conjugative elements can efficiently recombine chromosomal DNA between strains of Novosphingobium aromaticivorans but not between different Novosphingobium species. We conclude that ICE-mediated chromosomal recombination can be an important adaptive mechanism within a species, due to its ability to recombine nearby chromosomal alleles, but also serves to delineate species-specific gene pools as a result of its limited phylogenetic range.

BACKGROUNDEarly detection of colorectal cancer (CRC) is essential for reducing disease-related mortality; however, the invasiveness and resource intensity of colonoscopy limit its widespread use in population screening. Although several non-invasive screening modalities are available, their sensitivity remains suboptimal, particularly for advanced precancerous lesions (APL). Stool-derived host RNA biomarkers represent a promising approach to improve screening performance. However, numerous technical challenges are associated with extracting and quantifying this RNA, including an abundance of PCR inhibitors, a high microbial background, and varying levels of RNA degradation. METHODSWe developed an extraction method for the isolation of host RNA from stool with improved sensitivity of detection and better diagnostic performance than the conventional phenol-chloroform-based extraction. ECB-Extract (Enrichment Capture By hybridization), uses hybridization capture of RNA under denaturing conditions by adding locked nucleic acid bases to the probes. The improved hybridization characteristics allow us to directly isolate host RNA from stool lysate. RESULTSECB-Extract significantly improved PCR inhibitor removal, enabling scale-up of stool input and resulting in increased diagnostic sensitivity. We evaluated ECB-Extract against a conventional phenol-chloroform-based extraction method using the same panel of biomarkers in a pilot clinical cohort (N = 73). Using ECB-Extract, the sensitivity for detecting CRC and Advanced Precancerous Lesions (APL) was 96% and 50%, respectively, compared with 56% and 21% for the phenol-chloroform comparator at 96% specificity. In a larger cohort of 359 samples, ECB-Extract combined with a panel of the eight CRC-associated mRNA biomarkers achieved sensitivities of 95.7% for CRC and 51.2% for APL at a specificity greater than 90%. CONCLUSIONThese results demonstrate that ECB-Extract substantially improves stool host RNA extraction performance and, consequently, enhances diagnostic sensitivity for CRC and APL.

MmpL proteins play an important role in the various mechanisms associated with mycobacterial virulence. Identification of interacting protein partners required for a detailed understanding of their role remains hampered because of their large size (> 100 kDa) and the presence of twelve transmembrane domains by classical methods. In this study, we used two independent biotin proximity labelling assays (APEX2 and BioID) to define the proxisome of MmpL11 in M. smegmatis. Indeed, these techniques are performed directly in the organism of interest, allowing the detection of potentially transient or weak interactions in multiprotein complexes and preserving the subcellular structures and the presence of cofactors or post-translational modifications that can also impact protein-protein interactions. BioID leads to the biotinylation of lysine residues, whereas APEX2 leads to the biotinylation of mainly tyrosine residues; they have also been shown to have different effective labelling radii. On one hand, an interaction was detected between the cytoplasmic C-terminal domain of MmpL11 and MSMEG_0240, a protein of unknown function, using BioID. This interaction was confirmed using both MmpL11 and MSMEG_0240 as fusions with BirA and was corroborated by AlphaFold3 prediction. On the other hand, APEX2 failed to detect an interaction between MmpL11 and MSMEG_0240, probably due to the absence of accessible tyrosines. However, both approaches identified MSMEG_0940 as an additional interactant with MmpL11 that also depends on the C-terminal domain. Overall, this study demonstrates that APEX2 and BioID as complementary tools for defining the proxisome of mycobacterial proteins.

Global initiatives emphasize the need for harmonized soil biodiversity assessments. Efficient DNA extraction methods that accommodate larger soil volumes are essential for capturing higher trophic levels than bacteria and fungi and supporting extensive sampling campaigns. We developed and evaluated a scalable, cost-efficient, and automation-ready soil DNA isolation technique alongside commercial protocols. Three starting soil amounts (0.25 g, 2.5 g, and 5 g) were tested using widely used Qiagen kits, the developed isolation method, or combinations thereof. Lysis volumes ranged from 800 {micro}l to 15 ml, and purification employed either silica membrane or carboxyl-coated magnetic beads. Four different types of soil, both agricultural and forest soil, samples were sequenced on an Illumina MiSeq platform using universal eukaryotic primers targeting the 18S rRNA SSU region, enabling detection of non-fungal eukaryotes such as soil mesofauna and protozoa. The developed protocol, which combined a tenfold increase in sample volume with hybrid purification steps, yielded the highest DNA recovery and consistently improved detected richness in several soil types. Species richness patterns varied by soil type and organism group: for eukaryotes and protozoa, commercial maxiprep methods along with the combination methods outperformed the miniprep approach in agricultural soils, while the developed technique excelled in coarse xeric forest soils. For metazoans, larger extraction volumes were associated with higher richness in forest soils. Our findings indicate that at least a tenfold increase in soil input compared to conventional 0.2-0.3 g is required to reliably capture mesofaunal diversity, with preliminary evidence suggesting further benefits at 20-fold volumes. We confirm that extraction volume is a key factor shaping detection of both soil metazoan and protozoan community compositions, with effects varying by soil type and organism group. The developed scalable approach offers a practical solution for large-scale soil biodiversity assessments, aligning with global monitoring goals and enabling integration of higher trophic levels into eDNA-based frameworks.

Marine non-cyanobacterial diazotrophs (NCDs) are recognized as globally distributed, however, few representatives have been isolated in pure cultures. As a result, understanding the physiology, growth rate, substrate preference and dinitrogen (N2) fixation capabilities proves difficult. Thalassolituus haligoni. sp. nov., BB40 was isolated from a fjord-like inlet within Kjipuktuk (Halifax), Nova Scotia. The fully sequenced genome displayed all necessary genes required for N2 fixation, and various carbon uptake pathways. The gram-negative flagellated rod shape bacterium displayed significantly higher growth rates in medium amended with nitrate (NO3-) or ammonia (NH3), compared to dissolved N2, as the sole nitrogen source. Biological N2 fixation rates were detectable across all conditions, measuring a range from 9.34 x 10-6 to 1.4 x 10-1 fmol N cell-1 day-1. Growth of the isolate was successful between 4 {degrees}C up to 35 {degrees}C, with a Topt of 20 {degrees}C for N2, and between 27 - 30 {degrees}C for fixed nitrogen (NO3- and NH3). The closest relatives to T. haligoni, were found to be the uncultured Arc-gamma-03 (99% average nucleotide identity (ANI)) and Oceanobacter antarcticus (81% ANI). T. haligoni also displays versatile capabilities for growth on various carbon, and nitrogen sources, and antibiotics. Collectively this study provides an in-depth physiological assessment of an Oceanospirillales diazotrophic species which we presently have limited knowledge of.

The genus Exiguobacterium comprises Gram-positive, non-spore-forming, facultative anaerobic bacteria known for their remarkable adaptability to extreme environments, including soils, hot springs, glaciers, and the gastrointestinal tracts of certain organisms. Despite their unique adaptations for surviving in extreme environments, their pathogenicity is well documented. Here, we analyzed the phenotypical traits of two Mexican strains of Exiguobacterium--JVH47, isolated from contaminated urban sediments in Mexico City, and P4526, from the less human-impacted Cuatro Cienegas Basin. Furthermore, strains were related via comparative genomics using publicly available genomes. Phenotypic characterization demonstrated that both strains thrive across a wide range of temperatures (20-50 {degrees}C), pH (7-11), and salinity (up to 7% NaCl). Although sensitive to erythromycin, the JVH47 strain exhibited higher erythromycin resistance and harbored antibiotic resistance genes. This study underscores the ecological versatility of Exiguobacterium and its potential role as a reservoir for antibiotic resistance genes. While rarely associated with human infections, its ability to survive in extreme conditions and form biofilms raises concerns for immunocompromised individuals. These findings highlight the need for careful consideration of Exiguobacterium in biotechnological applications and its implications under the One Health framework.

Miniature inverted-repeat transposable elements (MITEs) are nonautonomous mobile genetic elements (MGEs) that can be mobilized by transposases provided by the relevant autonomous MGEs. MITEs originating from Tn3-family transposons were previously termed Tn3-derived inverted-repeat miniature elements (TIMEs). Composite transposon-like structures bounded by two copies of TIME, called TIME-COMPs, were shown to mobilize the intervening sequences. However, their association with antibiotic resistance genes (ARGs) has not yet been systematically studied. This study thus aimed to identify new TIME-COMP-like structures containing ARGs in the genomic sequences of the clinically important bacterial family Enterobacteriaceae in public databases. TIME-COMP-like structures were first searched for in the plasmid database PLSDB, focusing on small plasmids, using a self-against-self blastn approach to identify repeated elements. Then, newly and previously identified MITEs (including TIMEs) were searched for in the NCBI core nucleotide database to identify TIME-COMP-like structures located on other replicons. Bioinformatic analysis identified multiple previously unreported TIME-COMPs containing ARGs, which are bounded by directly or inversely oriented TIMEs, namely, IS101, MITESen1, and a novel 244-bp TIME termed TIME244. TIME244 contains a putative resolution site related to that of Tn21. These TIMEs were predominantly detected in plasmids and very rarely in chromosomes. The ARGs embedded in newly identified TIME-COMPs were blaKPC-2, floR, qnrS1, and tet(A). Notably, the blaKPC-2 carbapenemase gene was found in TIME-COMPs bounded by TIME244 and a TIME-COMP bounded by IS101. These findings highlight a potential role for TIMEs in the spread of diverse ARGs. IMPACT STATEMENTBacterial miniature inverted-repeat transposable elements (MITEs) are a group of short (50 bp-500 bp) nonautonomous transposable elements that are thought to have originated from insertion sequences or transposons. Although MITEs can theoretically mobilize antibiotic resistance genes (ARGs) in the presence of transposases, only a few studies have reported their association with ARGs, probably due to difficulties in identifying MITEs in genomic sequences. This study provides evidence, based on bioinformatic analysis of public Enterobacteriaceae genomes, that a subset of MITEs, called Tn3-derived inverted-repeat miniature elements (TIMEs), mobilizes ARGs by forming composite transposon-like structures. A novel 244-bp TIME, designated TIME244, was present in more than 100 Enterobacteriaceae plasmids in the current RefSeq database, suggesting its further transmission in bacterial populations through horizontal gene transfer. This study reveals that TIMEs were often overlooked when analyzing the genetic contexts of ARGs in previous studies. These findings highlight the importance of TIMEs in bacterial gene acquisition and underscore the need for new tools that can detect TIMEs in bacterial genomes for ARG surveillance. DATA SUMMARYAccession numbers of sequence data analyzed in this study are provided within the article or in supplementary data files.

Plasmids are extrachromosomal mobile genetic elements that can facilitate rapid bacterial adaptation by transferring genes between individuals. While plasmids are known to exist in diverse habitats and encode a range of traits, most of our knowledge about plasmids comes from clinically-associated antimicrobial resistance (AMR) plasmids that have already been recruited as vectors of drug resistance and have likely been shaped by strong selection for plasmid-encoded resistance. Here, we investigated 26 plasmids from the pQBR collection -- a set of large, co-existing mercury resistance environmental plasmids isolated in Pseudomonas spp. from a field in Oxfordshire in the 1990s -- and explored the ability of pQBR plasmids to mobilise novel chromosomally-encoded traits. New whole genome sequences for 25 plasmids confirmed that these soil-isolated plasmids are generally very large (140-588 kb), constitute at least five distinct genetic groups, and have relatives in various other Pseudomonas species and habitats. Despite significant nucleotide-level divergence, Groups I (pQBR103-like, [~]406 kb) and IV (pQBR57-like, [~]328 kb) showed remarkable ancient similarities in synteny and gene content both with one other, and with the PInc-2 family of plasmids known to mobilise clinically significant drug resistance in Pseudomonas aeruginosa. None of the pQBR plasmids sequenced to date harboured known AMR determinants, but putative phage defence systems and metal resistances were evident. Transposable elements, including the Tn5042 mercury resistance transposon, were responsible for significant structural variation within plasmid groups, consistent with a predominant role of transposons in rapidly remodelling plasmids. To experimentally test the ability of pQBR plasmids to spread new traits, we developed a novel transposon mobilisation assay which showed that certain Group IV pQBR plasmids were especially effective at acquiring the chromosomally-encoded transposon Tn6291, and that this mobilisation was likely due to specific plasmid factors rather than generic conjugation rate. Our work presents a tractable set of sequenced plasmids suitable for exploring the evolution and dynamics of gene acquisition by pre-AMR plasmids, and provides a key case study highlighting the pervasive interplay between plasmids and transposable elements that can drive microbial genome evolution. Repositories: github.com/jpjh/PQBR_PLASMIDS Impact statementPlasmids can drive microbial evolution by acting as vectors for horizontal gene transfer. Because of their central role in disseminating antimicrobial resistance (AMR), plasmids are mainly explored as vehicles for AMR traits, meaning that our knowledge of the diversity and evolutionary dynamics of non-AMR plasmids is more limited. Here, we explore sequences from a set of mercury resistance plasmids isolated in Pseudomonas spp. from pristine agricultural land that lack AMR determinants. By providing new whole genome sequencing analyses we expand the set of sequenced pQBR plasmids to 26, finding globally dispersed relatives from clinical, environmental, and industrial settings, and identifying an ancient plasmid backbone shared amongst divergent modern environmental and clinical AMR plasmids. We experimentally verify the role of pQBR plasmids in readily mobilising chromosomal traits using a novel transposon mobilisation assay, which suggests that specific plasmid-transposon interactions may drive trait spread. Overall, our work expands our understanding of the role of environmental plasmids in mobilising and disseminating adaptive traits.

1Opportunistic, biofilm-forming pathogens such as Pseudomonas aeruginosa can employ an array of strategies to reduce the impact of antibiotics on their survival. The biofilm matrix can prevent antibiotics from reaching bacteria embedded within it; general changes in metabolic activity alter susceptibility to specific drugs dependent on the target; changes in the membrane and the expression of channel or pump proteins embedded within it affect drug uptake and efflux; and production of antibiotic-degrading enzymes can remove the threat. In this study, we report that biofilm-deficient mutants of two well-studied lab strains of P. aeruginosa (PA14 and PAO1) have wild-type (WT) levels of tolerance to colistin and meropenem when allowed to establish mature populations in an ex vivo pig lung model of cystic fibrosis lung infection. The biofilm defects in the mutants were confirmed using SEM, and cryoSEM was used to visualise the hydrated biofilm matrix in the WT. Using RNA sequencing of the PA14 WT and an isogenic mutant lacking the pel polysaccharide, we were able to identify a small number of differences in the responses of the two genotypes to the lung environment and to exposure to sub-bactericidal colistin in the lung model. Notably, there was differential upregulation of the MexXY-OprM and MexEF-OprN multidrug efflux pumps. However, the relative roles of biofilm matrix versus cellular changes in physiology in conferring antibiotic tolerance in this environment remain to be fully elucidated.

Understanding diverse microbial communities is important due to their ecological and medical significance. Bacterial cells are genetically and phenotypically heterogeneous, making their interactions in the communities complex. The heterogeneity and interactions of cells contribute to the formation of specific spatial structures, such as biofilms, and the spread of antibiotic resistance. Here, we describe a novel single-cell approach for studying the cellular heterogeneity and spatial interactions in microbial communities that combines polyacrylamide bead encapsulation of cells and split-pool-barcoding. We demonstrate the method by determining artificially imposed interactions and connecting the taxonomic information in a mock three-species bacterial community with a species-specific genomic target. The method can be utilised for the spatial analysis of microbial communities as well as, once fully optimised for single-cell resolution, linking genetic traits to single cells.

BackgroundAcinetobacter baumannii is a common bacterial pathogen in nosocomial infections. It has become one of the greatest threats to human health for its growing resistance to last resort antibiotics, which has led to a revival of phage therapy as a potential treatment. However, conventional methods for isolating A. baumannii-infecting phages are labor-intensive and often unsuccessful. MethodsOur approach involves a computational pipeline to identify temperate phages (prophages) integrated into A. baumannii genomes, followed by mitomycin C (MMC) induction of those strains to screen for active prophages. ResultsHere we show a prophage analysis for nearly 900 A. baumannii genomes. We observed MMC-triggered excision of nine prophages from eight A. baumannii strains by PCR and sequencing. Further we show four prophage form virions detectable by transmission electron microscopy, and two which can plaque on other A. baumannii isolates. ConclusionThis work demonstrates the utility and diversity of prophages for further development as therapeutics for antibiotic resistant A. baumannii.

Aryl polyenes (APEs) are specialized polyunsaturated outer membrane lipids that protect their producers from oxidative stress and contribute to biofilm formation. APEs are produced by an abundant biosynthetic gene cluster (BGC) family conserved across Gram-negative bacterial clades. The APE biosynthesis pathway involves 11 different enzymes and cumulates in the attachment of APEs to an anchor molecule in the Gram-negative outer membrane. Unlike most other small molecule BGCs, the APE BGC does not contain a dedicated regulatory gene that controls production of its metabolically costly compounds. Building from our prior observations of APEs role in acute oxidative stress protection, we here use a uropathogenic Escherichia coli (UPEC) strain to show that APE expression conveys a potential competitive advantage characterized by increased early-stage growth, sensitization of the bacterial oxidative stress response, and dampening of the redox stress of innate immune cells after in vitro infection. Our data indicate that APEs could act as a UPEC fitness factor, and in future work we aim to study their contribution to overall bacterial pathogenicity and survival, as well as how APEs could facilitate the transition from an oxygen poor environment such as the gut to the oxygen rich environment of the urinary tract.