Evolutionary changes of an intestinal Lactobacillus reuteri during probiotic manufacture

A plasmid with the bla CTX-M gene enhances the fitness of Escherichia coli strains under laboratory conditions

Antimicrobial resistance (AMR) is a major threat to global public health that continues to grow owing to selective pressure caused by the use and overuse of antimicrobial drugs. Resistance spread by plasmids is of special concern, as they can mediate a wide distribution of AMR genes, including those encoding extended-spectrum β-lactamases (ESBLs). The CTX-M family of ESBLs has rapidly spread worldwide, playing a large role in the declining effectiveness of third-generation cephalosporins. This rapid spread across the planet is puzzling given that plasmids carrying AMR genes have been hypothesized to incur a fitness cost to their hosts in the absence of antibiotics. Here, we focus on a WT plasmid that carries the bla CTX-M 55 ESBL gene. We examine its conjugation rates and use head-to-head competitions to assay its associated fitness costs in both laboratory and wild Escherichia coli strains. We found that the wild strains exhibit intermediate conjugation levels, falling between two high-conjugation and two low-conjugation laboratory strains, the latter being older and more ancestral. We also show that the plasmid increases the fitness of both WT and lab strains when grown in lysogeny broth and Davis-Mingioli media without antibiotics, which might stem from metabolic benefits conferred on the host, or from interactions between the host and the rifampicin-resistant mutation we used as a selective marker. Laboratory strains displayed higher conjugation frequencies compared to WT strains. The exception was a low-passage K-12 strain, suggesting that prolonged laboratory cultivation may have compromised bacterial defences against plasmids. Despite low transfer rates among WT E. coli, the plasmid carried low fitness cost in minimal medium but conferred improved fitness in enriched medium, indicating a complex interplay between plasmids, host genetics and environmental conditions. Our findings reveal an intricate relationship between plasmid carriage and bacterial fitness. Moreover, they show that resistance plasmids can confer adaptive advantages to their hosts beyond AMR. Altogether, these results highlight that a closer study of plasmid dynamics is critical for developing a secure understanding of how they evolve and affect bacterial adaptability that is necessary for combating resistance spread.

Reevaluating human-microbiota symbiosis: Strain-level insights and evolutionary perspectives across animal species

The prevailing consensus in scientific literature underscores the mutualistic bond between the microbiota and the human host, suggesting a finely tuned coevolutionary partnership that enhances the fitness of both parties. This symbiotic relationship has been extensively studied, with certain bacterial attributes being construed as hallmarks of natural selection favoring the benefit of the human host. Some scholars go as far as equating the intricate interplay between humans and their intestinal microbiota to that of endosymbiotic relationships, even conceptualizing microbiota as an integral human organ. However, amidst the prevailing narrative of bacterial species being categorized as beneficial or detrimental to human health, a critical oversight often emerges - the inherent functional diversity within bacterial strains. Such reductionist perspectives risk oversimplifying the complex dynamics at play within the microbiome. Recent genomic analysis at the strain level is highly limited, which is surprising given that strain information provides critical data about selective pressures in the intestine. These pressures appear to focus more on the well-being of bacteria rather than human health. Connected to this is the extent to which animals depend on metabolic activity from intestinal bacteria, which varies widely across species. While omnivores like humans exhibit lower dependency, certain herbivores rely entirely on bacterial activity and have developed specialized compartments to house these bacteria.

Laboratory domestication of Lactiplantibacillus plantarum alters some phenotypic traits but causes non-novel genomic impact

AIMS

Laboratory domestication has been negligibly examined in lactic acid bacteria (LAB). Lactiplantibacillus plantarum is a highly studied and industrially relevant LAB. Here, we passaged L. plantarum JGR2 in a complex medium to study the effects of domestication on the phenotypic properties and the acquisition of mutations.

METHODS AND RESULTS

Lactiplantibacillus plantarum JGR2 was passaged in mMRS medium (deMan Rogossa Sharpe supplemented with 0.05% w/v L-cysteine) in three parallel populations for 70 days. One pure culture from each population was studied for various phenotypic properties and genomic alterations. Auto-aggregation of the evolved strains was significantly reduced, and lactic acid production and ethanol tolerance were increased. Other probiotic properties and antibiotic sensitivity were not altered. Conserved synonymous and non-synonymous mutations were observed in mobile element proteins (transposases), β-galactosidase, and phosphoketolases in all three isolates. The evolved strains lost all the repeat regions and some of the functions associated with them. Most of the conserved mutations were found in the genomes of other wild-type strains available in a public database, indicating the non-novel genomic impact of laboratory passaging.

CONCLUSIONS

Laboratory domestication can affect the phenotypic and genotypic traits of L. plantarum and similar studies are necessary for other important species of LAB.

Salmonella grows massively and aerobically in chicken faecal matter

The use of wastewater for irrigation and animal manure as fertilizer can cause transmission of intestinal pathogens, conditions frequently observed in low- and middle-income countries (LMICs). Here, we tested the ability of Salmonella to grow in the faecal matter. We inoculated freshly isolated Salmonella strains (from chickens) in chicken faecal matter and incubated for 1 to 12 days, under aerobic and anaerobic conditions. We found that both Salmonella and Escherichia coli multiplied massively in faecal matter outside a host and significantly higher in aerobic conditions. Our results have critical implications in waste management, as we demonstrate that aerobic treatments may not be the best to reduce the number of Salmonella in the environment.

Evolutionary changes of an intestinal Lactobacillus reuteri during probiotic manufacture

Probiotic bacteria are frequently used to treat intestinal diseases or to improve health; however, little is known about the evolutionary changes of these bacteria during probiotic manufacture and the bacterial ability to colonize the intestine. It has been observed that when bacteria adapt to a new environment, they lose some traits required to thrive in the original niche. In this study, a strain of Lactobacillus reuteri was isolated from mouse duodenum and subjected to 150 serial passes in milk to simulate the industrial propagation of probiotic bacteria. The strains adapted to milk outperformed their ancestor when grown in milk; we also showed evidence of reduced intestinal colonization of milk‐adapted strains. Whole‐genome sequencing showed that bacterial adaptation to milk selects mutants with altered metabolic functions.