Targeted IS-element sequencing uncovers transposition dynamics during selective pressure in enterococci.

Insertion sequences (IS) are simple transposons implicated in the genome evolution of diverse pathogenic bacterial species. Enterococci have emerged as important human intestinal pathogens with newly adapted virulence potential and antibiotic resistance. These genetic features arose in tandem with large-scale genome evolution mediated by mobile elements. Pathoadaptation in enterococci is thought to be mediated in part by the IS element IS256 through gene inactivation and recombination events. However, the regulation of IS256 and the mechanisms controlling its activation are not well understood. Here, we adapt an IS256-specfic deep sequencing method to describe how chronic lytic phage infection drives widespread diversification of IS256 in E. faecalis and how antibiotic exposure is associated with IS256 diversification in E. faecium during a clinical human infection. We show through comparative genomics that IS256 is primarily found in hospital-adapted enterococcal isolates. Analyses of IS256 transposase gene levels reveal that IS256 mobility is regulated at the transcriptional level by multiple mechanisms in E. faecalis, indicating tight control of IS256 activation in the absence of selective pressure. Our findings reveal that stressors such as phages and antibiotic exposure drives rapid genome-scale transposition in the enterococci. IS256 diversification can therefore explain how selective pressures mediate evolution of the enterococcal genome, ultimately leading to the emergence of dominant nosocomial lineages that threaten human health.

Identification of a novel cationic glycolipid in Streptococcus agalactiae that contributes to brain entry and meningitis.

Bacterial membrane lipids are critical for membrane bilayer formation, cell division, protein localization, stress responses, and pathogenesis. Despite their critical roles, membrane lipids have not been fully elucidated for many pathogens. Here, we report the discovery of a novel cationic glycolipid, lysyl-glucosyl-diacylglycerol (Lys-Glc-DAG), which is synthesized in high abundance by the bacterium Streptococcus agalactiae (Group B Streptococcus, GBS). To our knowledge, Lys-Glc-DAG is more positively charged than any other known lipids. Lys-Glc-DAG carries 2 positive net charges per molecule, distinct from the widely described lysylated phospholipid lysyl-phosphatidylglycerol (Lys-PG) that carries one positive net charge due to the presence of a negatively charged phosphate moiety. We use normal phase liquid chromatography (NPLC) coupled with electrospray ionization (ESI) high-resolution tandem mass spectrometry (HRMS/MS) and genetic approaches to determine that Lys-Glc-DAG is synthesized by the enzyme MprF in GBS, which covalently modifies the neutral glycolipid Glc-DAG with the cationic amino acid lysine. GBS is a leading cause of neonatal meningitis, which requires traversal of the endothelial blood-brain barrier (BBB). We demonstrate that GBS strains lacking mprF exhibit a significant decrease in the ability to invade BBB endothelial cells. Further, mice challenged with a GBSΔmprF mutant developed bacteremia comparably to wild-type (WT) infected mice yet had less recovered bacteria from brain tissue and a lower incidence of meningitis. Thus, our data suggest that Lys-Glc-DAG may contribute to bacterial uptake into host cells and disease progression. Importantly, our discovery provides a platform for further study of cationic lipids at the host-pathogen interface.

Correction: CRISPR-based antimicrobials to obstruct antibiotic-resistant and pathogenic bacteria.

[This corrects the article DOI: 10.1371/journal.ppat.1009672.].

Enterococcus faecalis Strains with Compromised CRISPR-Cas Defense Emerge under Antibiotic Selection for a CRISPR-Targeted Plasmid.

Enterococcus faecalis is a Gram-positive bacterium that natively colonizes the human gastrointestinal tract and opportunistically causes life-threatening infections. Multidrug-resistant (MDR) E. faecalis strains have emerged that are replete with mobile genetic elements (MGEs). Non-MDR E. faecalis strains frequently possess CRISPR-Cas systems, which reduce the frequency of MGE acquisition. We demonstrated in previous studies that E. faecalis populations can transiently maintain both a functional CRISPR-Cas system and a CRISPR-Cas target. In this study, we used serial passage and deep sequencing to analyze these populations. In the presence of antibiotic selection for the plasmid, mutants with compromised CRISPR-Cas defense and enhanced ability to acquire a second antibiotic resistance plasmid emerged. Conversely, in the absence of selection, the plasmid was lost from wild-type E. faecalis populations but not E. faecalis populations that lacked the cas9 gene. Our results indicate that E. faecalis CRISPR-Cas can become compromised under antibiotic selection, generating populations with enhanced abilities to undergo horizontal gene transfer. IMPORTANCE Enterococcus faecalis is a leading cause of hospital-acquired infections and disseminator of antibiotic resistance plasmids among Gram-positive bacteria. We have previously shown that E. faecalis strains with an active CRISPR-Cas system can prevent plasmid acquisition and thus limit the transmission of antibiotic resistance determinants. However, CRISPR-Cas is not a perfect barrier. In this study, we observed populations of E. faecalis with transient coexistence of CRISPR-Cas and one of its plasmid targets. Our experimental data demonstrate that antibiotic selection results in compromised E. faecalis CRISPR-Cas function, thereby facilitating the acquisition of additional resistance plasmids by E. faecalis.

Streptococcus pneumoniae, S. pyogenes and S. agalactiae membrane phospholipid remodelling in response to human serum.

Streptococcus pneumoniae, S. pyogenes (Group A Streptococcus; GAS) and S. agalactiae (Group B Streptococcus; GBS) are major aetiological agents of diseases in humans. The cellular membrane, a crucial site in host-pathogen interactions, is poorly characterized in streptococci. Moreover, little is known about whether or how environmental conditions influence their lipid compositions. Using normal phase liquid chromatography coupled with electrospray ionization MS, we characterized the phospholipids and glycolipids of S. pneumoniae, GAS and GBS in routine undefined laboratory medium, streptococcal defined medium and, in order to mimic the host environment, defined medium supplemented with human serum. In human serum-supplemented medium, all three streptococcal species synthesize phosphatidylcholine (PC), a zwitterionic phospholipid commonly found in eukaryotes but relatively rare in bacteria. We previously reported that S. pneumoniae utilizes the glycerophosphocholine (GPC) biosynthetic pathway to synthesize PC. Through substrate tracing experiments, we confirm that GAS and GBS scavenge lysoPC, a major metabolite in human serum, thereby using an abbreviated GPC pathway for PC biosynthesis. Furthermore, we found that plasmanyl-PC is uniquely present in the GBS membrane during growth with human serum, suggesting GBS possesses unusual membrane biochemical or biophysical properties. In summary, we report cellular lipid remodelling by the major pathogenic streptococci in response to metabolites present in human serum.

The Integrity of Heme Is Essential for Reproducible Detection of Metronidazole-Resistant Clostridioides difficile by Agar Dilution Susceptibility Tests.

Metronidazole resistance in clinical Clostridioides difficile is often described as unstable, since resistant strains reportedly appear susceptible following freezer storage or brief passage. This has presented a conundrum for adopting susceptibility testing to accurately evaluate the connection between metronidazole resistance and decreased clinical efficacy of metronidazole in patients with C. difficile infections (CDIs). We discovered that supplementation of microbiological media with the metalloporphyrin heme is crucial for detection of metronidazole-resistant C. difficile using the agar dilution susceptibility testing method. Known metronidazole-resistant strains appeared susceptible to metronidazole in media lacking heme. Similarly, these resistant strains exhibited increased susceptibility to metronidazole when tested on heme-containing agars that were exposed to room light for more than 1 day, likely due to heme photodecomposition. In parallel experiments, resistance was reproducibly detected when heme-containing agars were either prepared and used on the same day or protected from light and then used on subsequent days. Notably, heme did not influence the susceptibilities of drug-susceptible strains that were of the same ribotype as the resistant strains. These findings firmly show that the consistent detection of metronidazole-resistant C. difficile is dependent upon heme and its protection from light. Studies are warranted to determine the extent to which this heme-associated metronidazole-resistant phenotype affects the clinical efficacy of metronidazole in CDI and the underlying genetic and biochemical mechanisms.

Mammalian cell response and bacterial adhesion on titanium healing abutments: effect of multiple implantation and sterilization cycles.

OBJECTIVE: Multiple implantations of the implant healing abutment (IHA) could adversely impact its surface properties in vivo. Furthermore, the effect of sterilization and reuse of the IHA on soft tissue viability and bacterial contamination has not been extensively studied. The goal of this study was to perform an in vitro analysis of mammalian cell viability and bacterial adhesion on the surfaces of retrieved IHA after single and multiple implantations and repetitive cycles of sterilization. MATERIALS AND METHODS: IHA surface morphology was studied using optical microscopy. Cell viability of gingival fibroblasts (HGF-1) and oral keratinocytes (HOKg) in indirect contact with IHAs was assessed for 3 and 7 days. Immersion in bacterial culture was performed with a polyculture of Streptococcus species for 3 days and Streptococcus species with Fusobacterium nucleatum for 7 days. RESULTS: IHAs exhibited signs of surface damage even after a single exposure to the oral cavity. Fibroblasts did not show a significant preference towards control IHAs over used IHAs, whereas keratinocytes exhibited a significant decrease in viability when exposed to IHAs after multiple implantation cycles as compared with controls. Adherent bacterial count increased with increasing number of IHA implantations for both polycultures. CONCLUSIONS: Reusing of IHAs in vivo promoted surface degradation in addition to adversely impacting host cell viability and oral bacterial attachment in vitro. These findings show IHA reuse might potentially affect its clinical performance. CLINICAL RELEVANCE: Careful consideration should be taken when reusing IHAs in patients because this practice can result in permanent surface changes that might affect soft tissue integration during the healing period and promote bacterial colonization.

Chromosomal Resistance to Metronidazole in Clostridioides difficile Can Be Mediated by Epistasis between Iron Homeostasis and Oxidoreductases.

Chromosomal resistance to metronidazole has emerged in clinical Clostridioides difficile isolates, but the genetic mechanisms remain unclear. This is further hindered by the inability to generate spontaneous metronidazole-resistant mutants in the lab to interpret genetic variations in clinical isolates. We therefore constructed a mismatch repair mutator in nontoxigenic ATCC 700057 to survey the mutational landscape for de novo resistance mechanisms. In separate experimental evolutions, the mutator adopted a deterministic path to resistance, with truncation of the ferrous iron transporter FeoB1 as a first-step mechanism of low-level resistance. Deletion of feoB1 in ATCC 700057 reduced the intracellular iron content, appearing to shift cells toward flavodoxin-mediated oxidoreductase reactions, which are less favorable for metronidazole's cellular action. Higher-level resistance evolved from sequential acquisition of mutations to catalytic domains of pyruvate-ferredoxin/flavodoxin oxidoreductase (PFOR; encoded by nifJ), a synonymous codon change to putative xdh (xanthine dehydrogenase; encoded by CD630_31770), likely affecting mRNA stability, and last, frameshift and point mutations that inactivated the iron-sulfur cluster regulator (IscR). Gene silencing of nifJ, xdh, or iscR with catalytically dead Cas9 revealed that resistance involving these genes occurred only when feoB1 was inactivated; i.e., resistance was seen only in the feoB1 deletion mutant and not in the isogenic wild-type (WT) parent. Interestingly, metronidazole resistance in C. difficile infection (CDI)-associated strains carrying mutations in nifJ was reduced upon gene complementation. This observation supports the idea that mutation in PFOR is one mechanism of metronidazole resistance in clinical strains. Our findings indicate that metronidazole resistance in C. difficile is complex, involving multigenetic mechanisms that could intersect with iron-dependent and oxidoreductive metabolic pathways.

Constitutive expression of the cryptic vanGCd operon promotes vancomycin resistance in Clostridioides difficile clinical isolates.

OBJECTIVES: To describe, for the first time (to the best of our knowledge), the genetic mechanisms of vancomycin resistance in clinical isolates of Clostridioides difficile ribotype 027. METHODS: Clinical isolates and laboratory mutants were analysed: genomically to identify resistance mutations; by transcriptional analysis of vanGCd, the vancomycin resistance operon encoding lipid II d-alanine-d-serine that is less bound by vancomycin than native lipid II d-alanine-d-alanine; by imaging of vancomycin binding to cell walls; and for changes in vancomycin bactericidal activity and autolysis. RESULTS: Vancomycin-resistant laboratory mutants and clinical isolates acquired mutations to the vanSR two-component system that regulates vanGCd. The substitutions impaired VanSR's function, resulting in constitutive transcription of vanGCd. Resistance was reversed by silencing vanG, encoding d-alanine-d-serine ligase in the vanGCd operon. In resistant cells, vancomycin was less bound to the cell wall septum, the site where vancomycin interacts with lipid II. Vancomycin's bactericidal activity was reduced against clinical isolates and laboratory mutants (64 and ≥1024 mg/L, respectively) compared with WT strains (4 mg/L). Truncation of the potassium transporter TrkA occurred in laboratory mutants, which were refractory to autolysis, accounting for their survival in high drug concentrations. CONCLUSIONS: Ribotype 027 evolved first-step resistance to vancomycin by constitutively expressing vanGCd, which is otherwise silent. Experimental evolutions and bactericidal assays show that ribotype 027 can acquire mutations to drastically enhance its tolerance to vancomycin. Thus, further epidemiological studies are warranted to examine the extent to which vancomycin resistance impacts clinical outcomes and the potential for these strains to evolve higher-level resistance, which would be devastating.

Phosphatidylcholine Biosynthesis in Mitis Group Streptococci via Host Metabolite Scavenging.

The mitis group streptococci include the major human pathogen Streptococcus pneumoniae and the opportunistic pathogens Streptococcus mitis and Streptococcus oralis, which are human oral cavity colonizers and agents of bacteremia and infective endocarditis in immunocompromised patients. Bacterial membrane lipids play crucial roles in microbe-host interactions; for many pathogens, however, the composition of the membrane is poorly understood. In this study, we characterized the lipidomes of selected species of mitis group streptococci and investigated the mechanistic basis for biosynthesis of the phospholipid phosphatidylcholine (PC). PC is a major lipid in eukaryotic cellular membranes, but it is considered to be comparatively rare in bacterial taxa. Using liquid chromatography-mass spectrometry in conjunction with stable isotope tracing, we determined that mitis group streptococci synthesize PC via a rare host-metabolite-scavenging pathway, the glycerophosphocholine (GPC) pathway, which is largely uncharacterized in bacteria. Our work demonstrates that mitis group streptococci, including S. pneumoniae, remodel their membranes in response to the major human metabolites GPC and lysophosphatidylcholine.IMPORTANCE We lack fundamental information about the composition of the cellular membrane even for the best-studied pathogens of critical significance for human health. The mitis group streptococci are closely linked to humans in health and disease, but their membrane biology is poorly understood. Here, we demonstrate that these streptococci scavenge major human metabolites and use them to synthesize the membrane phospholipid PC. Our work is significant because it identifies a mechanism by which the major human pathogen S. pneumoniae and the primary human oral colonizers S. mitis and S. oralis remodel their membranes in response to host metabolites.

Bacteriophage Resistance Alters Antibiotic-Mediated Intestinal Expansion of Enterococci.

Enterococcus faecalis is a human intestinal pathobiont with intrinsic and acquired resistance to many antibiotics, including vancomycin. Nature provides a diverse and virtually untapped repertoire of bacterial viruses, or bacteriophages (phages), that could be harnessed to combat multidrug-resistant enterococcal infections. Bacterial phage resistance represents a potential barrier to the implementation of phage therapy, emphasizing the importance of investigating the molecular mechanisms underlying the emergence of phage resistance. Using a cohort of 19 environmental lytic phages with tropism against E. faecalis, we found that these phages require the enterococcal polysaccharide antigen (Epa) for productive infection. Epa is a surface-exposed heteroglycan synthesized by enzymes encoded by both conserved and strain-specific genes. We discovered that exposure to phage selective pressure favors mutation in nonconserved epa genes both in culture and in a mouse model of intestinal colonization. Despite gaining phage resistance, epa mutant strains exhibited a loss of resistance to cell wall-targeting antibiotics. Finally, we show that an E. faecalisepa mutant strain is deficient in intestinal colonization, cannot expand its population upon antibiotic-driven intestinal dysbiosis, and fails to be efficiently transmitted to juvenile mice following birth. This study demonstrates that phage therapy could be used in combination with antibiotics to target enterococci within a dysbiotic microbiota. Enterococci that evade phage therapy by developing resistance may be less fit at colonizing the intestine and sensitized to vancomycin, preventing their overgrowth during antibiotic treatment.

Conjugative Delivery of CRISPR-Cas9 for the Selective Depletion of Antibiotic-Resistant Enterococci.

The innovation of new therapies to combat multidrug-resistant (MDR) bacteria is being outpaced by the continued rise of MDR bacterial infections. Of particular concern are hospital-acquired infections (HAIs) that are recalcitrant to antibiotic therapies. The Gram-positive intestinal pathobiont Enterococcus faecalis is associated with HAIs, and some strains are MDR. Therefore, novel strategies to control E. faecalis populations are needed. We previously characterized an E. faecalis type II CRISPR-Cas system and demonstrated its utility in the sequence-specific removal of antibiotic resistance determinants. Here, we present work describing the adaption of this CRISPR-Cas system into a constitutively expressed module encoded on a pheromone-responsive conjugative plasmid that efficiently transfers to E. faecalis for the selective removal of antibiotic resistance genes. Using in vitro competition assays, we show that these CRISPR-Cas-encoding delivery plasmids, or CRISPR-Cas antimicrobials, can reduce the occurrence of antibiotic resistance in enterococcal populations in a sequence-specific manner. Furthermore, we demonstrate that deployment of CRISPR-Cas antimicrobials in the murine intestine reduces the occurrence of antibiotic-resistant E. faecalis by several orders of magnitude. Finally, we show that E. faecalis donor strains harboring CRISPR-Cas antimicrobials are immune to uptake of antibiotic resistance determinants in vivo Our results demonstrate that conjugative delivery of CRISPR-Cas antimicrobials may be adaptable for future deployment from probiotic bacteria for exact targeting of defined MDR bacteria or for precision engineering of polymicrobial communities in the mammalian intestine.

A Type I Restriction-Modification System Associated with Enterococcus faecium Subspecies Separation.

The gastrointestinal colonizer Enterococcus faecium is a leading cause of hospital-acquired infections. Multidrug-resistant (MDR) E. faecium isolates are particularly concerning for infection treatment. Previous comparative genomic studies revealed that subspecies referred to as clade A and clade B exist within E. faecium MDR E. faecium isolates belong to clade A, while clade B consists of drug-susceptible fecal commensal E. faecium isolates. Isolates from clade A are further grouped into two subclades, clades A1 and A2. In general, clade A1 isolates are hospital-epidemic isolates, whereas clade A2 isolates are isolates from animals and sporadic human infections. Such phylogenetic separation indicates that reduced gene exchange occurs between the clades. We hypothesize that endogenous barriers to gene exchange exist between E. faecium clades. Restriction-modification (R-M) systems are such barriers in other microbes. We utilized a bioinformatics analysis coupled with second-generation and third-generation deep-sequencing platforms to characterize the methylomes of two representative E. faecium strains, one from clade A1 and one from clade B. We identified a type I R-M system that is clade A1 specific, is active for DNA methylation, and significantly reduces the transformability of clade A1 E. faecium Based on our results, we conclude that R-M systems act as barriers to horizontal gene exchange in E. faecium and propose that R-M systems contribute to E. faecium subspecies separation.IMPORTANCEEnterococcus faecium is a leading cause of hospital-acquired infections around the world. Rising antibiotic resistance in certain E. faecium lineages leaves fewer treatment options. The overarching aim of this work was to determine whether restriction-modification (R-M) systems contribute to the structure of the E. faecium species, wherein hospital-epidemic and non-hospital-epidemic isolates have distinct evolutionary histories and highly resolved clade structures. R-M provides bacteria with a type of innate immunity to horizontal gene transfer (HGT). We identified a type I R-M system that is enriched in the hospital-epidemic clade and determined that it is active for DNA modification activity and significantly impacts HGT. Overall, this work is important because it provides a mechanism for the observed clade structure of E. faecium as well as a mechanism for facilitated gene exchange among hospital-epidemic E. faecium isolates.

Modulators of Enterococcus faecalis Cell Envelope Integrity and Antimicrobial Resistance Influence Stable Colonization of the Mammalian Gastrointestinal Tract.

The Gram-positive bacterium Enterococcus faecalis is both a colonizer of the gastrointestinal tract (GIT) and an agent of serious nosocomial infections. Although it is typically required for pathogenesis, GIT colonization by E. faecalis is poorly understood. E. faecalis tolerates high concentrations of GIT antimicrobials, like cholate and lysozyme, leading us to hypothesize that resistance to intestinal antimicrobials is essential for long-term GIT colonization. Analyses of E. faecalis mutants exhibiting defects in antimicrobial resistance revealed that IreK, a determinant of envelope integrity and antimicrobial resistance, is required for long-term GIT colonization. IreK is a member of the PASTA kinase protein family, bacterial transmembrane signaling proteins implicated in the regulation of cell wall homeostasis. Among several determinants of cholate and lysozyme resistance in E. faecalis, IreK was the only one found to be required for intestinal colonization, emphasizing the importance of this protein to enterococcal adaptation to the GIT. By studying ΔireK suppressor mutants that recovered the ability to colonize the GIT, we identified two conserved enterococcal proteins (OG1RF_11271 and OG1RF_11272) that function antagonistically to IreK and interfere with cell envelope integrity, antimicrobial resistance, and GIT colonization. Our data suggest that IreK, through its kinase activity, inhibits the actions of these proteins. IreK, OG1RF_11271, and OG1RF_11272 are found in all enterococci, suggesting that their effect on GIT colonization is universal across enterococci. Thus, we have defined conserved genes in the enterococcal core genome that influence GIT colonization through their effect on enterococcal envelope integrity and antimicrobial resistance.

EfrEF and the Transcription Regulator ChlR Are Required for Chlorhexidine Stress Response in Enterococcus faecalis V583.

Enterococcus faecalis is an opportunistic pathogen and leading cause of health care-associated infections. Daily chlorhexidine gluconate (CHG) bathing of patients is generally regarded as an effective strategy to reduce the occurrence of health care-associated infections. It is likely that E. faecalis is frequently exposed to inhibitory and subinhibitory concentrations of CHG in clinical settings. The goal of this study was to investigate how the vancomycin-resistant strain E. faecalis V583 transcriptionally responds to and tolerates stress from CHG. We used transcriptome (microarray) analysis to identify genes upregulated by E. faecalis V583 in response to CHG. The genes efrE (EF2226) and efrF (EF2227), encoding a heterodimeric ABC transport system, were the most highly upregulated genes. efrEF expression was induced by CHG at concentrations several 2-fold dilutions below the MIC. Deletion of efrEF increased E. faecalis V583 susceptibility to CHG. We found that ChlR, a MerR-like regulator encoded by a sequence upstream of efrEF, mediated the CHG-dependent upregulation of efrEF, and deletion of chlR also increased chlorhexidine susceptibility. Overall, our study gives insight into E. faecalis stress responses to a commonly used antiseptic.

Loss-of-Function Mutations in epaR Confer Resistance to ϕNPV1 Infection in Enterococcus faecalis OG1RF.

Enterococcus faecalis is a Gram-positive opportunistic pathogen that inhabits the human gastrointestinal tract. Because of the high frequency of antibiotic resistance among Enterococcus clinical isolates, interest in using phage to treat enterococcal infections and to decolonize high-risk patients for antibiotic-resistant Enterococcus is rising. Bacteria can evolve phage resistance, but there is little published information on these mechanisms in E. faecalis In this report, we identified genetic determinants of E. faecalis resistance to phage NPV1 (ϕNPV1). We found that loss-of-function mutations in epaR confer ϕNPV1 resistance by blocking phage adsorption. We attribute the inability of the phage to adsorb to the modification or loss of an extracellular polymer in strains with inactivated epaR Phage-resistant epaR mutants exhibited increased daptomycin and osmotic stress susceptibilities. Our results demonstrate that in vitro spontaneous resistance to ϕNPV1 comes at a cost in E. faecalis OG1RF.

Reduced Chlorhexidine and Daptomycin Susceptibility in Vancomycin-Resistant Enterococcus faecium after Serial Chlorhexidine Exposure.

Vancomycin-resistant Enterococcus faecium strains (VREfm) are critical public health concerns because they are among the leading causes of hospital-acquired bloodstream infections. Chlorhexidine (CHX) is a bisbiguanide cationic antiseptic that is routinely used for patient bathing and other infection control practices. VREfm are likely frequently exposed to CHX; however, the long-term effects of CHX exposure have not been studied in enterococci. In this study, we serially exposed VREfm to increasing concentrations of CHX for a period of 21 days in two independent experimental evolution trials. Reduced CHX susceptibility emerged (4-fold shift in CHX MIC). Subpopulations with reduced daptomycin (DAP) susceptibility were detected, which were further analyzed by genome sequencing and lipidomic analysis. Across the trials, we identified adaptive changes in genes with predicted or experimentally confirmed roles in chlorhexidine susceptibility (efrE), global nutritional stress response (relA), nucleotide metabolism (cmk), phosphate acquisition (phoU), and glycolipid biosynthesis (bgsB), among others. Moreover, significant alterations in membrane phospholipids were identified for some populations with reduced DAP susceptibility. Our results are clinically significant because they identify a link between serial subinhibitory CHX exposure and reduced DAP susceptibility. In addition, the CHX-induced genetic and lipidomic changes described in this study offer new insights into the mechanisms underlying the emergence of antibiotic resistance in VREfm.

Streptococcus mitis and S. oralis Lack a Requirement for CdsA, the Enzyme Required for Synthesis of Major Membrane Phospholipids in Bacteria.

Synthesis and integrity of the cytoplasmic membrane are fundamental to cellular life. Experimental evolution studies have hinted at unique physiology in the Gram-positive bacteria Streptococcus mitis and S. oralis These organisms commonly cause bacteremia and infectious endocarditis (IE) but are rarely investigated in mechanistic studies of physiology and evolution. Unlike in other Gram-positive pathogens, high-level (MIC ≥ 256 µg/ml) daptomycin resistance rapidly emerges in S. mitis and S. oralis after a single drug exposure. In this study, we found that inactivating mutations in cdsA are associated with high-level daptomycin resistance in S. mitis and S. oralis IE isolates. This is surprising given that cdsA is an essential gene for life in commonly studied model organisms. CdsA is the enzyme responsible for the synthesis of CDP-diacylglycerol, a key intermediate for the biosynthesis of all major phospholipids in prokaryotes and most anionic phospholipids in eukaryotes. Lipidomic analysis by liquid chromatography-mass spectrometry (LC-MS) showed that daptomycin-resistant strains have an accumulation of phosphatidic acid and completely lack phosphatidylglycerol and cardiolipin, two major anionic phospholipids in wild-type strains, confirming the loss of function of CdsA in the daptomycin-resistant strains. To our knowledge, these daptomycin-resistant streptococci represent the first model organisms whose viability is CdsA independent. The distinct membrane compositions resulting from the inactivation of cdsA not only provide novel insights into the mechanisms of daptomycin resistance but also offer unique opportunities to study the physiological functions of major anionic phospholipids in bacteria.

Indole production provides limited benefit to Escherichia coli during co-culture with Enterococcus faecalis.

Escherichia coli lives in the gastrointestinal tract and elsewhere, where it coexists within a mixed population. Indole production enables E. coli to grow with other gram-negative bacteria as indole inhibits N-acyl-homoserine lactone (AHL) quorum regulation. We investigated whether E. coli indole production enhanced competition with gram-positive Enterococcus faecalis, wherein quorum signaling is mediated by small peptides. During planktonic co-culture with E. faecalis, the fitness and population density of E. coli tnaA mutants (unable to produce indole) equaled or surpassed that of E. coli wt. During biofilm growth, the fitness of both populations of E. coli stabilized around 100 %, whereas the fitness of E. faecalis declined over time to 85-90 %, suggesting that biofilm and planktonic populations have different competition strategies. Media supplementation with indole removed the competitive advantage of E. coli tnaA in planktonic populations but enhanced it in biofilm populations. E. coli wt and tnaA showed similar growth in Luria-Bertani (LB) broth. However, E. coli growth was inhibited in the presence of filter-sterilized spent LB from E. faecalis, with inhibition being enhanced by indole. Similarly, there was also an inhibition of E. faecalis growth by proteinaceous components (likely bacteriocins) from spent culture media from both E. coli strains. We conclude that E. coli indole production is not a universal competition strategy, but rather works against gram-negative, AHL-producing bacteria.