A partial reconstitution implicates DltD in catalyzing lipoteichoic acid d-alanylation

Lipoteichoic acid biosynthesis by Staphylococcus aureus is controlled by the MspA protein

Staphylococcus aureus produces a plethora of virulence factors critical to its ability to establish an infection and cause disease. We have previously characterized a small membrane protein, MspA, which has pleiotropic effects on virulence and contributes to S. aureus pathogenicity in vivo. Here we report that mspA inactivation triggers overaccumulation of the essential cell wall component, lipoteichoic acid (LTA), which, in turn, decreases autolytic activity and leads to increased cell size due to a delay in cell separation. We show that MspA directly interacts with the enzymes involved in LTA biosynthesis (LtaA, LtaS, UgtP, and SpsB), interfering with their normal activities. MspA, in particular, interacts with the type I signal peptidase SpsB, limiting its cleavage of LtaS into its active form. These findings suggest that MspA contributes to maintaining a physiological level of LTA in the cell wall by interacting with and inhibiting the activity of SpsB, thereby uncovering a critical role for the MspA protein in regulating cell envelope biosynthesis and pathogenicity.IMPORTANCEThe S. aureus cell envelope, comprising the cytoplasmic membrane, a thick peptidoglycan layer, and the anionic polymers lipoteichoic acid and wall teichoic acids, is fundamental for bacterial growth and division, as well as being the main interface between the pathogen and the host. It has become increasingly apparent that the synthesis and turnover of cell envelope components also affect the virulence of S. aureus. In this study, we show that MspA, an effector of S. aureus virulence, contributes to the maintenance of normal levels of lipoteichoic acid in the cell wall, with implications on cell cycle and size. These findings further our understanding of the connections between envelope synthesis and pathogenicity and suggest that MspA represents a promising target for the development of future therapeutic strategies.

Structural insights into the transporting and catalyzing mechanism of DltB in LTA D-alanylation

DltB, a model member of the Membrane-Bound O-AcylTransferase (MBOAT) superfamily, plays a crucial role in D-alanylation of the lipoteichoic acid (LTA), a significant component of the cell wall of gram-positive bacteria. This process stabilizes the cell wall structure, influences bacterial virulence, and modulates the host immune response. Despite its significance, the role of DltB is not well understood. Through biochemical analysis and cryo-EM imaging, we discover that Streptococcus thermophilus DltB forms a homo-tetramer on the cell membrane. We further visualize DltB in an apo form, in complex with DltC, and in complex with its inhibitor amsacrine (m-AMSA). Each tetramer features a central hole. The C-tunnel of each protomer faces the intratetramer interface and provides access to the periphery membrane. Each protomer binds a DltC without changing the tetrameric organization. A phosphatidylglycerol (PG) molecule in the substrate-binding site may serve as an LTA carrier. The inhibitor m-AMSA bound to the L-tunnel of each protomer blocks the active site. The tetrameric organization of DltB provides a scaffold for catalyzing D-alanyl transfer and regulating the channel opening and closing. Our findings unveil DltB's dual function in the D-alanylation pathway, and provide insight for targeting DltB as a anti-virulence antibiotic.

Bactericidal and biofilm eradication efficacy of a fluorinated benzimidazole derivative, TFBZ, against methicillin-resistant Staphylococcus aureus

Methicillin-resistant Staphylococcus aureus (MRSA) is a major inducement of nosocomial infections and its biofilm formation render the high tolerance to conventional antibiotics, which highlights the requirement to develop new antimicrobial agents urgently. In this study, we identified a fluorinated benzimidazole derivative, TFBZ, with potent antibacterial efficacy toward planktonic MRSA (MIC = 4 μg/mL, MBC = 8 μg/mL) and its persistent biofilms (≥99%, MBEC = 8 μg/mL). TFBZ manifested significant irreversible time-dependent killing against MRSA as characterized by diminished cell viability, bacterial morphological change and protein leakage. Furthermore, the results from CBD devices, crystal violet assay in conjunction with live/dead staining and scanning electron microscopy confirmed that TFBZ was capable of eradicating preformed MRSA biofilms with high efficiency. Simultaneously, TFBZ reduced the bacterial invasiveness and exerted negligible hemolysis and cytotoxicity toward mammalian cells, which ensuring the robust therapeutic effect on mouse skin abscess model. The transcriptome profiling and quantitative RT-PCR revealed that a set of encoding genes associated with cell adhesion, biofilm formation, translation process, cell wall biosynthesis was consistently downregulated in MRSA biofilms upon exposure to TFBZ. In conclusion, TFBZ holds promise as a valuable candidate for therapeutic applications against MRSA chronic infections.

Antimicrobial activities of lavandulylated flavonoids in Sophora flavences against methicillin-resistant Staphylococcus aureus via membrane disruption

INTRODUCTION

The continuous emergence and rapid spread of multidrug-resistant bacteria have accelerated the demand for the discovery of alternative antibiotics. Natural plants contain a variety of antibacterial components, which is an important source for the discovery of antimicrobial agents.

OBJECTIVE

To explore the antimicrobial activities and related mechanisms of two lavandulylated flavonoids, sophoraflavanone G and kurarinone in Sophora flavescens against methicillin-resistant Staphylococcus aureus.

METHODS

The effects of sophoraflavanone G and kurarinone on methicillin-resistant Staphylococcus aureus were comprehensively investigated by a combination of proteomics and metabolomics studies. Bacterial morphology was observed by scanning electron microscopy. Membrane fluidity, membrane potential, and membrane integrity were determined using the fluorescent probes Laurdan, DiSC3(5), and propidium iodide, respectively. Adenosine triphosphate and reactive oxygen species levels were determined using the adenosine triphosphate kit and reactive oxygen species kit, respectively. The affinity activity of sophoraflavanone G to the cell membrane was determined by isothermal titration calorimetry assays.

RESULTS

Sophoraflavanone G and kurarinone showed significant antibacterial activity and anti-multidrug resistance properties. Mechanistic studies mainly showed that they could target the bacterial membrane and cause the destruction of the membrane integrity and biosynthesis. They could inhibit cell wall synthesis, induce hydrolysis and prevent bacteria from synthesizing biofilms. In addition, they can interfere with the energy metabolism of methicillin-resistant Staphylococcus aureus and disrupt the normal physiological activities of the bacteria. In vivo studies have shown that they can significantly improve wound infection and promote wound healing.

CONCLUSION

Kurarinone and sophoraflavanone G showed promising antimicrobial properties against methicillin-resistant Staphylococcus aureus, suggesting that they may be potential candidates for the development of new antibiotic agents against multidrug-resistant bacteria.

Rapid and simultaneous multiepitope antigen-based detection of Enterococcus by microscale thermophoresis and immunomagnetic separation

Background

Generally, enterococci bacteria cause nosocomial infections and are major indicators of bacterial contamination in marine bathing beach. However, a method for the rapid and simultaneous detection of multiple pathogenic enterococci has not been developed on account of the wide variety of pathogenic enterococci and their existence in complex matrices.

Methods

Immunoinformatics tools were used to design a multi-epitope antigen for the detection of various pathogenic enterococci by using the sequence of dltD gene on enterococci lipoteichoic acid (LTA) surface, which is associated with toxicological effects. The multi-epitopes included enterococci such as Enterococcus faecalis, E. gallinarum, E. raffinosus, E. durans, E. faecium, E. hirae, E. thailandicus, E. casseliflavus, E. avium, E. mundtii, E. lactis, E. solitarius, E. pseudoavium, and E. malodoratum. Microscale thermophoresis (MST) and western blot were carried out to detect the affinity between multi-epitope antigens and antibodies and between multi-epitope antibodies and bacteria. Furthermore, the detection of pathogenic enterococci was carried out by using immunomagnetic beads (IMBs) and immune chromatographic test strip (ICTS).

Results

The multi-epitope antibody had a satisfactory affinity to the antigen and enterococci. IMBs and ICTS were detected with a minimum of 101 CFU/mL and showed incompatibility for Vibrio parahemolyticus, V. vulnifcus, V. harveyi, V. anguillarum, and Edwardsiella tarda.

Implication

The present study demonstrated that the multi-epitope antigens exhibited excellent specificity and sensitivity, making them highly suitable for efficient on-site screening of enterococci bacteria in marine bathing beaches.

Proteomic Analysis of Staphylococcus aureus Treated with ShangKeHuangShui

Staphylococcus aureus (SAU) stands as the prevailing pathogen in post-traumatic infections, with the emergence of antibiotic resistance presenting formidable treatment hurdles. The pressing need is to explore novel antibiotics to address this challenge. ShangKeHuangShui (SKHS), a patented traditional Chinese herbal formula, has gained widespread use in averting post-traumatic infections, but its biological effects remain incomplete understanding. This study's primary objective was to delve into the antibacterial properties, potential antibacterial compounds within SKHS, and their associated molecular targets. In vitro SKHS antibacterial assays demonstrated that the minimum inhibitory concentration (MIC) was 8.625 mg/mL and the minimum bactericide concentration (MBC) was 17.25 mg/mL. Proteomic analysis based on tandem mass tag (TMT) showed significant changes in the expression level of 246 proteins in SKHS treated group compared to control group, with 79 proteins upregulated and 167 proteins downregulated (>1.5-fold, p < 0.05). Subsequently, thirteen target proteins related to various biological processes and multiple metabolic pathways were selected to conduct parallel reaction monitoring (PRM) and molecular docking screen. In protein tyrosine phosphatase PtpA (ptpA) docking screening, phellodendrine and obacunone can bind to ptpA with the binding energy of - 8.4 and - 8.3 kcal/mol, respectively. This suggests their potential impact on antibacterial activity by modulating the two-component system of SAU. The discovery lays a groundwork for future research endeavors for exploring new antibacterial candidates and elucidating specific active chemical components within SKHS that match target proteins. Further investigations are imperative to unveil the biological effects of these monomers and their potential synergistic actions.

Interactions of Neutrophils with the Polymeric Molecular Components of the Biofilm Matrix in the Context of Implant-Associated Bone and Joint Infections

In the presence of orthopedic implants, opportunistic pathogens can easily colonize the biomaterial surfaces, forming protective biofilms. Life in biofilm is a central pathogenetic mechanism enabling bacteria to elude the host immune response and survive conventional medical treatments. The formation of mature biofilms is universally recognized as the main cause of septic prosthetic failures. Neutrophils are the first leukocytes to be recruited at the site of infection. They are highly efficient in detecting and killing planktonic bacteria. However, the interactions of these fundamental effector cells of the immune system with the biofilm matrix, which is the true interface of a biofilm with the host cells, have only recently started to be unveiled and are still to be fully understood. Biofilm matrix macromolecules consist of exopolysaccharides, proteins, lipids, teichoic acids, and the most recently described extracellular DNA. The latter can also be stolen from neutrophil extracellular traps (NETs) by bacteria, who use it to strengthen their biofilms. This paper aims to review the specific interactions that neutrophils develop when they physically encounter the matrix of a biofilm and come to interact with its polymeric molecular components.

Systematic analysis of drug combinations against Gram-positive bacteria

Drug combinations can expand options for antibacterial therapies but have not been systematically tested in Gram-positive species. We profiled ~8,000 combinations of 65 antibacterial drugs against the model species Bacillus subtilis and two prominent pathogens, Staphylococcus aureus and Streptococcus pneumoniae. Thereby, we recapitulated previously known drug interactions, but also identified ten times more novel interactions in the pathogen S. aureus, including 150 synergies. We showed that two synergies were equally effective against multidrug-resistant S. aureus clinical isolates in vitro and in vivo. Interactions were largely species-specific and synergies were distinct from those of Gram-negative species, owing to cell surface and drug uptake differences. We also tested 2,728 combinations of 44 commonly prescribed non-antibiotic drugs with 62 drugs with antibacterial activity against S. aureus and identified numerous antagonisms that might compromise the efficacy of antimicrobial therapies. We identified even more synergies and showed that the anti-aggregant ticagrelor synergized with cationic antibiotics by modifying the surface charge of S. aureus. All data can be browsed in an interactive interface ( https://apps.embl.de/combact/ ).

Transposon sequencing identifies genes impacting Staphylococcus aureus invasion in a human macrophage model

Staphylococcus aureus is a facultative intracellular pathogen in many host cell types, facilitating its persistence in chronic infections. The genes contributing to intracellular pathogenesis have not yet been fully enumerated. Here, we cataloged genes influencing S. aureus invasion and survival within human THP-1 derived macrophages using two laboratory strains (ATCC2913 and JE2). We developed an in vitro transposition method to produce highly saturated transposon mutant libraries in S. aureus and performed transposon insertion sequencing (Tn-Seq) to identify candidate genes with significantly altered abundance following macrophage invasion. While some significant genes were strain-specific, 108 were identified as common across both S. aureus strains, with most (n = 106) being required for optimal macrophage infection. We used CRISPR interference (CRISPRi) to functionally validate phenotypic contributions for a subset of genes. Of the 20 genes passing validation, seven had previously identified roles in S. aureus virulence, and 13 were newly implicated. Validated genes frequently evidenced strain-specific effects, yielding opposing phenotypes when knocked down in the alternative strain. Genomic analysis of de novo mutations occurring in groups (n = 237) of clonally related S. aureus isolates from the airways of chronically infected individuals with cystic fibrosis (CF) revealed significantly greater in vivo purifying selection in conditionally essential candidate genes than those not associated with macrophage invasion. This study implicates a core set of genes necessary to support macrophage invasion by S. aureus, highlights strain-specific differences in phenotypic effects of effector genes, and provides evidence for selection of candidate genes identified by Tn-Seq analyses during chronic airway infection in CF patients in vivo.

Mechanism of D-alanine transfer to teichoic acids shows how bacteria acylate cell envelope polymers

Bacterial cell envelope polymers are often modified with acyl esters that modulate physiology, enhance pathogenesis and provide antibiotic resistance. Here, using the D-alanylation of lipoteichoic acid (Dlt) pathway as a paradigm, we have identified a widespread strategy for how acylation of cell envelope polymers occurs. In this strategy, a membrane-bound O-acyltransferase (MBOAT) protein transfers an acyl group from an intracellular thioester onto the tyrosine of an extracytoplasmic C-terminal hexapeptide motif. This motif shuttles the acyl group to a serine on a separate transferase that moves the cargo to its destination. In the Dlt pathway, here studied in Staphylococcus aureus and Streptococcus thermophilus, the C-terminal 'acyl shuttle' motif that forms the crucial pathway intermediate is found on a transmembrane microprotein that holds the MBOAT protein and the other transferase together in a complex. In other systems, found in both Gram-negative and Gram-positive bacteria as well as some archaea, the motif is fused to the MBOAT protein, which interacts directly with the other transferase. The conserved chemistry uncovered here is widely used for acylation throughout the prokaryotic world.

Probiotic characteristics of Lactobacillus gasseri TF08-1: A cholesterol-lowering bacterium, isolated from human gut

Lactobacillus contribute to maintain the human healthy and use for nutritional additives as probiotics. In this study, a cholesterol-lowering bacterium, Lactobacillus gasseri TF08-1, was isolated from the feces of a healthy adolescent, and its probiotic potentials were evaluated through genomic mining and in vitro test. The assembled draft genome comprised of 1,974,590 bp and was predicted total of 1,940 CDSs. The annotation of the genome revealed that L. gasseri TF08-1 harbored abundant categories of functional genes in metabolic and information processing. Moreover, strain TF08-1 has capacity to utilize D-Glucose, Sucrose, D-Maltose, Salicin, D-Xylose, D-Cellobiose, D-Mannose, and D-Trehalose, as the carbon source. The safety assessment showed strain TF08-1 contained few antibiotic resistance genes and virulence factors and was only resistant to 2 antibiotics detected by antimicrobial susceptibility test. A high bile salt hydrolase activity was found and a cholesterol-reducing effect was determined in vitro, which the result showed a remarkable cholesterol removal capability of L. gasseri TF08-1 with an efficiency of 84.40 %. This study demonstrated that the strain showed great capability of exopolysaccharide production, and tolerance to acid and bile salt. Therefore, these results indicate that L. gasseri TF08-1 can be considered as a safe candidate for probiotic, especially its potential in biotherapeutic for metabolic diseases.

Functional and structural characterization of Streptococcus pneumoniae pyruvate kinase involved in fosfomycin resistance

Glycolysis is the primary metabolic pathway in the strictly fermentative Streptococcus pneumoniae, which is a major human pathogen associated with antibiotic resistance. Pyruvate kinase (PYK) is the last enzyme in this pathway that catalyzes the production of pyruvate from phosphoenolpyruvate (PEP) and plays a crucial role in controlling carbon flux; however, while S. pneumoniae PYK (SpPYK) is indispensable for growth, surprisingly little is known about its functional properties. Here, we report that compromising mutations in SpPYK confer resistance to the antibiotic fosfomycin, which inhibits the peptidoglycan synthesis enzyme MurA, implying a direct link between PYK and cell wall biogenesis. The crystal structures of SpPYK in the apo and ligand-bound states reveal key interactions that contribute to its conformational change as well as residues responsible for the recognition of PEP and the allosteric activator fructose 1,6-bisphosphate (FBP). Strikingly, FBP binding was observed at a location distinct from previously reported PYK effector binding sites. Furthermore, we show that SpPYK could be engineered to become more responsive to glucose 6-phosphate instead of FBP by sequence and structure-guided mutagenesis of the effector binding site. Together, our work sheds light on the regulatory mechanism of SpPYK and lays the groundwork for antibiotic development that targets this essential enzyme.

The cell wall lipoprotein CD1687 acts as a DNA binding protein during deoxycholate-induced biofilm formation in Clostridioides difficile

The ability of bacterial pathogens to establish recurrent and persistent infections is frequently associated with their ability to form biofilms. Clostridioides difficile infections have a high rate of recurrence and relapses and it is hypothesized that biofilms are involved in its pathogenicity and persistence. Biofilm formation by C. difficile is still poorly understood. It has been shown that specific molecules such as deoxycholate (DCA) or metronidazole induce biofilm formation, but the mechanisms involved remain elusive. In this study, we describe the role of the C. difficile lipoprotein CD1687 during DCA-induced biofilm formation. We showed that the expression of CD1687, which is part of an operon within the CD1685-CD1689 gene cluster, is controlled by multiple transcription starting sites and some are induced in response to DCA. Only CD1687 is required for biofilm formation and the overexpression of CD1687 is sufficient to induce biofilm formation. Using RNAseq analysis, we showed that CD1687 affects the expression of transporters and metabolic pathways and we identified several potential binding partners by pull-down assay, including transport-associated extracellular proteins. We then demonstrated that CD1687 is surface exposed in C. difficile, and that this localization is required for DCA-induced biofilm formation. Given this localization and the fact that C. difficile forms eDNA-rich biofilms, we confirmed that CD1687 binds DNA in a non-specific manner. We thus hypothesize that CD1687 is a component of the downstream response to DCA leading to biofilm formation by promoting interaction between the cells and the biofilm matrix by binding eDNA.

Insights into the Roles of Lipoteichoic Acids and MprF in Bacillus subtilis

Gram-positive bacterial cells are protected from the environment by a cell envelope that is comprised of a thick layer of peptidoglycan that maintains cell shape and teichoic acid polymers whose biological function remains unclear. In Bacillus subtilis, the loss of all class A penicillin-binding proteins (aPBPs), which function in peptidoglycan synthesis, is conditionally lethal. Here, we show that this lethality is associated with an alteration of lipoteichoic acids (LTAs) and the accumulation of the major autolysin LytE in the cell wall. Our analysis provides further evidence that the length and abundance of LTAs act to regulate the cellular level and activity of autolytic enzymes, specifically LytE. Importantly, we identify a novel function for the aminoacyl-phosphatidylglycerol synthase MprF in the modulation of LTA biosynthesis in both B. subtilis and Staphylococcus aureus. This finding has implications for our understanding of antimicrobial resistance (particularly to daptomycin) in clinically relevant bacteria and the involvement of MprF in the virulence of pathogens such as methicillin-resistant S. aureus (MRSA). IMPORTANCE In Gram-positive bacteria such as Bacillus subtilis and Staphylococcus aureus, the cell envelope is a structure that protects the cells from the environment but is also dynamic in that it must be modified in a controlled way to allow cell growth. In this study, we show that lipoteichoic acids (LTAs), which are anionic polymers attached to the membrane, have a direct role in modulating the cellular abundance of cell wall-degrading enzymes. We also find that the apparent length of the LTA is modulated by the activity of the enzyme MprF, previously implicated in modifications of the cell membrane leading to resistance to antimicrobial peptides. These findings are important contributions to our understanding of how bacteria balance cell wall synthesis and degradation to permit controlled growth and division. These results also have implications for the interpretation of antibiotic resistance, particularly for the clinical treatment of MRSA infections.

DltC acts as an interaction hub for AcpS, DltA and DltB in the teichoic acid D-alanylation pathway of Lactiplantibacillus plantarum

Teichoic acids (TA) are crucial for the homeostasis of the bacterial cell wall as well as their developmental behavior and interplay with the environment. TA can be decorated by different modifications, modulating thus their biochemical properties. One major modification consists in the esterification of TA by d-alanine, a process known as d-alanylation. TA d-alanylation is performed by the Dlt pathway, which starts in the cytoplasm and continues extracellularly after d-Ala transportation through the membrane. In this study, we combined structural biology and in vivo approaches to dissect the cytoplasmic steps of this pathway in Lactiplantibacillus plantarum, a bacterial species conferring health benefits to its animal host. After establishing that AcpS, DltB, DltC1 and DltA are required for the promotion of Drosophila juvenile growth under chronic undernutrition, we solved their crystal structure and/or used NMR and molecular modeling to study their interactions. Our work demonstrates that the suite of interactions between these proteins is ordered with a conserved surface of DltC1 docking sequentially AcpS, DltA and eventually DltB. Altogether, we conclude that DltC1 acts as an interaction hub for all the successive cytoplasmic steps of the TA d-alanylation pathway.

Identification of the antibacterial action mechanism of diterpenoids through transcriptome profiling

Effective antibacterial substances of Aralia continentalis have anti-biofilm and bactericidal activity to the oral pathogen Streptococcus mutans. In this study, three compounds extracted from A. continentalis were identified as acanthoic acid, continentalic acid, and kaurenoic acid by NMR and were further investigated how these diterpenoids affect the physiology of the S. mutans. When S. mutans was exposed to individual or mixed fraction of diterpenoids, severe growth defects and unique morphology were observed. The proportion of unsaturated fatty acids in the cell membrane was increased compared to that of saturated fatty acids in the presence of diterpenoids. Genome-wide gene expression profiles with RNA-seq were compared to reveal the mode of action of diterpenoids. Streptococcus mutans commonly enhanced the expression of 176 genes in the presence of the individual diterpenoids, whereas the expression of 232 genes was considerably reduced. The diterpenoid treatment modulated the expression of genes or operon(s) involved in cell membrane synthesis, cell division, and carbohydrate metabolism of S. mutans. Collectively, these findings provide novel insights into the antibacterial effect of diterpenoids to control S. mutans infection, which causes human dental caries.

D-alanylation of lipoteichoic acid contributes to biofilm formation and acidogenesis capacity of Streptococcusmutans

BACKGROUND

D-alanylation of Lipoteichoic acid (LTA) is considered to be essential for virulence factors expression in Gram-positive microorganism. The effects of the D-alanylation of LTA on biofilm formation and acidogenesis of Streptococcus mutans (S. mutans) are still not clearly understood.

AIM

This study was designed to investigate the impact of D-alanylation of LTA on biofilm formation and acidogenesis of S. mutans and explore the related mechanisms.

METHODS AND MATERIAL

We compared the biofilm formation process by fluorescence microscope observation of LTA D-alanylation blocking strain with that of the wildtype strain. Auto-aggregation, cell surface charge, and polysaccharide production assays were performed to investigate the related mechanisms. pH drop assay and glycolysis pH drop-down analysis were carried out to evaluate the acidogenesis capacity of S. mutans after LTA D-alanylation blocking. To identify the biofilm formation and adhesive-related genes expressions of S. mutans mutant, qRT-PCR was performed.

RESULTS

After blocking off the D-alanylation of LTA, S. mutans could not form the three-dimensional structural biofilm, in which cells were scattered on the substratum as small clusters. The auto-aggregation was prompted due to the mutant strain cell morphology change (*p < 0.05). Furthermore, more negative charges were found on the mutant strain cells surfaces and fewer water-insoluble glucans were produced in mutant biofilm (*p < 0.05). The adhesion capacity of the S. mutans biofilm was impaired after LTA D-alanylation blocking (*p < 0.05). Biofilm formation and adhesive-related genes expressions decreased (*p < 0.05), especially at the early stages of biofilm formation. S. mutans mutant strains exhibited suppressed acidogenesis because its glycolytic activity was impaired.

CONCLUSION

The results of this study suggest that blocking of LTA D-alanylation disrupts normal biofilm formation in S. mutans predominantly if not entirely by altering intercellular auto-aggregation, cell adhesion, and extracellular matrix formation. Moreover, our study results suggest that the LTA D-alanylation plays an important role in S. mutans acidogenesis by altering glycolytic activity. These findings add to the knowledge about mechanisms underlying biofilm formation and acid tolerance in S. mutans.

Antibacterial activities and action mode of anti-hyperlipidemic lomitapide against Staphylococcus aureus

Background

The increasing emergence of multidrug-resistant Gram-positive bacterial infections necessitates new antibacterial agents with novel mechanisms of action that can be used to treat these infections. Lomitapide has been approved by FDA for years in reducing levels of low-density lipoprotein (LDL) in cases of familial hypercholesterolemia, whereas the antibacterial effect of lomitapide remains elusive. In this study, the inhibitory activities of lomitapide against Gram-positive bacteria were the first time explored. Quantitative proteomics analysis was then applied to investigate the mechanisms of action of lomitapide.

Results

The minimum inhibitory concentration (MIC) values of lomitapide against Gram-positive bacteria including both methicillin sensitive and resistant Staphylococcus aureus, Staphylococcus epidermidis, Enterococcus faecalis, Enterococcus faecium, and Streptococcus agalactiae were range 12.5–50 μM. Moreover, lomitapide also inhibited anti-biofilm activity against clinical S. aureus isolates. A total of 106 proteins with > 1.5-fold changes in expression were identified upon 1/2 × MIC lomitapide exposure, including 83 up-regulated proteins and 23 down-regulated proteins. Based on bioinformatics analysis, the expression of cell wall damage response proteins including two-component system VraS/VraR, lipoteichoic acid (LPA) D-alanylnation related proteins D-alanyl carrier protein (dltC) and carrier protein ligase (dltA), methionine sulfoxide reductases (mrsA1 and mrsB) were up-regulated. Moreover, the expression of SaeS and multiple fibrinogen-binding proteins (SAOUHSC_01110, FnBPB, SAOUHSC_02802, SdrC, SdrD) which were involved in the bacterial adhesion and biofilm formation, was inhibited by lomitapide. Furthermore, VraS/VraR deletion mutant (ΔvraSR) showed an enhanced lomitapide sensitivity phenotype.

Conclusion

Lomitapide displayed broad antimicrobial activities against Gram-positive bacteria. The antibacterial effect of lomitapide may be caused by cell wall destruction, while the anti-biofilm activity may be related to the inhibition of surface proteins.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12866-022-02535-9.

d-Alanylation of Lipoteichoic Acids in Streptococcus suis Reduces Association With Leukocytes in Porcine Blood

Streptococcus suis (S. suis) is a common swine pathogen but also poses a threat to human health in causing meningitis and severe cases of streptococcal toxic shock-like syndrome (STSLS). Therefore, it is crucial to understand how S. suis interacts with the host immune system during bacteremia. As S. suis has the ability to introduce d-alanine into its lipoteichoic acids (LTAs), we investigated the working hypothesis that cell wall modification by LTA d-alanylation influences the interaction of S. suis with porcine blood immune cells. We created an isogenic mutant of S. suis strain 10 by in-frame deletion of the d-alanine d-alanyl carrier ligase (DltA). d-alanylation of LTAs was associated with reduced phagocytosis of S. suis by porcine granulocytes, reduced deposition of complement factor C3 on the bacterial surface, increased hydrophobicity of streptococci, and increased resistance to cationic antimicrobial peptides (CAMPs). At the same time, survival of S. suis was not significantly increased by LTA d-alanylation in whole blood of conventional piglets with specific IgG. However, we found a distinct cytokine pattern as IL-1β but not tumor necrosis factor (TNF)-α levels were significantly reduced in blood infected with the ΔdltA mutant. In contrast to TNF-α, activation and secretion of IL-1β are inflammasome-dependent, suggesting a possible influence of LTA d-alanylation on inflammasome regulation. Especially in the absence of specific antibodies, the association of S. suis with porcine monocytes was reduced by d-alanylation of its LTAs. This dltA-dependent phenotype was also observed with a non-encapsulated dltA double mutant indicating that it is independent of capsular polysaccharides. High antibody levels caused high levels of S. suis—monocyte—association followed by inflammatory cell death and strong production of both IL-1β and TNF-α, while the influence of LTA d-alanylation of the streptococci became less visible. In summary, the results of this study expand previous findings on d-alanylation of LTAs in S. suis and suggest that this pathogen specifically modulates association with blood leukocytes through this modification of its surface.

Structural and functional analysis of the D-alanyl carrier protein ligase DltA from Staphylococcus aureus Mu50

D-Alanylation of the teichoic acids of the Gram-positive bacterial cell wall plays crucial roles in bacterial physiology and virulence. Deprivation of D-alanine from the teichoic acids of Staphylococcus aureus impairs biofilm and colony formation, induces autolysis and ultimately renders methicillin-resistant S. aureus highly susceptible to antimicrobial agents and host defense peptides. Hence, the D-alanylation pathway has emerged as a promising antibacterial target against drug-resistant S. aureus. D-Alanylation of teichoic acids is mediated via the action of four proteins encoded by the dlt operon, DltABCD, all four of which are essential for the process. In order to develop novel antimicrobial agents against S. aureus, the D-alanyl carrier protein ligase DltA, which is the first protein in the D-alanylation pathway, was focused on. Here, the crystal structure of DltA from the methicillin-resistant S. aureus strain Mu50 is presented, which reveals the unique molecular details of the catalytic center and the role of the P-loop. Kinetic analysis shows that the enantioselectivity of S. aureus DltA is much higher than that of DltA from other species. In the presence of DltC, the enzymatic activity of DltA is increased by an order of magnitude, suggesting a new exploitable binding pocket. This discovery may pave the way for a new generation of treatments for drug-resistant S. aureus.

[Effect of Lipoteichoic Acid Synthesis-Related Gene dltD on Acid Tolerance of Highly Cariogenic Strains of Streptococcus mutans]

Objective

To study the role and possible mechanism of dltD in the acid tolerance of Streptococcus mutans 593 (SM593), and to provide a theoretical basis for the ecological prevention and control of dental caries by constructing the dltD gene deletion strain of SM593 (SM593-ΔdltD).

Methods

1) SM593-Δ dltD was constructed by homologous recombination. 2) The growth curve of SM593 dltD and SM593-Δ dltD under different pH culture conditions was drawn by the automatic growth curve analyzer to compare their acid tolerance. Colony forming unit (CFU) at different time points was used to calculate the survival rate and to compare the acid tolerance response (ATR) of SM593 and SM593-Δ dltD. 3) Under different pH conditions, glycolysis experiments, proton permeability test and H +-ATPase activity test were conducted to make preliminary exploration into the mechanisms of how dltD gene deletion may affect acid tolerance.

Results

1) PCR and sequencing results showed that the SM593-Δ dltD was constructed successfully. 2) With decreasing pH value of the culture medium, the growth of SM593-Δ dltD slowed down. When the pH value of the culture medium was 5.0, SM593-Δ dltD was not allowed to grow, and its acid tolerance was lower than that of SM593. Compared with SM593, the ATR capability of SM593-Δ dltD was decreased. 3) SM593 dltD and SM593-Δ dltD did not show obvious difference in their glycolysis ability under different pH conditions. Compared with SM593 dltD, the proton permeability of SM593-Δ dltD under different pH conditions was increased significantly (P<0.05), and H +-ATPase activity decreased significantly (P<0.05).

Conclusion

Compared with SM593 dltD, SM593-Δ dltD showed obvious decrease in acid tolerance, which may be caused by the significant increase in proton permeability and significant decrease in the H +-ATPase activity induced by the deletion of the dltD gene, hence reducing its ability to maintain intracellular pH homeostasis.

Systematic Assessment of Accessibility to the Surface of Staphylococcus aureus

Proteins from bacterial foes, antimicrobial peptides, and host immune proteins must navigate past a dense layer of bacterial surface biomacromolecules to reach the peptidoglycan (PG) layer of Gram-positive bacteria. A subclass of molecules (e.g., antibiotics with intracellular targets) also must permeate through the PG (in a molecular sieving manner) to reach the cytoplasmic membrane. Despite the biological and therapeutic importance of surface accessibility, systematic analyses in live bacterial cells have been lacking. We describe a live cell fluorescence assay that is robust, shows a high level of reproducibility, and reports on the permeability of molecules to and within the PG scaffold. Moreover, our study shows that teichoic acids impede the permeability of molecules of a wide range of sizes and chemical composition.

Inhibition of d-alanylation of teichoic acids overcomes resistance of methicillin-resistant Staphylococcus aureus

Background

MRSA are high-priority multidrug-resistant pathogens. Although there are still some antibiotics active against MRSA, continuous efforts to discover new antibiotics and treatment strategies are needed because resistance to these new drugs has already been reported.

Objectives

Here we explore if d-alanylation of teichoic acids (TAs) mediated by the dlt operon gene products might be a druggable target to overcome β-lactam-resistance of MRSA.

Methods

MICs and bactericidal effects of several β-lactam antibiotics were monitored in a panel of clinical MRSA strains with genetic or chemically induced deficiency in d-alanylation of TAs. Efficiency of the chemical inhibitor to rescue MRSA-infected larvae of Galleria mellonella as well as its ability to prevent or eradicate biofilms of S. aureus were analysed.

Results

Genetic inactivation of the Dlt system or its chemical inhibition re-sensitizes MRSA to β-lactams. Among the 13 strains, the most pronounced effect was obtained using the inhibitor with imipenem, reducing the median MIC from 16 to 0.25 mg/L. This combination was also bactericidal in some strains and significantly protected G. mellonella larvae from lethal MRSA infections. Finally, inactivation of d-alanylation potentiated the effect of imipenem on inhibition and/or eradication of biofilm.

Conclusions

Our combined results show that highly efficient inhibitors of d-alanylation of TAs targeting enzymes of the Dlt system should be promising therapeutic adjuvants, especially in combination with carbapenems, for restoring the therapeutic efficacy of this class of antibiotics against MRSA.

Genomic and molecular evidence reveals novel pathways associated with cell surface polysaccharides in bacteria

Amino acid (acyl carrier protein) ligases (AALs) are a relatively new family of bacterial amino acid adenylating enzymes with unknown function(s). Here, genomic enzymology tools that employ sequence similarity networks and genome context analyses were used to hypothesize the metabolic function(s) of AALs. In over 50% of species, aal and its cognate acyl carrier protein (acp) genes, along with three more genes, formed a highly conserved AAL cassette. AAL cassettes were strongly associated with surface polysaccharide gene clusters in Proteobacteria and Actinobacteria, yet were prevalent among soil and rhizosphere-associated α- and β-Proteobacteria, including symbiotic α- and β-rhizobia and some Mycolata. Based on these associations, AAL cassettes were proposed to encode a noncanonical Acp-dependent polysaccharide modification route. Genomic-inferred predictions were substantiated by published experimental evidence, revealing a role for AAL cassettes in biosynthesis of biofilm-forming exopolysaccharide in pathogenic Burkholderia and expression of aal and acp genes in nitrogen-fixing Rhizobium bacteroids. Aal and acp genes were associated with dltBD-like homologs that modify cell wall teichoic acids with d-alanine, including in Paenibacillus and certain other bacteria. Characterization of pathways that involve AAL and Acp may lead to developing new plant and human disease-controlling agents as well as strains with improved nitrogen fixation capacity.

Biological Effect of Streptococcus pyogenes-Released Extracellular Vesicles on Human Monocytic Cells, Induction of Cytotoxicity, and Inflammatory Response

Most bacteria naturally release spherical lipid-bilayered extracellular vesicles (EVs) containing proteins, nucleic acids, and virulence-related molecules, thus contributing to diverse biological functions including transport of virulence factors. The group A streptococcus, Streptococcus pyogenes (GAS), a major human pathogen, also releases EVs; however, it remains unclear how GAS EVs interact physiologically and pathologically with host cells, and what the differences are between invasive and non-invasive strains. The proteome profile in this study revealed that GAS EVs enclosed many virulence-related proteins such as streptolysin O and NAD-glycohydrolase, facilitating their pathogenicity, and invasive GAS EVs were more abundant than non-invasive counterparts. In terms of biological effects, invasive GAS EVs showed slo-dependent cytotoxic activity and the induction of cytokine expression, contributing to GAS pathogenicity directly. Although non-invasive GAS EVs did not show cytotoxic activity, they may be utilized as a means to prevent antibacterial mechanisms such as autophagy, leading to enhancement of their own survival in the intracellular environment after the infection. These results suggest that invasive and non-invasive GAS EVs play different roles in GAS infection strategy and pathogenicity. Our findings also indicate that EVs could be a key factor for GAS pathogenicity in GAS-host interactions.

β-Lactams against the Fortress of the Gram-Positive Staphylococcus aureus Bacterium

The biological diversity of the unicellular bacteria-whether assessed by shape, food, metabolism, or ecological niche-surely rivals (if not exceeds) that of the multicellular eukaryotes. The relationship between bacteria whose ecological niche is the eukaryote, and the eukaryote, is often symbiosis or stasis. Some bacteria, however, seek advantage in this relationship. One of the most successful-to the disadvantage of the eukaryote-is the small (less than 1 μm diameter) and nearly spherical Staphylococcus aureus bacterium. For decades, successful clinical control of its infection has been accomplished using β-lactam antibiotics such as the penicillins and the cephalosporins. Over these same decades S. aureus has perfected resistance mechanisms against these antibiotics, which are then countered by new generations of β-lactam structure. This review addresses the current breadth of biochemical and microbiological efforts to preserve the future of the β-lactam antibiotics through a better understanding of how S. aureus protects the enzyme targets of the β-lactams, the penicillin-binding proteins. The penicillin-binding proteins are essential enzyme catalysts for the biosynthesis of the cell wall, and understanding how this cell wall is integrated into the protective cell envelope of the bacterium may identify new antibacterials and new adjuvants that preserve the efficacy of the β-lactams.

Modifications of cell wall polymers in Gram-positive bacteria by multi-component transmembrane glycosylation systems

Secondary cell wall polymers fulfil diverse and important functions within the cell wall of Gram-positive bacteria. Here, we will provide a brief overview of the principles of teichoic acid and complex secondary cell wall polysaccharide biosynthesis pathways in Firmicutes and summarize the recently revised mechanism for the decoration of teichoic acids with d-alanines. Many cell wall polymers are decorated with glycosyl groups, either intracellularly or extracellularly. The main focus of this review will be on the extracellular glycosylation mechanism and recent advances that have been made in the identification of enzymes involved in this process. Based on the proteins involved, we propose to rename the system to multi-component transmembrane glycosylation system in place of three-component glycosylation system.

Wall teichoic acids: physiology and applications

Wall teichoic acids (WTAs) are charged glycopolymers containing phosphodiester-linked polyol units and represent one of the major components of Gram-positive cell envelope. WTAs have important physiological functions in cell division, gene transfer, surface adhesion, drug resistance and biofilm formation, and are critical virulence factors and vital determinants in mediating cell interaction with and tolerance to environmental factors. Here, we first briefly introduce WTA structure, biosynthesis and its regulation, and then summarize in detail four major physiological roles played by WTAs, i.e. WTA-mediated resistance to antimicrobials, virulence to mammalian cells, interaction with bacteriolytic enzymes and regulation of cell metabolism. We also review the applications of WTAs in these fields that are closely related to the human society, including antibacterial drug discovery targeting WTA biosynthesis, development of vaccines and antibodies regarding WTA-mediated pathogenicity, specific and sensitive detection of pathogens in food using WTAs as a surface epitope and regulation of WTA-related pathways for efficient microbial production of useful compounds. We also point out major problems remaining in these fields, and discuss some possible directions in the future exploration of WTA physiology and applications.

Experimental Evolution In Vivo To Identify Selective Pressures during Pneumococcal Colonization

Experimental evolution is a powerful technique to understand how populations evolve from selective pressures imparted by the surrounding environment. With the advancement of whole-population genomic sequencing, it is possible to identify and track multiple contending genotypes associated with adaptations to specific selective pressures. This approach has been used repeatedly with model species in vitro, but only rarely in vivo Herein we report results of replicate experimentally evolved populations of Streptococcus pneumoniae propagated by repeated murine nasal colonization with the aim of identifying gene products under strong selection as well as the population genetic dynamics of infection cycles. Frameshift mutations in one gene, dltB, responsible for incorporation of d-alanine into teichoic acids on the bacterial surface, evolved repeatedly and swept to high frequency. Targeted deletions of dltB produced a fitness advantage during initial nasal colonization coupled with a corresponding fitness disadvantage in the lungs during pulmonary infection. The underlying mechanism behind the fitness trade-off between these two niches was found to be enhanced adherence to respiratory cells balanced by increased sensitivity to host-derived antimicrobial peptides, a finding recapitulated in the murine model. Additional mutations that are predicted to affect trace metal transport, central metabolism, and regulation of biofilm production and competence were also selected. These data indicate that experimental evolution can be applied to murine models of pathogenesis to gain insight into organism-specific tissue tropisms.IMPORTANCE Evolution is a powerful force that can be experimentally harnessed to gain insight into how populations evolve in response to selective pressures. Herein we tested the applicability of experimental evolutionary approaches to gain insight into how the major human pathogen Streptococcus pneumoniae responds to repeated colonization events using a murine model. These studies revealed the population dynamics of repeated colonization events and demonstrated that in vivo experimental evolution resulted in highly reproducible trajectories that reflect the environmental niche encountered during nasal colonization. Mutations impacting the surface charge of the bacteria were repeatedly selected during colonization and provided a fitness benefit in this niche that was counterbalanced by a corresponding fitness defect during lung infection. These data indicate that experimental evolution can be applied to models of pathogenesis to gain insight into organism-specific tissue tropisms.

iTRAQ-based quantitative analysis reveals the mechanism underlying the changes in physiological activity in a glutamate racemase mutant strain of Streptococcus mutans UA159

Streptococcus mutans UA159 is responsible for human dental caries with robust cariogenic potential. Our previous study noted that a glutamate racemase (MurI) mutant strain (designated S. mutans FW1718), with the hereditary background of UA159, displayed alterations of morphogenesis, attenuated stress tolerance, and weakened biofilm-forming capabilities, accompanying with unclear mechanisms. In this study, we applied isobaric tags for relative and absolute quantitation (iTRAQ)-based proteomics to characterize the proteome profiles of the murI mutant strain vs. the wild-type strain in chemically defined media to elucidate the mechanisms by which S. mutans copes with MurI deficiency. Whole-cell proteins of S. mutans FW1718 and UA159 were assessed by iTRAQ-coupled LC-ESI-MS/MS. Furthermore, differentially expressed proteins (DEPs) were identified by Mascot, Gene Ontology (GO) annotation, Cluster of Orthologous Groups of proteins (COG), and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analyses. Finally, a protein-protein interaction (PPI) network was established using the Search Tool for the Retrieval of Interacting Genes/Proteins (STRING). Among 1173 total bacterial proteins identified, 112 DEPs exhibited altered expression patterns in S. mutans UA159 with or without the murI mutation. The ΔmurI cells displayed an increase in the relative expression of 93 proteins (fold change ≥ 1.2, p < 0.05) and a decrease in 29 proteins (fold change ≤ 0.833, p < 0.05) compared with the wild-type cells. PPI analysis revealed a complex network of DEPs containing 191 edges and 122 nodes. The DEPs significantly upregulated after murI knockout had roles in diverse functional processes spanning cell-wall biosynthesis, energy production, and DNA replication and repair. We identified distinct variations and diverse modulators caused by murI mutation in the proteome of S. mutans, indicating that the modification of cell membrane structure, redistribution of energy metabolism and enhanced nucleic acid machinery contributed to the S. mutans response to specific environmental contexts.

Phosphoglycerol-type wall and lipoteichoic acids are enantiomeric polymers differentiated by the stereospecific glycerophosphodiesterase GlpQ

The cell envelope of Gram-positive bacteria generally comprises two types of polyanionic polymers linked to either peptidoglycan (wall teichoic acids; WTA) or to membrane glycolipids (lipoteichoic acids; LTA). In some bacteria, including Bacillus subtilis strain 168, both WTA and LTA are glycerolphosphate polymers yet are synthesized through different pathways and have distinct but incompletely understood morphogenetic functions during cell elongation and division. We show here that the exolytic sn-glycerol-3-phosphodiesterase GlpQ can discriminate between B. subtilis WTA and LTA. GlpQ completely degraded unsubstituted WTA, which lacks substituents at the glycerol residues, by sequentially removing glycerolphosphates from the free end of the polymer up to the peptidoglycan linker. In contrast, GlpQ could not degrade unsubstituted LTA unless it was partially precleaved, allowing access of GlpQ to the other end of the polymer, which, in the intact molecule, is protected by a connection to the lipid anchor. Differences in stereochemistry between WTA and LTA have been suggested previously on the basis of differences in their biosynthetic precursors and chemical degradation products. The differential cleavage of WTA and LTA by GlpQ reported here represents the first direct evidence that they are enantiomeric polymers: WTA is made of sn-glycerol-3-phosphate, and LTA is made of sn-glycerol-1-phosphate. Their distinct stereochemistries reflect the dissimilar physiological and immunogenic properties of WTA and LTA. It also enables differential degradation of the two polymers within the same envelope compartment in vivo, particularly under phosphate-limiting conditions, when B. subtilis specifically degrades WTA and replaces it with phosphate-free teichuronic acids.

Mechanisms of bactericidal action and resistance of polymyxins for Gram-positive bacteria

Polymyxins are cationic antimicrobial peptides used as the last-line therapy to treat multidrug-resistant Gram-negative bacterial infections. The bactericidal activity of polymyxins against Gram-negative bacteria relies on the electrostatic interaction between the positively charged polymyxins and the negatively charged lipid A of lipopolysaccharide (LPS). Given that Gram-positive bacteria lack an LPS-containing outer membrane, it is generally acknowledged that polymyxins are less active against Gram-positive bacteria. However, Gram-positive bacteria produce negatively charged teichoic acids, which may act as the target of polymyxins. More and more studies suggest that polymyxins have potential as a treatment for Gram-positive bacterial infection. This mini-review discusses recent advances in the mechanism of the antibacterial activity and resistance of polymyxins in Gram-positive bacteria.Key Points• Teichoic acids play a key role in the action of polymyxins on Gram-positive bacteria.• Polymyxin kills Gram-positive bacteria by disrupting cell surface and oxidative damage.• Modification of teichoic acids and phospholipids contributes to polymyxin resistance in Gram-positive bacteria.• Polymyxins have potential as a treatment for Gram-positive bacterial infection.

Genetic and pharmacological inactivation of d-alanylation of teichoic acids sensitizes pathogenic enterococci to β-lactams

Background

Enterococci intrinsically resistant to cephalosporins represent a major cause of healthcare-associated infections, and the emergence of MDR makes therapeutic approaches particularly challenging.

Objectives

Teichoic acids are cell wall glycopolymers present in Gram-positive bacteria. Teichoic acids can be modified by d-alanylation, which requires four proteins encoded by the dltABCD operon. Our objective was to evaluate the Dlt system as a druggable target to treat enterococcal infections.

Methods

The susceptibility of a d-alanylation-deficient strain of Enterococcus faecalis to β-lactam antibiotics individually and/or in combination was analysed. Moreover, a DltA inhibitor was synthesized to test pharmacological inhibition of d-alanylation in vivo and in host using the animal model Galleria mellonella with different clinical isolates of E. faecalis and Enterococcus faecium.

Results

Most cephalosporins used as mono treatment had no impact on survival of the parental strain, but were slightly lethal for the dltA mutant of E. faecalis. Addition of a very low concentration of amoxicillin significantly increased killing of the dltA mutant under these conditions. The most spectacular effect was obtained with a combination of cefotaxime (1 mg/L) and amoxicillin (0.03 mg/L). In the presence of the inhibitor, the WT strain was as susceptible to this combination treatment as the dltA mutant. This molecule associated with the antibiotics was also effective in killing other E. faecalis clinical isolates and successfully prevented death of Galleria infected with either E. faecalis or E. faecium.

Conclusions

The combined results support the potential usefulness of the Dlt system as a target to potentiate antibiotic combination therapies for the treatment of drug-resistant enterococci.

Identification of Pneumococcal Factors Affecting Pneumococcal Shedding Shows that the dlt Locus Promotes Inflammation and Transmission

Streptococcus pneumoniae (the pneumococcus) is a common cause of respiratory tract and invasive infection. The overall effectiveness of immunization with the organism’s capsular polysaccharide depends on its ability to block colonization of the upper respiratory tract and thereby prevent host-to-host transmission. Because of the limited coverage of current pneumococcal vaccines, we carried out an unbiased in vivo transposon mutagenesis screen to identify pneumococcal factors other than its capsular polysaccharide that affect transmission. One such candidate was expressed by the dlt locus, previously shown to add d-alanine onto the pneumococcal lipoteichoic acid present on the bacterial cell surface. This modification protects against host antimicrobials and augments host inflammatory responses. The latter increases secretions and bacterial shedding from the upper respiratory tract to allow for transmission. Thus, this study provides insight into a mechanism employed by the pneumococcus to successfully transit from one host to another.

Host-to-host transmission is a necessary but poorly understood aspect of microbial pathogenesis. Herein, we screened a genomic library of mutants of the leading respiratory pathogen Streptococcus pneumoniae generated by mariner transposon mutagenesis (Tn-Seq) to identify genes contributing to its exit or shedding from the upper respiratory tract (URT), the limiting step in the organism’s transmission in an infant mouse model. Our analysis focused on genes affecting the bacterial surface that directly impact interactions with the host. Among the multiple factors identified was the dlt locus, which adds d-alanine onto lipoteichoic acids (LTA) and thereby increases Toll-like receptor 2-mediated inflammation and resistance to antimicrobial peptides. The more robust proinflammatory response in the presence of d-alanylation promotes secretions that facilitate pneumococcal shedding and allows for transmission. Expression of the dlt locus is controlled by the CiaRH system, which senses cell wall stress in response to antimicrobial activity, including in response to lysozyme, the most abundant antimicrobial along the URT mucosa. Accordingly, in a lysM^−/− host, there was no longer an effect of the dlt locus on pneumococcal shedding. Thus, our findings demonstrate how a pathogen senses the URT milieu and then modifies its surface characteristics to take advantage of the host response for transit to another host.