The Peptidoglycan Pattern of Staphylococcus carnosus TM300-Detailed Analysis and Variations Due to Genetic and Metabolic Influences

Combined mutations of the penA with ftsX genes contribute to ceftriaxone resistance in Neisseria gonorrhoeae and peptide nucleic acids targeting these genes reverse ceftriaxone resistance

OBJECTIVES

To investigate the gene mutations associated with ceftriaxone (CRO) resistance among gonococcal isolates, and to determine the effects of the mutated genes on CRO minimum inhibitory concentrations (MICs) with transformation assays and antisense peptide nucleic acids (asPNAs).

METHODS

Ceftriaxone-resistant (CROR) and ceftriaxone-susceptible (CROS) isolates were identified using EUCAST and paired according to similarity in their MICs to other antimicrobials. The two groups of gonococci were sequenced and analysed. Mutated genes that showed a statistical difference between the two groups were transformed into gonococcal reference strains to determine their functions. AsPNAs were designed and transformed into the former transformant to further confirm the effects of the mutated genes.

RESULTS

Twenty-two paired CROR and CROS isolates were obtained. The incidence of the penA-A501T and penA-G542S mutations individually, as well as combined mutations (penA-A501T and ftsX-R251H, penA-G542S and ftsX R251H), was statistically different between the two groups. The MIC of ATCC43069 (A43) increased 2 times following transformation with penA-A501T, and the MICs of A43 and ATCC49226 (A49) increased 32 times and 2 times following transformation with penA-A501T and ftsX-R251H, respectively. Antisense PNA-P3 reduced the MIC of the A43 transformant most significantly when transformed individually. PNA-P3 and PNA-F1 (asPNAs of the penA and ftsX) restored CRO susceptibility.

CONCLUSIONS

PenA-A501T and penA-G542S mutations are important in CRO resistance among gonococci isolates. The ftsX-R251H mutation is also related to CRO resistance, and combined mutations of ftsX-R251H and penA-A501T comediate a significant reduction in CRO susceptibility. The combined application of PNA-P3 and PNA-F1 could effectively reverse the resistance to CRO in N. gonorrhoeae.

The peptidoglycan and biofilm matrix of Staphylococcus epidermidis undergo structural changes when exposed to human platelets

Staphylococcus epidermidis is a bacterium frequently isolated from contaminated platelet concentrates (PCs), a blood product used to treat bleeding disorders in transfusion patients. PCs offer an accidental niche for colonization of S. epidermidis by forming biofilms and thus avoiding clearance by immune factors present in this milieu. Using biochemical and microscopy techniques, we investigated the structural changes of the peptidoglycan (PG) and the biofilm matrix of S. epidermidis biofilms formed in whole-blood derived PCs compared to biofilms grown in glucose-supplemented trypticase soy broth (TSBg). Both, the PG and the biofilm matrix are primary mechanisms of defense against environmental stress. Here we show that in PCs, the S. epidermidis biofilm matrix is mainly of a proteinaceous nature with extracellular DNA, in contrast to the predominant polysaccharide nature of the biofilm matrix formed in TSBg cultures. PG profile studies demonstrated that the PG of biofilm cells remodels during PC storage displaying fewer muropeptides variants than those observed in TSBg. The PG muropeptides contain two chemical modifications (amidation and O-acetylation) previously associated with resistance to antimicrobial agents by other staphylococci. Our study highlights two key structural features of S. epidermidis that are remodeled when exposed to human platelets and could be used as targets to reduce septic transfusions events.

LipL21 lipoprotein binding to peptidoglycan enables Leptospira interrogans to escape NOD1 and NOD2 recognition

Leptospirosis is a widespread zoonosis, potentially severe in humans, caused by spirochetal bacteria, Leptospira interrogans (L. interrogans). Host defense mechanisms involved in leptospirosis are poorly understood. Recognition of lipopolysaccharide (LPS) and lipoproteins by Toll-Like Receptors (TLR)4 and TLR2 is crucial for clearance of leptospires in mice, yet the role of Nucleotide Oligomerization Domain (NOD)-like receptors (NOD)1 and NOD2, recognizing peptidoglycan (PG) fragments has not previously been examined. Here, we show that pathogenic leptospires escape from NOD1 and NOD2 recognition both in vitro and in vivo, in mice. We found that leptospiral PG is resistant to digestion by certain hydrolases and that a conserved outer membrane lipoprotein of unknown function, LipL21, specific for pathogenic leptospires, is tightly bound to the PG. Leptospiral PG prepared from a mutant not expressing LipL21 (lipl21^-) was more readily digested than the parental or complemented strains. Muropeptides released from the PG of the lipl21^- mutant, or prepared using a procedure to eliminate the LipL21 protein from the PG of the parental strain, were recognized in vitro by the human NOD1 (hNOD1) and NOD2 (hNOD2) receptors, suggesting that LipL21 protects PG from degradation into muropeptides. LipL21 expressed in E. coli also resulted in impaired PG digestion and NOD signaling. We found that murine NOD1 (mNOD1) did not recognize PG of L. interrogans. This result was confirmed by mass spectrometry showing that leptospiral PG was primarily composed of MurTriDAP, the natural agonist of hNOD1, and contained only trace amounts of the tetra muropeptide, the mNOD1 agonist. Finally, in transgenic mice expressing human NOD1 and deficient for the murine NOD1, we showed enhanced clearance of a lipl21^- mutant compared to the complemented strain, or to what was observed in NOD1KO mice, suggesting that LipL21 facilitates escape from immune surveillance in humans. These novel mechanisms allowing L. interrogans to escape recognition by the NOD receptors may be important in circumventing innate host responses.

Leptospirosis is a widespread zoonosis caused by spirochetal bacteria, Leptospira interrogans (L. interrogans). L. interrogans are primarily extracellular pathogens although some reports suggest they may replicate within macrophages. In humans, leptospirosis can cause mild or severe disease, potentially leading to death, although rats or mice, which constitute the reservoir, are asymptomatic carriers. Host defense mechanisms involved in leptospirosis remain poorly understood. Toll-Like Receptor (TLR)2 and TLR4 are crucial for the clearance of L. interrogans, but the role of the cytosolic NOD receptors in leptospirosis is unknown. Here, we report that pathogenic leptospires escape the sensing of bacterial peptidoglycan through the NOD response. We found that an outer membrane lipoprotein of L. interrogans binds to and protects the peptidoglycan from degradation into muropeptides, thereby blocking signaling through NOD proteins. Moreover, in absence of this lipoprotein, the peptidoglycan of L. interrogans is properly sensed by human NOD1 but not by murine NOD1. This is due to the near absence of muramyl tetrapeptide, the murine NOD1 agonist, in the peptidoglycan of pathogenic leptospires. These novel mechanisms of NOD avoidance may facilitate the escape of leptospires from the innate immune system of their hosts.