Muramyl Endopeptidase Spr Contributes to Intrinsic Vancomycin Resistance in Salmonella enterica Serovar Typhimurium

Abstract

The impermeability barrier provided by the outer membrane of enteric bacteria, a feature lacking in Gram-positive bacteria, plays a major role in maintaining resistance to numerous antimicrobial compounds and antibiotics. Here we demonstrate that mutational inactivation of spr, coding for a muramyl endopeptidase, significantly sensitizes Salmonella enterica serovar Typhimurium to vancomycin without any accompanying apparent growth defect or outer membrane destabilization. A similar phenotype was not achieved by deleting the genes coding for muramyl endopeptidases MepA, PbpG, NlpC, YedA, or YhdO. The spr mutant showed increased autolytic behavior in response to not only vancomycin, but also to penicillin G, an antibiotic for which the mutant displayed a wild-type MIC. A screen for suppressor mutations of the spr mutant phenotype revealed that deletion of tsp (prc), encoding a periplasmic carboxypeptidase involved in processing Spr and PBP3, restored intrinsic resistance to vancomycin and reversed the autolytic phenotype of the spr mutant. Our data suggest that Spr contributes to intrinsic antibiotic resistance in S. Typhimurium without directly affecting the outer membrane permeability barrier. Furthermore, our data suggests that compounds targeting specific cell wall endopeptidases might have the potential to expand the activity spectrum of traditional Gram-positive antibiotics.

Peptidoglycan Endopeptidase Spr of Uropathogenic Escherichia coli Contributes to Kidney Infections and Competitive Fitness During Bladder Colonization

“…Accordingly, we speculate that the spr deletion-induced phenotypes may not necessarily be present in the yebA or ydhO mutant although they exhibit a similar endopeptidase function. Consistent with this speculation, it has been reported that deletion of spr impairs intrinsic vancomycin resistance in S. enterica serovar Typhimurium but deletion of yebA or ydhO does not (Vesto et al, 2018). Therefore, it would be worthy to extend the investigation on the roles of yebA and ydhO in E. coli infections.…”

“…It is known that high copy number plasmids impose metabolic burden in E. coli ( Togna et al, 1993 ; Silva et al, 2012 ). Given that pUC19 is a high copy number plasmid and spr deletion interferes with bacterial physiology ( Hara et al, 1996 ; Vesto et al, 2018 ), the plasmid may induce a substantial fitness burden in the spr mutant in bladders and kidneys. In the same co-infection experiment ( Figure 3B ), the competing strain was a spr mutant with the pUC19-derived pFlhDC (Δ spr -UTI89/pFlhDC).…”

“…Escherichia coli without spr fails to resist osmotic shock at high temperature ( Hara et al, 1996 ). Additionally, disruption of spr sensitizes Salmonella enterica serovar Typhimurium to the glycopeptide antibiotic vancomycin ( Vesto et al, 2018 ). These findings suggest that Spr-deficiency may interfere with the integrity of the bacterial envelope, and thus attenuate bacterial resistance against harsh environments.…”

Peptidoglycan: Structure, Synthesis, and Regulation

“…This is notably different from the characteristic septal blebbing and leakage of cytosolic contents resulting from the lack of aPBP activity. Endopeptidases have now been shown to be crucial for peptidoglycan synthesis in the Gram-negative bacteria Salmonella spp., and V. cholerae, as well as the Gram-positive bacteria B. subtilis and Streptococcus pneumoniae (223)(224)(225)(226). To maintain the continuum and integrity of peptidoglycan, hydrolysis must be tightly coupled to synthesis; however, the underlying mechanism of the coupling process was not clearly understood until now.…”

“…Peptidoglycan hydrolysis is indispensable for cell viability in several Gram-positive and -negative bacteria (221,(223)(224)(225)(226)(281)(282)(283). Although hydrolysis is fundamental for peptidoglycan growth, it is obvious that such hydrolytic activity has to be stringently regulated to maintain the structural integrity of the sacculus.…”