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Antibiotic resistance mechanisms

WTAs get tailored

PLoS Pathog. 10.1371/journal.ppat.1004919

The cell wall of the bacterial pathogen Listeria monocytogenes (Lm) contains a peptidoglycan layer that serves as a scaffold for proteins, glycopolymers and teichoic acids (called wall teichoic acids, WTAs). Among their various functions, WTAs protect bacteria from the antibiotic effects of antimicrobial peptides (AMPs). Modification of WTAs by D-alanylation is a known mechanism by which some Gram-positive pathogens can evade killing by AMPs. Because Lm WTAs are decorated strictly with monosaccharides, Carvalho et al. explored whether glycosylation could contribute to AMP resistance by Lm. They focused on a previously identified gene cluster they named rmlABCD, whose homologs in other bacteria are responsible for L-rhamnose biosynthesis, after they found that transcription of this cluster is increased throughout infection. As well, a significant number of bacteria that harbor a rmlABCD gene cluster are pathogenic. The authors found that Lm rmlABCD deletion strains completely lacked L-rhamnose and that their WTA composition was perturbed. They found a similar result—that WTAs contained no L-rhamnose—upon deletion of an upstream gene, rmlT, which they had considered to be important because of its homology to glycosyltransferases. Both rmlABCD and rmlT mutants were more susceptible to killing by AMPs, which the authors determined was due not to a change in the cell surface charge (a known defense mechanism against AMPs) or to an improved binding efficiency of the AMPs, but rather to an increased ability of the AMPs to cross the cell wall. Accordingly, Lm plasma membrane integrity—the efficacy target of AMPs—was more easily compromised in the mutant than in the wild-type strains. Lastly, the authors also found that the two mutant strains were less virulent in an animal model of Lm infection. These results suggest that L-rhamnosylation of Lm WTAs has a protective effect against penetration by AMPs and is critical for Lm pathogenesis.

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Bucci, M. WTAs get tailored. Nat Chem Biol 11, 448 (2015). https://doi.org/10.1038/nchembio.1851

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