A multifaceted cellular damage repair and prevention pathway promotes high‐level tolerance to β‐lactam antibiotics

Understanding tolerance to cell wall–active antibiotics

“…It thus appears that VxrAB's role in tolerance is at least partially to tone down an out‐of‐control iron starvation response that would otherwise overload β‐lactam–stressed cells with iron, causing downstream toxic events like the generation of ROS. Consistent with this model, deleting iron transporters partially restored high tolerance levels to a ∆ vxrAB mutant 128 . The VxrAB system thus aptly illustrates the complex physiological consequences of β‐lactam exposure and tolerance pathways, where cells not only have to overcome cell wall loss, but also remediate toxic iron influx and potentially downstream accumulation of toxic ROS.…”

“…A ∆ vxrAB mutant exhibits ∼10,000‐fold decreased tolerance against CWA antibiotics compared with the wild‐type strain; importantly, this is not the consequence of enhanced lysis, but rather reflects reduced recovery of cell wall–deficient spheroplasts that are formed in response to CWA antibiotics in this organism 17 . VxrAB positively controls genes encoding cell wall synthesis enzymes (including the entire intracellular precursor pathway), type VI secretion, and biofilm formation, and negatively controls motility and iron acquisition genes 128,166,222,223 . Both positive control over PG synthesis and negative control over iron acquisition contribute to CWA antibiotic tolerance 128 .…”

“…While PG synthesis induction is an intuitive way for a cell to recover from cell wall loss, control of iron acquisition was at first puzzling. Intriguingly, V. cholerae accumulates high intracellular iron levels upon β‐lactam exposure and this is exacerbated in the ∆ vxrAB mutant 128 . Further, β‐lactam exposure resulted in induction of the Fur iron starvation response, likely due to direct oxidation of its Fe 2+ corepressor by β‐lactam–induced H 2 O 2 production 128 .…”

“…For unknown reasons, the heat shock response (via the alternative sigma factor RpoH) is induced by CWA antibiotics in phylogenetically diverse bacteria, 130,131,241–243 suggesting a conserved pathway for induction. The signal for heat shock might be protein misfolding due to oxidation, which has been reported to occur in after exposure to β‐lactam antibiotics 128,129 . The exact degree to which heat shock contributes to tolerance is poorly understood; however, artificially upregulating RpoH prevented β‐lactam–induced lysis in E. coli , 244 suggesting a potential role in surviving β‐lactam exposure.…”

“…Importantly, L‐forms and spheroplast‐like forms have been isolated from patients treated with CWA antibiotics, 249,250 and this form of tolerance might thus be an under‐recognized reason for antibiotic treatment failure 250 . The exact mechanisms of spheroplast formation and maintenance are poorly understood, but physiological and genetic factors promoting this type of tolerance have been identified 87,128,165 . In V. cholerae , survival of, and recovery from, the spheroplast state requires induction of PG synthesis functions, remediation of toxic iron influx, likely detoxification of ROS, and induction of cell envelope stress responses, as detailed above 128 .…”