Tuning of the Lethal Response to Multiple Stressors with a Single-Site Mutation during Clinical Infection by Staphylococcus aureus

Abstract

The agr system of Staphylococcus aureus promotes invasion of host tissues, and as expected, agents that block agr quorum sensing have anti-infective properties. Paradoxically, agr-defective mutants are frequently recovered from patients, especially those persistently infected with S. aureus. We found that an agr deficiency increased survival of cultured bacteria during severe stress, such as treatment with gentamicin, ciprofloxacin, heat, or low pH. With daptomycin, deletion of agr decreased survival. Therefore, agr activity can be either detrimental or protective, depending on the type of lethal stress. Deletion of agr had no effect on the ability of the antimicrobials to block bacterial growth, indicating that agr effects are limited to lethal action. Thus, the effect of an agr deletion is on bacterial tolerance, not resistance. For gentamicin and daptomycin, activity can be altered by agr-regulated secreted factors. For ciprofloxacin, a detrimental function was downregulation of glutathione peroxidase (bsaA), an enzyme responsible for defense against oxidative stress. Deficiencies in agr and bsaA were epistatic for survival, consistent with agr having a destructive role mediated by reactive oxygen species. Enhanced susceptibility to lethal stress by wild-type agr, particularly antimicrobial stress, helps explain why inactivating mutations in S. aureus agr commonly occur in hospitalized patients during infection. Moreover, the agr quorum-sensing system of S. aureus provides a clinically relevant example in which a single-step change in the response to severe stress alters the evolutionary path of a pathogen during infection.

Oxidative stress drives the selection of quorum sensing mutants in the Staphylococcus aureus population

“…However, it does not explain the fitness advantage of the RNAIII mutant. AgrA mutants were previously found to be more resistant toward certain kinds of oxidative stress which was linked to higher expression of the glutathione peroxidase BsaA (51,59). The role of BsaA could not be confirmed here.…”

“…Ciprofloxacin leads to induction of the SOS response and, thus, an increased mutation rate. However, ciprofloxacin might also favor agr mutations because they are better adapted to survive oxidative stress imposed by the antibiotic (59). Thus, ROS formation by the bacterium's own products (PSMs), ciprofloxacin, or other infection-related stresses, such as phagocytosis may favor the selection of QS mutants.…”

Genome Plasticity of agr -Defective Staphylococcus aureus during Clinical Infection

“…The various stages of invasive infection reflect an array of environmental challenges to S. aureus . Experimental ( 44 – 47 ) and observational ( 8 , 48 – 51 ) work suggests that mutation of global regulators constitutes a “one-step” mechanism of adaptation that drives adaptive leaps made by microbes. The present comprehensive identification of variations using complete genome assemblies revealed that the number of mutations in S. aureus clone pairs having a loss of agr function is increased compared to uniformly wild-type controls.…”

Quorum Sensing-Regulated Phenol-Soluble Modulins Limit Persister Cell Populations in Staphylococcus aureus

“…In these cases, the PSM production will be absent and persisters are more likely to be present. Even short-term tolerance has been reported to be improved in strains lacking agr when exposed to lethal stressors such as ciprofloxacin or gentamicin ( Kumar et al, 2017 ), however, it remains to be determined if these findings can be explained by lack of PSM production. In summary, we propose that S. aureus by producing PSMs limit the persister population under conditions of acute infection by increasing their susceptibility to antibiotics via membrane modulation whereas under chronic conditions the reduced PSM production allows the presence of persister cell with greatly reduced susceptibility to antibiotics.…”