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

The system of promotes invasion of host tissues, and as expected, agents that block quorum sensing have anti-infective properties. Paradoxically, -defective mutants are frequently recovered from patients, especially those persistently infected with We found that an deficiency increased survival of cultured bacteria during severe stress, such as treatment with gentamicin, ciprofloxacin, heat, or low pH. With daptomycin, deletion of decreased survival. Therefore, activity can be either detrimental or protective, depending on the type of lethal stress. Deletion of had no effect on the ability of the antimicrobials to block bacterial growth, indicating that effects are limited to lethal action. Thus, the effect of an deletion is on bacterial tolerance, not resistance. For gentamicin and daptomycin, activity can be altered by -regulated secreted factors. For ciprofloxacin, a detrimental function was downregulation of glutathione peroxidase (), an enzyme responsible for defense against oxidative stress. Deficiencies in and were epistatic for survival, consistent with having a destructive role mediated by reactive oxygen species. Enhanced susceptibility to lethal stress by wild-type , particularly antimicrobial stress, helps explain why inactivating mutations in commonly occur in hospitalized patients during infection. Moreover, the quorum-sensing system of 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. When phenotypes produced in response to an environmental stress are inadequate to buffer against that stress, changes that do buffer may become genetically encoded by natural selection. A clinically relevant example is seen with mutants that are deficient in the key virulence regulator Paradoxically, defects in are selected during serious hospital infection and have been associated with worse outcome. The current work helps resolve this paradox: mutants are often less readily killed by lethal stressors without affecting MIC, a phenomenon known as tolerance. Our results indicate that tolerance, which would not be detected as resistance, can be selected in clinical settings. The data also support the ideas that (i) broadly hedges against environmental change and stress through genome plasticity, (ii) reactive oxygen can be involved in the self-destructive response in bacteria, and (iii) therapeutic targeting of and virulence can be counterproductive.