Development of stress resistance in Staphylococcus aureus after exposure to sublethal environmental conditions

The ability of Staphylococcus aureus to develop stress resistance responses was investigated. Exponential growth phase cells of S. aureus CECT 4459 were exposed to sublethal conditions (acid and alkaline pH, hydrogen peroxide, and heat) and then the acquisition of resistance to acid (pH 2.5), alkali (pH 12.0), hydrogen peroxide (50 mM), and heat (58 °C) was determined. Conditions resulting in the maximum development of homologous resistance (tolerance to the same stress), while preventing lethal effects in the population, were pH 4.5 (2 h), pH 9.5 (30 min), 0.05 mM H2O2 (30 min), and 45 °C (2 h). Under these adaptation conditions, times for the first decimal reduction (TFDC) to a lethal treatment at acid pH, alkaline pH, hydrogen peroxide, and heat were increased by a factor of 1.6, 2, 2, and 6, respectively. The presence of chloramphenicol or rifampicin in the adaptation medium completely abolished the increase in homologous resistance to acid pH and to hydrogen peroxide. By contrast, the development of homologous resistance to alkaline pH resulted independently of the presence of either chloramphenicol or rifampicin. S. aureus heat resistance increased in the presence of the inhibitors during the heat shock, but only partially. In some cases, the exposure to a given stress induced cross-protection against other agents. Protective combinations of sublethal stress and lethal agents were: acid pH–heat, acid pH–hydrogen peroxide, alkaline pH–hydrogen peroxide, heat–acid pH, and heat–hydrogen peroxide. These combinations of agents applied sequentially should be avoided in food-processing environments.