A Competitive Microflora Increases the Resistance of Salmonella typhimurium to Inimical Processes: Evidence for a Suicide Response

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Abstract
The presence of a viable competitive microflora at cell densities of 10 8 CFU ml −1 protects an underlying population of 10 5 CFU of Salmonella typhimurium ml −1 against freeze injury. The mechanism of enhanced resistance was initially postulated to be via an RpoS-mediated adaptive response. By using an spvRA::luxCDABE reporter we have shown that although the onset of RpoS-mediated gene expression was brought forward by the addition of a competitive microflora, the time taken for induction was measured in hours. Since the protective effect of a competitive microflora is essentially instantaneous, the stationary-phase adaptive response is excluded as the physiological mechanism. The only instantaneous effect of the competitive microflora was a reduction in the percent saturation of oxygen from 100% to less than 10%. For both mild heat treatment (55°C) and freeze injury this change in oxygen tension affords Salmonella a substantive (2 orders of magnitude) enhancement in survival. By reducing the levels of dissolved oxygen through active respiration, a competitive microflora reduces oxidative damage to exponential-phase cells irrespective of the inimical treatment. These results have led us to propose a suicide hypothesis for the destruction of rapidly growing cells by inimical processes. In essence, the suicide hypothesis proposes that a mild inimical process leads to the growth arrest of exponential-phase cells and to the decoupling of anabolic and catabolic metabolism. The result of this is a free radical burst which is lethal to unadapted cells.