A COMPETITIVE MICROFLORA INCREASES THE RESISTANCE OF SALMONELLA-TYPHIMURIUM TO INIMICAL PROCESSES - EVIDENCE FOR A SUICIDE RESPONSE

The presence of a viable competitive microflora at cell densities of 1 0(8) CFU ml(-1) protects an underlying population of 10(5) CFU of Salm onella typhimurium ml(-1) against freeze injury, The mechanism of enha nced resistance was initially postulated to be via an RpoS-mediated ad aptive response. By using an spvRA::luxCDABE reporter we have shown th at although the onset of RpoS-mediated gene expression was brought for ward by the addition of a competitive microflora, the time taken for i nduction was measured in hours, Since the protective effect of a compe titive microflora is essentially instantaneous, the stationary-phase a daptive 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 degrees C) and freeze injury this change in ox ygen tension affords Salmonella a substantive (2 orders of magnitude) enhancement in survival, By reducing the levels of dissolved oxygen th rough active respiration, a competitive microflora reduces oxidative d amage to exponential-phase cells irrespective of the inimical treatmen t, These results have led us to propose a suicide hypothesis for the d estruction 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 radic al burst which is lethal to unadapted cells.