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 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 degrees 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.
Aldsworth
T G
TG
Department of Applied Biochemistry and Food Science, University of Nottingham, Loughborough, Leicestershire, United Kingdom.
Sharman
R L
RL
Dodd
C E
CE
Stewart
G S
GS
D001426:Bacterial Proteins / Q000235:genetics; D015169:Colony Count, Microbial; D004926:Escherichia coli / Q000166:cytology / Q000235:genetics / Q000502:physiology; D005615:Freezing; D015870:Gene Expression; D005798:Genes, Bacterial; D017930:Genes, Reporter; D008954:Models, Biological; D009154:Mutation; D018384:Oxidative Stress; D012486:Salmonella typhimurium / Q000166:cytology / Q000235:genetics / Q000502:physiology*; D012808:Sigma Factor / Q000235:genetics
D001426:Bacterial Proteins; D012808:Sigma Factor; C069567:sigma factor KatF protein, Bacteria