. Several strains of were markedly sensitised to killing at H 2.5 or 3.5 when the ColV, I-K94 virulence plasmid was introduced into them. For strain 1829, the effect on acid sensitivity was due to the presence of plasmid in the previously resistant strain rather than to its introduction into an acid-sensitive variant already in the population. Acid sensitivity was also conferred by the ColV-K30 and ColB-K98 plasmids and the resistance plasmid R124-F2; other plasmids tested had no marked effect. Studies of ColV strains carrying mutant plasmids indicated that it was the presence of ColV-encoded transfer components that made ColV, I-K94 strains acidsensitive. Organisms in the exponential phase of growth were more sensitive to acid than were those from stationary phase cultures and this difference was more marked for ColV, I-K94 strains than for Col ones. Moreover, ColV strains exposed to conditions of low H for short periods subsequently grew less well than the Col parent and appeared to be sensitised by the damage to the effects of HO. These results indicate that some ColV strains may be more sensitive to gastric acid and to phagocytic acidity than are Col strains. ColV, I-K94 strains grew as well as Col ones in broth or urine at H 4.5-6.0 which suggests that the presence of the plasmid would not be detrimental to bacterial growth in the urinary tract. The presence of transfer components in the outer membrane of ColV, I-K94 bacteria may destabilise the lipopolysaccharide layer allowing increased penetration of hydrogen ions.


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