The effect of environmental pH on the physiology and surface structures of Salmonella serotype Enteritidis phage type 4

The incidence of food-poisoning caused by Salmonella serotype Enteritidis PT4 has increased. Implicated food products display pH levels between 4 and 9. Accordingly, the effect of growth at extremes of pH on the presence of surface structures and the carriage of a 38-MDa plasmid was determined by growing a clinical isolate of Enteritidis PT4 in a chemostat. Steady-state growth was possible over the pH range 4.35-9.45, corresponding to the pH extremes associated with key reservoirs implicated in outbreaks. Without pH control, cultures stabilised at pH 7.10. Growth at extremes of pH had significant effects on the distribution of cell surface structures; at pH 9.45, only 3% of cells were fimbriate compared with 52% at pH 7.10 and 20% at pH 4.35. The proportion of motile cells and the presence of flagella was also reduced at extremes of pH. A 38-MDa plasmid was present in cells grown in the chemostat at pH 7.10, but not in cells grown at pH 4.35 or pH 9.45. Thus, environmental pH may have a significant impact on the virulence potential of Enteritidis PT4.