Osmotic stress on dilution of acid injured Escherichia coli O157 : H7

To determine surviving numbers of Escherichia coli from cultures or food sy stems, dilution with 0.1% peptone is regularly used. Higher numbers of surv ivors could be obtained from an acid-treated culture if 0.5 mol l(-1) sucro se was added to the 0.1% peptone. Sorbitol, glucose or sodium chloride, but not glycerol, could be used in place of sucrose, Using electron microscopy distinct differences could be seen between acid-treated and untreated cell s. The osmolarity of the diluents ranged from 5 to 500 mosmol kg(-1) H2O fo r the 0.5 mol l(-1) sugar or glycerol solutions, to about 1000 mosmol kg(-1 ) H2O for the salt solution. Maximum recovery diluent has an osmolarity of about 300 mosmol kg(-1) H2O and resulted in recovery of similar numbers of injured cells as a 0.5 mol l(-1) solution of sugar in 0.1% peptone. Taking into account the observed damage to acid-treated cells and the differences in osmolarity of the diluents, it is likely that dilution in 0.1% peptone i mposed additional stress on the acid-injured cells which caused further cel l damage. Dilution in a more osmotically stable solution alleviated this os motic stress.