Differential survival of Salmonella typhi, Escherichia coli, and Enterobacter aerogenes on lettuce in the field.

Artificial contamination of young lettuce plants with Salmonella typhi (ST), Escherichia coli (EC), and Enterobacter aerogenes (EA) in the field in winter and summer resulted in presence of the pathogen and increased density of the two indicator bacteria (IB) in harvested produce. Viable counts of the three bacteria/g fresh weight declined at a decreasing rate with increasing time after contamination. The overall pattern of variation of the ST/IB ratio was correlated more closely with changes in the ST/EA than in the ST/EC ratio values. When viable counts were expressed as a proportion of the contaminating dose at different times after contamination, however, a closer similarity existed between ST and EC than between ST and EA values throughout winter and through the early part of summer experiments.