Rapid isolation of salmonellae from faeces

Abstract

Factors and techniques leading to the more rapid isolation of salmonellae from faeces that are negative by direct plating were examined. The aim was to increase the number of specimens on which positive reports might be given within 24 hours of receipt. Salmonellae were isolated from 279 of the specimens studied. Brilliant green MacConkey agar was found to be the most satisfactory solid selective medium both for direct plating and as a subculture medium, giving large characteristic colonies after 24 hours' incubation. Selenite F medium, inoculated with undiluted faeces, incubated at 43 degrees C., and subcultured after six hours' incubation on to brilliant green MacConkey agar, was the most successful rapid method of enrichment, though the results were considerably inferior to those obtained after 24 hours' incubation. Seventy-six of the positive specimens examined were negative by direct plating on three different selective media; 24 (31.6%) of these were positive on subculture from selenite F incubated for six hours at 43 degrees C. and 21 (27.6%) when incubation was at 37 degrees C. The use of six-hour subcultures from 43 degrees C. selenite F as a supplement to direct plating added significantly to the number of positive results. Other techniques that were tried but found to be of no practical value were dilution of the faeces before inoculation on solid and fluid media, and replica plating from one solid medium to another after a few hours' incubation.