Observations on the growth in nutrient broth of mixtures of Escherichia coli strains, and of faecal specimens harbouring pathogenic Escherichia coli serotypes.

Abstract

Experiments were made with five pathogenic serotypes and five non-pathogenicEsch. colistrains to study the competitive growth of mixed coli cultures in nutrient broth. In associations incubated at either 37 or 44°C. the organism inoculated in smallest numbers increased proportionately after 18 or more hours' incubation. The greater the initial disparity, the greater was the increase. Broth culture favoured the pathogenic strains only when these were numerically inferior in the inoculum. The tendency to numerical equation increased with increasing length of incubation.A study was made of the use of nutrient broth as an aid to the isolation of pathogenicEsch. colistrains from the faeces of infants. 120 positive specimens were examined. Nutrient broth incubated at 44° C. was the most successful method, and when used as a complement to direct plating resulted in an additional 19·6% of positive specimens. The main value of nutrient broth as an enrichment method is in the examination of the faeces of suspected carriers and convalescent patients, in which the pathogen is likely to be outnumbered by non-pathogenicEsch. colistrains.I wish to thank Prof. Scott Thomson for his advice and helpful criticism, and Dr R. W. S. Harvey for his interest and encouragement.