The use of phage-sensitivity patterns for tracing heat labile toxin-positive (LT⁺) Escherichia coli

The heat-labile toxin (LT) positive Escherichia coli colonies from 785 stool specimens obtained during a cholera vaccine trial were examined for their phage-sensitivity pattern to 31 E. coli phages. These specimens originated from 105 index cases and their contacts. Isolates with common phage-sensitivity pattern were grouped and were studied further both serologically and for their plasmid profile. The largest group (42 isolates) belonged to serogroup O78 and the second largest group (19 isolates) belonged to serogroup O6. There were 23 index cases which had E. coli with the same phage-sensitivity pattern as some of their contact strains. The identity of isolates from 16 index cases with strains from their respective contacts could be verified serologically. For the remaining seven index cases and their contacts, the isolates did not agglutinate with available antisera. However, subsequent studies demonstrated that, when the phage-sensitivity pattern among the strains matched, the plasmid profiles of these strains also matched. This further indicates the ability of phage-sensitivity patterns to serve as markers in tracing strains.