Discrimination by multiple typing of isolates of Shigella sonnei in Dundee (1971-6)

Different strains of Shigella sonnei present in Dundee from 1971 to 1976 were identified by a "multiple typing' method in which resistotyping, used as the main method of differentiation, was supplemented by colicine typing, antibiogram testing and biotyping. At least 19 different "multiple types' (MTs) were identified by combining information from the four typing techniques. The relation of the different types and their possible derivation from each other are discussed. The practical value of multiple typing was demonstrated in a study of 247 isolates, of three distinct MTs, recovered from 178 persons involved in an extended outbreak centred primarily on day nurseries. A few episodes that yielded isolates of different resistotypes were analysed to determine whether the cultures were: isolates of the same strain different in resistotype as a result of in vivo or in vitro variation of resistotype characters, or isolates of distinct strains of different resistotypes. The multiple typing approach clarified the way in which different MTs emerged, persisted, disappeared or co-existed in the community during the 6 years of the study.