Comparison of four methods of differential typing of isolates ofShigella sonnei

Abstract

Summary: An epidemiological study of Sonne dysentery in Dundee during the years 1971–6 was made by examining, in respect of 1420 isolates ofShigella sonnei, the discriminating power of colicine typing, antibiogram testing, biotyping and resistotyping and the stability of the markers they provided.Colicine typing identified nine colicine types, including four not previously described. However, because types 4 and 4 var., determined bycolIb, and type U, producing no colicines, accounted for 96 % of the isolates, discrimination with colicine typing was poor. In antibiotic sensitivity tests, 13 different antibiogram patterns were noted. Less than 1 % of the isolates were sensitive to all of the eight antibiotics tested; most were multiply drug-resistant. Resistance to kanamycin, neomycin and paromomycin (KNP) was apparently due to a single resistance determinant, widely distributed in a majority (53%) of the isolates. When definitive times were chosen for reading each biotyping test, only maltose and rhamnose of the 13 ‘sugars’ tested differentiated isolates into prompt- and late-fermenting types. Though the ability to ferment rhamnose was a stable property, it discriminated only 1·5% of the minority, late-fermenting type. Resistotyping with six chemicals discriminated eight epidemiologically valid resistotypes, including three new types. However, 93 % of the isolates belonged to only three resistotypes.Analysis of the data for isolates from 286 epidemiologically distinct episodes showed that the variability of colicine and antibiogram characters, found among isolates within, respectively, 40 and 28 % of the episodes, was generally associated with loss or gain of a plasmid (‘colIb-KNP’) which determined production of colicine Ib and KNP resistance. These characters varied bothin vivoandin vitro. Variability of resistotype characters, on the other hand, was observed in only 28 (9%) episodes, 14 of which possibly represented examples of mixed or sequential infections.For accurate epidemiological tracing of strains ofSh. sonneiin a community, resistotyping, the technique showing the greatest discrimination and least variability of the four tested, should be included as the principal typing method.