Colicin types of Shigella sonnei in relation to antibiotic resistance

Colicin typing of 436 strains of Shigella sonneri isolated in Dallas during a 10 year period was performed to determine whether resistance to ampicillin was associated with a single strain or was widespread among all S. sonnei types. One hundred ninety three strains collected during a period of eight and one half years when ampicillin resistance was 1.6% were available for retrospective review. During the last one and one half years of study, 20.6% of the 243 strains that represented almost all S. sonnei isolated in Dallas were resistant to ampicillin. More than 50% of the strains collected annually were colicin type 9, 22% were untypable, and seven other types were less frequently encountered. Resistance to ampicillin increased slightly from 1.5% to 9.5% among the type 9 strains, and one type 8 strain (2.9%) was resistant. All other ampicillin resistance was found in untypable strains (65.5%), and multiple resistance to seven antibiotics tested was found mainly in untypable strains. Thus it appears that the abrupt increase in ampicillin resistance in mid 1972 was due principally to resistance in a single biotype, and that resistance is not widespread among all types of S. sonnei.