Colicin types of Shigella sonnei in relation to antibiotic resistance

Colicin typing of 436 strains of Shigella sonnei 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 resistaiple 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.