Molecular Genetic Analysis of 675 Group A Streptococcus Isolates Collected in a Carrier Study at Lackland Air Force Base, San Antonio, Texas

Contemporary molecular genetic analysis methods have not been used to study large samples of carriage isolates of group A Streptococcus. To determine the emm types causing asymptomatic carriage and pharyngitis in a closed population, we analyzed 675 isolates recovered from a population-based surveillance study of 10,634 recruits at Lackland Air Force Base, Texas, during 4 months in 1993-1994. Strains with emml and emm6 alleles accounted for only 22% of the isolates recovered from asymptomatic recruits at entrance to training. However, these 2 emm types caused 69% of the pharyngitis cases identified during training and represented 51% of the isolates recovered from the throat on exit from training. Sequence analysis of the hypervariable sic gene documented that distinct emm 1 subclones disseminated in specific training groups called flights. The preferential increase in the prevalence of emm 1 and emm6 isolates during the 6-week training period indicates an enhanced ability of these strains to disseminate and cause disease in this population.