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

Abstract

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 emm1 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 emm1 subclones disseminated in specific training groups called flights. The preferential increase in the prevalence of emm1 and emm6 isolates during the 6-week training period indicates an enhanced ability of these strains to disseminate and cause disease in this population.

Virulence of serotype M3 Group AStreptococcusstrains in wax worms (Galleria mellonellalarvae)

“…GAs strains known to be highly pathogenic in mice and monkeys caused significantly lower survival and had significantly lower LD 50 s in wax worms than GAs strains associated with attenuated virulence or asymptomatic carriage. Furthermore, isogenic inactivation of proven virulence factors resulted in a significantly increased LD 50 and decreased lesion size compared to the wild-type strain, a finding that also strongly correlates with animal studies. Importantly, survival analysis and LD 50 determination in wax worms supported our hypothesis that a newly emerged GAs subclone that is epidemiologically associated with more human necrotizing fasciitis cases than its progenitor lineage has significantly increased virulence.…”

“…Results demonstrated that compared to subclone 5 strains, subclone 8 strains caused significantly lower survival over time (Fig. 5A) and have a significantly lower mean LD 50 (Fig. 5B).…”

“…(B) The 50% lethal dose (LD 50 ) was determined by probit analysis. The highly virulent reference strain MGAs315 (red bar) has a significantly lower LD 50 (that is, it is more virulent) than other invasive strains known to be less virulent in mice (various blue bars) or strains recovered from the oropharynx of asymptomatic carriers (various brown bars) (* indicates p < 0.05 compared to MGAs315, Mann-Whitney test). (c) Overall, GAs strains such as MGAs315, MGAs3382 and MGAs3392 which are known to be highly virulent in mice and/or monkeys (red checkered bar) have a significantly lower mean LD 50 in wax worm larvae (that is, they are more virulent) than GAs strains associated with lower virulence in animals (blue checkered bar) or asymptomatic carriage in humans (brown checkered bar) (* indicates p < 0.01 compared to MGAs315, Mann-Whitney test).…”

“…We found that GAs causes severe tissue damage and kills wax worms in a dosedependent manner. The virulence of genetically distinct GAs strains was compared by Kaplan-Meier survival analysis and determining 50% lethal doses (LD 50 ). host-pathogen interactions were further characterized using quantitative culture, histopathology and TaqMan assays.…”

Molecular genetic anatomy of inter- and intraserotype variation in the human bacterial pathogen group A Streptococcus

“…These four genomes were sequenced to closure, and an average base call error rate of Ͻ1 in 10,000 (Q40) throughout. Four strains, MGAS12501-MGAS12504, are asymptomatic carriage isolates recovered in a population-based carrier study of recruits at Lackland Air Force Base (San Antonio, TX) (31). An additional eight strains were recovered from January 1, 1992 to December 31, 2003 in an ongoing prospective populationbased surveillance study of GAS invasive infections in Ontario, Canada (http:͞͞microbiology.mtsinai.on.ca͞research͞gas.shtml).…”