MOLECULAR TYPING OF ISOLATES FROM AN OUTBREAK ASSOCIATED WITH ERYTHROMYCIN-RESISTANT B. PERTUSSIS. • 1030

Abstract

Between April 30, and October 1, 1994, 75 probable and confirmed cases of pertussis were identified in Yuma County, AZ. A strain of Bordetella pertussis highly resistant to erythromycin (MIC >64 μg/ml, usual MIC range: 0.02 to 0.1 μg/ml) was isolated from a 2-month-old infant during the outbreak. We performed pulsed-field gel electrophoresis (PFGE) typing on 49 isolates obtained from cases in the affected county and the surrounding area during the same period to (1) determine whether the resistant strain was of a unique genetic type. (2) define the distribution of B. pertussis genetic types in the region, and (3) compare the genetic types of isolates from the region with a random sample of isolates from other states in the U.S.. PFGE and antimicrobial susceptibility testing was performed on 7 isolates from Yuma, 24 from other Arizona counties, and 18 from San Diego County, CA. The 49 isolates yielded 12 distinct PFGE patterns. Two PFGE types. C (N=23) and G (N=11) predominated and comprised 69% of all strains. Five of the 7 isolates from Yuma were PFGE type G compared with 6 of 42 isolates from other counties (OR 15.0, 95% CI=1.9-150). Only the isolate from the index case was resistant to erythromycin and was PFGE type G. For comparison, 87 additional B. pertussis strains isolated during 1993-94 in 7 geographically separate states were typed. Only three of 87 strains isolated elsewhere, compared with 11 of 49 strains from southwestern U.S., were identified as type G (OR 8.1, 95% CI=1.9-39). Four type G strains were isolated during 1989-90 in Missouri; none of the 23 Missouri strains from 1994 were type G. Preliminary data suggest that the mechanism of erythromycin resistance in the index isolate may not involve an easily transmissible genetic element and may be associated with alterations in the chromosomal DNA. These data suggest that B. pertussis can develop resistance to erythromycin. This change in phenotype may not be associated with a detectable change in the PFGE pattern of the strain. Additionally, type G was the dominant strain in the outbreak. This genetic type of B. pertussis has circulated in other parts of the U.S. in the past and thus may not have been imported recently from another country.