Frequency of antimicrobial resistance among invasive and colonizing Group B Streptococcal isolates

Background: Group B Streptococcus (GBS) remains susceptible to penicillin, however, resistance to second-line antimicrobials, clindamycin and erythromycin, has increased since 1996. We describe the age-specific antibiotic susceptibility profile and capsular type distribution among invasive and colonizing GBS strains. Methods: We tested 486 invasive GBS isolates from individuals of all ages collected by a Wisconsin surveillance system between 1998 and 2002 and 167 colonizing strains collected from nonpregnant college students during 2001 in Michigan. Antimicrobial susceptibility testing was performed by disk diffusion or Etest and capsular typing was performed using DNA dot blot hybridization Results: 20.0% (97/486) of invasive and 40.7% (68/167) of colonizing isolates were resistant to clindamycin ( P <.001) and 24.5% (119/486) of invasive and 41.9% (70/167) of colonizing isolates were resistant to erythromycin ( P <.001). Similarly, 19.8% (96/486) of invasive and 38.3% (64/167) of colonizing isolates were resistant to both antimicrobial agents ( P <.001). 29.4% (5/17) of invasive isolates from persons 18 - 29 years of age and 24.3% (17/70) of invasive isolates from persons 30 49 years of age were resistant to clindamycin. Similarly, 35.3% (6/17) of invasive isolates from persons 18 - 29 years of age and 31.4% (22/70) of invasive isolates from persons 30 - 49 years of age were resistant to erythromycin. 34.7% (26/75) of invasive isolates from persons < 1 year of age were capsular type Ia, whereas capsular type V predominated among isolates from adults. Conclusion: Clindamycin and erythromycin resistance rates were high among isolates colonizing nonpregnant college students and invasive GBS isolates, particularly among the colonizing isolates. Susceptibility profiles were similar by age although the proportion of clindamycin and erythromycin resistance among invasive isolates was highest among persons 18 - 49 years of age. Increasing antimicrobial resistance has implications for GBS disease treatment and intrapartum prophylaxis among penicillin intolerant patients.