Determinants of co-colonization with group B Streptococcus among heterosexual college couples

Background. Group B Streptococcus causes morbidity and mortality among newborns, pregnant women, and nonpregnant adults. Among adults, sexual and fecal-oral routes of transmission are hypothesized; this study addresses whether sexual transmission occurs. Methods. Our outcome of interest was group B Streptococcus. From out investigation of the heterosexual transmission of urinary tract infections among college students at the University of Michigan, conducted in 1996-1999, we identified 120 couples in which one or both partners carried group B Streptococcus. Each partner completed a questionnaire regarding potential risk factors' for colonization. Results. Co-colonization with the identical group B Streptococcus strain (as determined by pulsed-field gel electrophoresis) occur-red in 86% of the 57 co-colonized couples. When the male sex partner carried group B Streptococcus, 64% of female partners also were colonized; conversely, 49% of male partners of colonized females were colonized with identical strains. Among behaviors predicting co-colonization within the partnership, male-to-female oral sex was a risk factor among both women (odds ratio [OR] 2.9; 95% confidence interval [CI] = 1.2-6.9) and men (OR 2.5; CI = 1.1-5.6). First sex at age 20 years or older was associated with an increased risk among women (OR = 2.1; CI = 0.7-6.4) and among men (OR = 3.0; CI = 1.0-9.3), and four or more lifetime sex partners was associated with a decreased risk of co-colonization among women (OR = 0.6; CI = 0.2-1.5) and among men (OR = 0.4; CI = 0.2-1.0). Conclusions. Among heterosexual college couples, sexual activity, particularly male-to-female oral sex, increases the risk of co-colonization with an identical group B Streptococcus strain. Future studies should evaluate the role of the pharynx and examine the effects of both bacterial characteristics and host response on transmission.