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

Group B 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.

Our outcome of interest was group B. From our 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. Each partner completed a questionnaire regarding potential risk factors for colonization.

Co-colonization with the identical group B strain (as determined by pulsed-field gel electrophoresis) occurred in 86% of the 57 co-colonized couples. When the male sex partner carried group B, 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).

Among heterosexual college couples, sexual activity, particularly male-to-female oral sex, increases the risk of co-colonization with an identical group B strain. Future studies should evaluate the role of the pharynx and examine the effects of both bacterial characteristics and host response on transmission.