In vitro ability of the group B streptococci to inhibit gram-positive and gram-variable constituents of the bacterial flora of the female genital tract.

Objective: The purpose of this study was to analyze the ability of septicemic and nonsepticemic isolates of group B streptococci (GBS) to inhibit in vitro the principal bacterial groups found in the normal bacterial flora of the female genital tract.

Methods: The target groups were composed of 1) 10 strains each of the following: viridans streptococci, nonhemolytic streptococci (not group B or D), group A streptococci, GBS, peptostreptococci, coagulase-negative staphylococci, Staphylococcus aureus, and Gardnerella vaginalis; 2) 9 strains of enterococci; 3) 9 strains of group C or G streptococci; 4) 7 strains of lactobacilli; and 5) 7 strains of diphtheroids. All target groups were tested for inhibition by a test panel of either a group of 10 or 41 GBS isolates. If the GBS isolates failed to inhibit a target group, that group was tested for its ability to inhibit the GBS test panel.

Results: The GBS test panel did not inhibit the growth of coagulase-negative staphylococci or S. aureus but uniformly inhibited groups A, B, C, and G streptococci, lactobacilli, and G. vaginalis. One of the 7 strains of diphtheroids was inhibited by 37 of the 41 GBS isolates; the other 6 strains of diphtheroids were uniformly inhibited. Variable inhibition by GBS was observed with viridans streptococci, nonhemolytic (not group B or D) streptococci, peptostreptococci, and enterococci; however, inhibition or noninhibition was uniform for a given target strain against the entire GBS test panel. The 23 GBS isolates obtained from septicemic neonates or adults did not differ from the 18 nonsepticemic isolates in their ability to inhibit other species of streptococci or other gram-positive or gram-variable constituents of the bacterial flora of the female genital tract. When converse testing was done, all 10 GBS isolates were uniformly inhibited by coagulase-negative staphylococci and by the majority of enterococci, but were not inhibited by S. aureus.

Conclusions: These studies suggest that GBS may be significant regulators of other beta-hemolytic streptococci, diphtheroids, lactobacilli, and G. vaginalis within the bacterial flora of the female genital tract. Moreover, the absence of GBS in the vaginal flora may be the result of mediation by coagulase-negative staphylococci and selected strains of enterococci.