Group B Streptococcal Vaginal Colonization in Older Women

The prevalence of Group B Streptococcus agalactiae (GBS) vaginal colonization, antibiotic susceptibility, and molecular patterns are comparable between older postmenopausal women and pregnant women, according to the results of a study published in BMC Infectious Diseases.

Noting that the incidence of invasive GBS disease continues to increase in nonpregnant adults but data on GBS colonization in older women are scarce, a team of investigators conducted a prospective, observational, cross-sectional study (ISRCTN registry, ISRCTN15468519) at 2 teaching hospitals in Switzerland from 2017 to 2019 to determine the rate of vaginal GBS colonization among women older than 60 years. The researchers also aimed to determine the proportion of known risk factors linked to invasive GBS diseases and to identify specific phenotypes and genotypes associated with these clusters in women with and without GBS colonization.

A total of 263 samples were collected from 259 patients; included in the analysis were 255 unique samples from vaginal swabs taken during routine examination. Eligible participants also completed a questionnaire including data on ethnicity, medical conditions, menstrual history, and sexual history.

A total of 44 samples (17%) were positive for GBS, 36 of which were available for phenotype and genotype characterization to determine antibiotic resistance profiles, capsular serotypes, and clone sequence types.

Susceptibility to penicillin was noted for all 36 GBS isolates, and the minimal inhibitory concentrations ranged from 0.032 mg/L to 0.094 mg/L; 8 and 5 isolates were not susceptible to erythromycin or clindamycin, respectively. An additional 3 isolates that were considered clindamycin-susceptible by E-test showed the macrolide-lincosamide-streptogramin B (MLS_B) phenotype based on the double disk diffusion test, indicating inducible resistance to clindamycin. Clindamycin and erythromycin susceptibility was confirmed in 27 GBS isolates.

Upon capsular serotyping, serotypes 3, 5, and 1a were the most prevalent serotypes. Following multilocus sequence typing, ST1 and ST19 were found to be the most common sequence types. Compared with non-ST1 isolates, ST1 (serotype 5) isolates were found to be significantly associated with resistance to clindamycin and erythromycin, as well as to the MLS_B phenotype; however, these associations were based on a small sample size.

The investigators noted that previous studies have reported an approximately 20% prevalence of GBS colonization among pregnant women, which is similar to the 17% prevalence found among the postmenopausal women included in the present study.

“We found no associations with patient characteristics, comorbidities, menstrual history, menopause, or sexual activity,” the investigators wrote.

“Our results indicate that the prevalence of colonization, the antibiotic susceptibility, and the molecular patterns are similar in pregnant and elderly women,” they concluded, adding that “[t]he observed increase in invasive GBS infections in elderly women may be for reasons other than the colonization rate.”

Reference

Baldan R, Droz S, Casanova C, et al. Group B streptococcal colonization in elderly women. BMC Infect Dis. Published online May 3, 2021. doi:10.1186/s12879-021-06102