Optimisation of prenatal group B streptococcal screening

Comparison of different sampling techniques and of different culture methods for detection of group B streptococcus carriage in pregnant women

Background

Streptococcus agalactiae (group B streptococcus; GBS) is a significant cause of perinatal and neonatal infections worldwide. To detect GBS colonization in pregnant women, the CDC recommends isolation of the bacterium from vaginal and anorectal swab samples by growth in a selective enrichment medium, such as Lim broth (Todd-Hewitt broth supplemented with selective antibiotics), followed by subculture on sheep blood agar. However, this procedure may require 48 h to complete. We compared different sampling and culture techniques for the detection of GBS.

Methods

A total of 300 swabs was taken from 100 pregnant women at 35-37 weeks of gestation. For each subject, one rectovaginal, one vaginal and one rectal ESwab were collected. Plating onto Columbia CNA agar (CNA), group B streptococcus differential agar (GBSDA) (Granada Medium) and chromID Strepto B agar (CA), with and without Lim broth enrichment, were compared. The isolates were confirmed as S. agalactiae using the CAMP test on blood agar and by molecular identification with tDNA-PCR or by 16S rRNA gene sequence determination.

Results

The overall GBS colonization rate was 22%. GBS positivity for rectovaginal sampling (100%) was significantly higher than detection on the basis of vaginal sampling (50%), but not significantly higher than for rectal sampling (82%). Direct plating of the rectovaginal swab on CNA, GBSDA and CA resulted in detection of 59, 91 and 95% of the carriers, respectively, whereas subculturing of Lim broth yielded 77, 95 and 100% positivity, respectively. Lim broth enrichment enabled the detection of only one additional GBS positive subject. There was no significant difference between GBSDA and CA, whereas both were more sensitive than CNA. Direct culture onto GBSDA or CA (91 and 95%) detected more carriers than Lim broth enrichment and subculture onto CNA (77%). One false negative isolate was observed on GBSDA, and three false positives on CA.

Conclusions

In conclusion, rectovaginal sampling increased the number GBS positive women detected, compared to vaginal and/or rectal sampling. Direct plating on CA and/or GBSDA provided rapid detection of GBS that was at least as sensitive and specific as the CDC recommended method of Lim broth subcultured onto non chromogenic agar.

Genotyping of Streptococcus agalactiae (group B streptococci) isolated from vaginal and rectal swabs of women at 35-37 weeks of pregnancy

BACKGROUND

Group B streptococci (GBS), or Streptococcus agalactiae, are the leading bacterial cause of meningitis and bacterial sepsis in newborns. Here we compared different culture media for GBS detection and we compared the occurrence of different genotypes and serotypes of GBS isolates from the vagina and rectum.

METHODS

Streptococcus agalactiae was cultured separately from both rectum and vagina, for a total of 150 pregnant women, i) directly onto Columbia CNA agar, or indirectly onto ii) Granada agar resp. iii) Columbia CNA agar, after overnight incubation in Lim broth.

RESULTS

Thirty six women (24%) were colonized by GBS. Of these, 19 harbored GBS in both rectum and vagina, 9 only in the vagina and 8 exclusively in the rectum. The combination of Lim broth and subculture on Granada agar was the only culture method that detected all GBS positive women. Using RAPD-analysis, a total of 66 genotypes could be established among the 118 isolates from 32 women for which fingerprinting was carried out. Up to 4 different genotypes in total (rectal + vaginal) were found for 4 women, one woman carried 3 different genotypes vaginally and 14 women carried two 2 different genotypes vaginally. Only two subjects were found to carry strains with the same genotype, although the serotype of both of these strains was different.Eighteen of the 19 subjects with GBS at both sites had at least one vaginal and one rectal isolate with the same genotype.We report the presence of two to four different genotypes in 22 (61%) of the 36 GBS positive women and the presence of identical genotypes in both sites for all women but one.

CONCLUSION

The combination of Lim broth and subculture on Granada medium provide high sensitivity for GBS detection from vaginal and rectal swabs from pregnant women. We established a higher genotypic diversity per individual than other studies, with up to four different genotypes among a maximum of 6 isolates per individual picked. Still, 18 of the 19 women with GBS from both rectum and vagina had at least one isolate from each sampling site with the same genotype.

Antimicrobial susceptibility of group B streptococci collected in two Belgian hospitals

OBJECTIVE

to determine the in vitro susceptibility of group B streptococci (GBS) to antibiotics, used for intrapartum chemoprophylaxis and treatment of infections; to determine the rate of resistance to erythromycin and clindamycin and the phenotype distribution of GBS strains.

METHODS

262 GBS strains from pregnant women at 35-37 weeks' gestation were collected in University Hospital Gasthuisberg (Leuven) and Imelda Hospital (Bonheiden). The minimum inhibitory concentrations (MICs) of penicillin G, amoxicillin, cefazolin, cefotaxime, erythromycin, clindamycin, gentamicin, vancomycin and linezolid were determined by the agar dilution method, according to NCCLS guidelines.

RESULTS

all isolates were susceptible to penicillin, amoxicillin, cefazolin, cefotaxime, vancomycin and linezolid. We found resistance rates of 16.7% to erythromycin and 11.0% to clindamycin. Of all erythromycin-resistant strains, 63.6% had the cMLSB phenotype, 20.5% the iMLSB phenotype and 15.9% the M-phenotype. For 25% of erythromycin-resistant strains, the resistance was of a very high level (MICs ranging from 128 microg/mL to 256 microg/mL). All these isolates belong to the cMLSB phenotype. For the remaining 75% the resistance to erythromycin was of low level (MICs ranging from 1 microg/mL to 4 microg/mL). These isolates had the cMLSB phenotype (38.6%), the iMLSB phenotype (20.5%) and the M-phenotype (15.9%).

CONCLUSION

the susceptibility of GBS to the beta-lactam antibiotics supports the continued use of penicillin for intrapartum chemoprophylaxis. For women who are allergic to penicillin, clindamycin or erythromycin are considered to be the alternatives, however resistance rates to these antibiotics are significant.

Use of unmodified starches and partial removal of serum to improve Granada medium stability

The use of 1% unmodified rice starch and 1% horse serum instead of 2% soluble starch and 5% serum in Granada medium is described. These components result in a medium of increased stability, preventing spoilage after a few days of storage at room temperature.