Virulence and pathogenicity factors of S. agalactiae strains isolated from pregnant women and newborns

Abstract

BACKGROUND: Obstetric and neonatal infections caused by Steptococcus agalactiae are among the most significant perinatal infections. To date, intrapartum antibiotic prophylaxis is used to prevent the transmission of the pathogen to the child, however, the growth of antibiotic resistance and ineffectiveness of therapy against late-onset neonatal infection are its limitations. Vaccination is considered to be the most effective method for preventing diseases caused by S. agalactiae in both pregnant women and newborn babies. To identify promising vaccine targets and to develop alternative prevention approaches, it is necessary to study the virulence factors of S. agalactiae strains and their variability in the population.
 AIM: The aim of this study was to evaluate the variability of virulence and pathogenicity factors (capsular polysaccharides, pili, hypervirulent sequence type ST-17, biofilm-forming ability, antibiotic resistance) of S. agalactiae isolated from pregnant women and newborn infants in St. Petersburg, Russia.
 MATERIALS AND METHODS: We studied isolates of S. agalactiae out of clinical material samples obtained from pregnant women and newborns at the D.O. Ott Research Institute of Obstetrics, Gynecology, and Reproductology in 2018-2020. The PCR method was used to determine the types of capsular polysaccharides, pili, and strain affiliation with the hypervirulent sequencing type ST-17. Biofilm-forming ability was determined by the Christensen method. The antibiotic sensitivity was determined by disc diffusion.
 RESULTS: We examined 60 clinical isolates of S. agalactiae. The most common S. agalactiae serotypes were Ia, Ib, II, III, IV, and V; in total, these six serotypes accounted for 95.1% of all strains. The most common pili genotype was PI-1 + PI-2a (60%). Resistance to erythromycin was found in 36.7% of the strains, and a similar number of the strains were resistant to clindamycin. The ability to form biofilms was detected in 68% of the strains, and the increased ability was associated with the PI-2b pili allele.
 CONCLUSIONS: A hexavalent vaccine based on capsular polysaccharides of types Ia, Ib, II, III, IV, and V would have a 95% efficacy in this region. Stable distribution of different pili types is an important factor when using pili as vaccine targets. The high level of resistance of S. agalactiae strains to erythromycin and clindamycin indicates that isolates should be tested for sensitivity to these antibiotics before their use, and regular regional monitoring of antibiotic resistance of the pathogen to update clinical guidelines should be performed.