Streptococcus agalactiae in pregnant women: Phenotypic and genotypic characters
Introduction
Streptococcus agalactiae (group B streptococcus, GBS) was first recognized as the cause of bovine mastitis, but since the 1970s emerged as an important human pathogen. Currently it is considered a leading cause of severe invasive infections among newborns.1 The vaginal colonization prevalence varies in European countries among pregnant women between 10 and 20%2, 3, 4 and the incidence of neonatal infections ranges from 0.5 to 2 per 1000 live births.2, 5 An Italian report of 1997 indicates a vertical transmission rate from colonized mothers to newborns of 41.5%.3 The outcome of neonatal infection may be bacterial sepsis, pneumonia and meningitis, that in Europe and United States have a mortality rate of 4–6%.2 In 1996 the U.S. Center for Disease Control and Prevention recommended a screening-based strategy to prevent the perinatal transmission of streptococcal disease from pregnant women to newborns during delivery. An update of prophylaxis recommendations based on retrospective studies was issued in 2002.6 The implementation of selective intra-partum chemoprophylaxis prompted a 70% reduction in early-onset diseases.2, 7, 8 The neonatal morbidity decrease allowed significant economies in terms of neonatal laboratory tests, antibiotic therapy and hospital days.9 GBS are susceptible to penicillin, which is still considered the first choice antibiotic for the treatment and prophylaxis of these infections; erythromycin or clindamycin are recommended alternatives for penicillin-allergic GBS carriers. However, probably due to the widespread use of these antibiotics, that are used in up to 20% of all deliveries, a high rate of resistance to these agents has been observed in the U.S. and in Spain.10, 11, 12 A study performed in Turkey showed that 21 and 9% of GBS strains isolated from pregnant women were resistant to erythromycin and clindamycin, respectively.13 Basic research to elucidate correlations between antibiotic resistance and phenotypic and genotypic characters is needed in order to control the emergence of antibiotic-resistant strains. Traditionally, serotyping methods based on the capsular polysaccharide antigens have been used to investigate GBS epidemiology in humans. To date, nine serotypes have been identified, even if recently a new serotype (IX), previously unrecognized, has been proposed.14 The Ia, Ib, II, III and V serotypes predominate in the western hemisphere among infants and pregnant women, serotypes VI and VIII have been more frequently found in Japan.15, 16 and the serotype IV is predominant in pregnant women in United Arab Emirates.17 A relationship between erythromycin resistance and serotype has been observed in serotype V18, 19 suggesting a clonal dissemination and a horizontal transfer of resistance genes.
Phenotypic methods, however, have limited accuracy and often result in some non-typeable isolates. Molecular typing overcomes many shortcomings of the phenotypic methods; a technique particularly suitable to identify the emergence and spread of new clones and to monitor the propagation of particular traits is the pulsed-field gel electrophoresis (PFGE).20, 21, 22, 23 This technique has been used extensively to delineate both endemic infections and epidemic outbreaks due to different bacterial species, and the interpretation of PFGE patterns is aided by published guidelines.24 The purpose of this study was to characterize GBS isolates collected from pregnant healthy women by analyzing phenotypic (serotype and antibiotic susceptibility) and genotypic (PFGE profile) characters.
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Patients
Four hundred pre-partum (35–37 weeks of gestation) women were screened for the presence of S. agalactiae in vaginal and rectal secretions during the years 2005–2006. No one had any symptom of infection at the moment of sampling. The procedure is mandatory and routinely performed at the Obstetrics Division of the Mauriziano Umberto I Hospital in Turin (Italy). Each patient gave informed written consent to the use of sample material for research purposes.
Isolation and typization of bacteria
Seventy-three single-patient S. agalactiae
Results
About 18% of the parturient women were found to carry GBS. All isolates were typeable and each one of the six different serotypes tested was represented; the strains belonging to the III, V and Ia serotypes were the most prevalent, but the difference was statistically significant for serotype III only (Student's t-test, p < 0.01) (Table 1). Based on PFGE profiles, the 73 GBS isolates were grouped into seven (A–G) different PFGE major profiles (Table 1, Fig. 1). Approximately 1/3 of the strains
Discussion
The prevalence (18%) of GBS vaginal or rectal carriage in parturient women observed by us is higher than that reported by Sensini et al. in 1997 (11.3%) and in the range of percentages reported by Brimil et al. (16%), and by Gonzalez et al. (10–18.5%).3, 29, 30 Our investigation shows that the III, V and Ia serotypes were the most prevalent and the most resistant to antibiotics, in agreement with the results of other authors.29, 31 These serotypes are more frequently responsible for neonatal
Acknowledgements
We thank Wyeth Pharmaceuticals (USA) for providing tigecycline discs. This work was partly supported by a grant from the Italian Ministry for University and Scientific Research (ex 60%).
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