Chorioamnionitis caused by gram-negative bacteria as an etiologic factor in preterm birth.

The role and microbiological causes of infection and inflammation of the chorioamnion were studied in 85 patients with spontaneous preterm deliveries (< 37 weeks) and in 85 control patients with full term deliveries. Microorganisms were isolated from the freshly separated chorioamnion in 55% of preterm and 26% of term deliveries (p < 0.001). Isolation rates of gram-negative enteric bacteria were significantly higher in preterm deliveries than in term deliveries (p < 0.001), whereas differences in the isolation of other bacterial species were not significant. Histological chorioamnionitis was noted in 49% of preterm and 14% of term deliveries (p < 0.001), and was strongly associated with a positive chorioamniotic culture in both groups (p < 0.001). Histological chorioamnionitis was noted in 94%, 54%, and 4% of membranes with gram-negative rods, other microbial species and negative cultures, respectively (p < 0.001). Preterm deliveries were also associated with significantly higher rates of bacterial vaginosis (38% vs. 14%) and isolation of vaginal pathogens (85% vs. 65%). In the case of the majority (88%) of chorioamniotic isolates the same species was isolated in the vagina. The findings suggest that gram-negative enteric rods are important placental pathogens responsible for sub-clinical chorioamnionitis and possibly preterm birth. The findings support the concept that microorganisms ascending from the lower genital tract produce local inflammation, which may result in preterm labour and delivery.