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 subclinical 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.