Failure of E. coli bacteria to induce preterm delivery in the rat

Female Sprague-Dawley rats on day 17 of gestation (normal term = 21–22 days) were inoculated into the uterus with either 2 × 109 – 7 × 1010 killed E. coli organisms, 1 – 4 × 108 live E. coli or sterile solution. These inoculations were made either via trans-cervical catheter or by direct intrauterine injection at laparotomy. Animals were then observed for delivery for variable periods up to term. Necropsies were performed and fetal viability was assessed.No rats delivered prematurely after bacterial exposure (27 animals observed for at least 48 hours), and all animals followed to term (n = 3) delivered live pups. No dams exhibited signs of systemic illness. There was a statistically significant but small negative effect of killed E. coli on fetal viability (100% of 80 fetuses from 6 control pregnancies and 93% of 182 fetuses from 14 bacterially-treated pregnancies were alive at necropsy, p = 0.014). Live bacteria had a larger effect on fetal viability, with only 64% of 14 fetuses, 47% of 28 fetuses and 32% of 31 fetuses surviving after trans-cervical administration of 7 × 107, 2 × 108 and 4 × 108 E. coli, respectively.Unlike mice, it has proven difficult to induce preterm labor in the rat using E. coli as a stimulating agent. The relevant literature is reviewed and hypotheses are offered to explain this phenomenon.Preterm birth is the major cause of neonatal morbidity and mortality in the developed world [1]. Infection within the gestational compartment is thought to be the primary cause of a large proportion of cases of preterm labor, accounting for as many as 50% or more of premature deliveries, especially at very early gestational ages. Given the complexity inherent in the processes of parturition and the obstacles to conducting properly controlled and prospective studies in human subjects, animal models have proven helpful in developing an understanding of the physiology and pathophysiology of parturition [2]. Novel insights have been generated in animal models