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

We sought to develop a model of bacterially induced preterm delivery in rats to parallel similar models in mice.

Female Sprague-Dawley rats on day 17 of gestation (normal term = 21-22 days) were inoculated into the uterus with either 2 x 10(9)-7 x 10(10) killed E. coli organisms, 1-4 x 10(8) 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 x 10(7), 2 x 10(8) and 4 x 10(8) 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.