Background: Necrotizing enterocolitis (NEC) is an inflammatory intestinal disorder affecting premature infants. NEC presumably involves colonization of the gut with opportunistic pathogens. One such pathogen, Enterobacter sakazakii (ES) has been associated with hospital outbreaks of NEC. It is also known that breast feeding protects from NEC. P-Glycoprotein (Pgp), a product of the multidrug resistance gene, Mdr1a, a transmembrane transporter found on intestinal epithelial cells, may protect against the effects of bacterial invasion by facilitating the efflux of xenobiotics and toxins. We have previously shown that breast milk induces Pgp in the neonatal intestinal epithelium and that Pgp-deficient mice have increased susceptibility to NEC. In this study, we tested the hypothesis that breast feeding protects from NEC via induction of Pgp.

Methods: WT and congenic Mdr1a-/- newborn mice were administered 107 cfu ES or equivalent amount of PBS and allowed to nurse with their mothers. Mortality was determined as frequency of death or events requiring euthanasia (abdominal distension, profuse bleeding or diarrhea, cyanosis, lethargy). Pathology of NEC was graded by examination of H&E-stained ileal samples by a pathologist blinded to the groups. Mortality, timing of disease, and pathology grades were compared using the Mann-Whitney test.

Results: There was no evidence of illness in the 2 groups not exposed to ES, and the average pathology grade was 0.19+/-0.25 in the WT and 0.75+/-0.75 in the Mdr1a-/- group. In the WT-ES group, 4 of 9 died on day of life (DOL) 3(1 pup), 4(2 pups) and 5 (1 pup) respectively, and the remainder were healthy by DOL 5. In the Mdr1a-//--ES group, all 8 died, with death occurring at DOL 2(2), 3(4), 4(1) and 5(1). Mean pathology scores in the two latter groups were 0.5+/-1.3and 2.5+/-1.4, respectively.

Conclusion: Mdr1a-/- mice had higher mortality, more rapid onset of the disease and higher NEC pathology grades than WT mice when challenged with ES. We conclude that P-glycoprotein is required for breast milk-induced protection from NEC.