RESUMEN
Adhesion of enteropathogenic Escherichia coli (EPEC) to HeLa cells is inhibited by human colostrum. In the present study we investigated the effect of colostrum on the stability of pMS49, an EPEC adherence plasmid coding for localized adhesion and ampicillin (Ap) resistance. The plasmid was highly stable after serial passage of bacterial cultures in Tryptic Soy Broth containing 67%, 50%, 10% (v/v) or no human colostrum. A few variants (0.4%) with a low adherence were observed regardless of the treatment given. Human colostrum did not enhance their emergence. No bactericidal or bacteriostatic effect of colostrum was observed under the experimental conditions used. A specific process regulating plasmid expression is supposed to occur in EPRC strains, giving rise to variants with a lower concentration of the outher-membrane adherence-related protein and consequently lower adherence ability. This process seems to also occur for Ap-resistance genes coded in the same plasmid