RESUMEN
PIP: 20 Mexico City and 23 Houston, Texas colostrum samples, and 21 Mexican and 25 Houston mature milk samples were analyzed by ELISA and Western blot, respectively, for antibodies against the virulence plasmid of Shigella flexneri serotype 5 strain M9OT. The method involved comparing water extracts of milk in ELISA and Western blot determinations of antigens against shigella flexneri strain M9OT which is fully virulent, to those against M9OT A2 which lacks the virulence plasmid. While there are at least 37 know distinct lipopolysaccharide antigens on different strains of the 4 species of Shigella, all contain the same plasmid conferring virulence, the ability of the bacteria to invade mammalian cells. This provided a universal test for antigens to Shigella. Western blots showed antibodies in all 21 Mexican women and in 40% of 25 Houston women. Plasmid antibodies were detected by ELISA in all 20 Mexican colostrum samples and in 52% of 23 Houston colostrum samples. After 8 days of lactation, 93% of the Mexican and 46% of the Houston milk samples were positive. The actual protective factor in human milk against Shigella bacteria is unknown: these findings suggest a mechanism for protection against all serotypes of shigella. The high prevalence of antibodies against Shigella found in Houston women was attributed to infection in the distant past.^ieng