The incidence of colonization by enteropathogenic, enterotoxigenic, enteroinvasive, and enterohemorrhagic Escherichia coli (detected by DNA hybridization with specific radiolabeled probes), Salmonella sp., Shigella sp., Campylobacter jejuni, and rotavirus was related to the presence of diarrhea in a cohort of 75 rural infants followed longitudinally during the first year of life. The study was carried out between August 1985 and February 1987 in the village of Lugar Sobre la Tierra Blanca, in the state of Morelos, 180 km southwest of Mexico City. Intestinal colonization by specific enteropathogens was followed with fecal cultures taken every fortnight and every time a child had diarrhea. Pathogens isolated from cultures taken in the 48 hours prior to the initiation of the diarrheal episode were considered to be associated with the disease. Diarrhea was detected in 82% of the children with initial isolation of enterohemorrhagic E. coli and in 64% of the children with enteropathogenic E. coli or Shigella sp. The risk of diarrhea associated with the initial isolation of other pathogens was lower, at 41% for rotavirus and approximately 25% for enterotoxigenic E. coli, Salmonella sp., and C. jejuni. Initial colonization by the enteropathogens studied, whether or not they were associated with diarrhea, prevented disease, but not colonization by the same organism, when the children were reinfected during the first year of life. Enteropathogenic E. coli adherence factor, human or porcine heat-stable enterotoxins, fimbrial colonization factor antigens, and Shiga-like toxins I and II were important pathogenic characteristics related to the presence of diarrhea and to protection against subsequent infection by the same organisms.

Multilocus enzyme electrophoresis (MLEE) of 397 Vibrio cholerae isolates, including 143 serogroup reference strains and 244 strains from Mexico and Guatemala, identified 279 electrophoretic types (ETs) distributed in two major divisions (I and II). Linkage disequilibrium was demonstrated in both divisions and in subdivision Ic of division I but not in subdivision Ia, which includes 76% of the ETs. Despite this evidence of relatively frequent recombination, clonal lineages may persist for periods of time measured in at least decades. In addition to the pandemic clones of serogroups O1 and O139, which form a tight cluster of four ETs in subdivision Ia, MLEE analysis identified numerous apparent clonal lineages of non-O1 strains with intercontinental distributions. A clone of serogroup O37 that demonstrated epidemic potential in the 1960s is closely related to the pandemic O1/O139 clones, but the nontoxigenic O1 Inaba El Tor reference strain is not. A strain of serogroup O22, which has been identified as the most likely donor of exogenous rfb region DNA to the O1 progenitor of the O139 clone, is distantly related to the O1/O139 clones. The close evolutionary relationships of the O1, O139, and O37 epidemic clones indicates that new cholera clones are likely to arise by the modification of a lineage that is already epidemic or is closely related to such a clone.

In our article, we suggested that non-O1 strains of identical or closely similar electrophoretic type (ET) collected on different continents represent clonal lineages (Tables 3 and 5 in the original article). Reconsideration of our data, however, indicates that for many of these sets of strains an equally plausible if not more likely explanation of multilocus genotypic similarity is the independent recombinational assembly of common alleles. This is the case for ET 196 and ET 128 (Table 3), each of which has a genotype consisting of alleles that occur in high or moderate frequency in populations, as well as for many of the ETs listed in Table 5, which pertains to sets of strains differing at single loci. However, for a number of ETs of non-O1 strains, a clonal relationship is probable because their genotypes include unique or rare alleles. Thus, for example, the genotype of ET 256, which was represented by a strain collected in India in 1979 and five strains recovered from patients in Mexico and Guatemala in the early 1990s, includes a unique allele of leucine aminopeptidase, an extremely rare indophenol oxidase allele, and uncommon alleles at four additional loci. Similarly, a clonal relationship is indicated for a pair of serotype O44 strains from India (1973) and Mexico (1991) that share a unique allele of nucleoside phosphorylase and a rare allele of phosphoglucomutase. Multilocus enzyme electrophoresis is of limited use in identifying clonal lineages because, as we have noted, electromorphs cannot be equated with isoalleles and convergence in electrophoretic mobility of an enzyme is not infrequent. To determine clonal relationships among strains with a high degree of confidence, sequence data for multiple housekeeping genes will be required. This reinterpretation of the likely status of certain groups of non-O1 strains does not affect any other aspect of the work reported in our study.

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