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The ipaH loci comprise a multicopy antigen gene family unique to Shigella species and enteroinvasive Escherichia coli (EIEC). DNA probes derived from the Shigella flexneri serotype 5 ipaH7.8 gene were used to compare the molecular arrangement of ipaH alleles in a variety of Shigella and EIEC strains. Multiple copies of ipaH-homologous sequences were detected in all invasion plasmids examined. Oligonucleotide probes covering discrete 24 bp segments of the ipaH7.8 gene and sequences flanking the ipaH4.5 (probe H25) and ipaH2.5 (probe H24) loci were used to define the extent of homology among invasion plasmid copies of ipaH in S. flexneri serotypes 1, 2 and 5 and in S. sonnei. IpaH alleles carried by these invasion plasmids were not structurally equivalent and showed sequence divergence at their amino- and carboxy-terminal ends. The H25 probe was shown to correspond to an IS629 sequence genetically linked to the ipaH alleles, while the H24 probe defined a DNA sequence found only in Shigella invasion plasmids. Chromosomal DNA from invasion plasmid-cured S. flexneri and S. sonnei strains hybridized a core ipaH7.8 gene segment, indicating that portions of the ipaH7.8 structural gene were reiterated and contained within the shigellae chromosomes. Based on the specificity of the ipaH7.8 core probe and the detection of ipaH sequences on the invasion plasmids and chromosomes of Shigella strains, three polymorphic groups within a collection of forty S. dysenteriae 1 isolates received by the United States Centers for Disease Control in 1988 were identified using this probe. These results suggest that ipaH restriction fragment length polymorphisms may be useful in genetic lineage and epidemiologic studies of virulent shigellae.

Shigellosis is a major cause of infant morbidity and mortality in developing countries. To find immunological correlates of specific protection against shigellosis, we examined chronological samples of sera, stool extracts, duodenal aspirates, and saliva samples from 39 adults and 22 children with shigellosis from Peru for the presence of specific antibody to invasion plasmid antigens (Ipa) common to all virulent Shigella strains, by using both a whole-organism enzyme-linked immunosorbent assay (ELISA) and a Western blot (immunoblot) assay. Antibody responses to lipopolysaccharide (LPS) from Shigella serotypes both homologous and heterologous to the infecting strain were also determined by ELISA. ELISAs showed that the highest serum immunoglobulin G (IgG) antibody titers to Shigella whole organisms both with and without surface Ipa were found in adults and malnourished children, the two groups with the shortest and longest durations of disease, respectively. Mucosal IgA antibody titers to Shigella strains decreased over time to a much greater extent than serum IgG titers, and IgA to Ipa in mucosal secretions was found in adults and well-nourished children but not in malnourished children. The presence of mucosal antibody to Ipa may limit the spread and severity of the infection, as indicated by the prolonged illness observed in malnourished children who have no significant mucosal antibody to Shigella Ipa. Serum antibody titers to the Ipa antigens were high relative to anti-Shigella LPS antibody titers, especially in pediatric patients. In contrast to the anti-Ipa responses observed, no differences in antibody responses to LPS in children compared by nutritional status were found. High levels of serum and mucosal cross-reacting antibody to heterologous serotype LPS were found between Shigella flexneri serotypes 1a and 2a. Different patterns of immune response to Ipa proteins and LPS that may aid in the definition of Shigella antigens important in host protection were observed in adults, well-nourished children, and malnourished children.

The products of the ipaB, ipaC, and ipaD genes are involved in the expression of the invasive phenotype in all species of Shigella and enteroinvasive Escherichia coli (EIEC). DNA probes derived from these genes are accurate indicators of the invasive phenotype (M. Venkatesan, J. M. Buysse, E. V. Vandendries, and D. J. Kopecko, J. Clin. Microbiol. 26:261-266, 1988); however, spontaneous loss of the invasion plasmid or selective deletion of invasion-associated genes may restrict the usefulness of such probes as general diagnostic tools. In this study, we report that laboratory-passaged strains of Shigella spp. and EIEC that were invasion and Sereny test negative were unable to hybridize to the ipaC DNA probe. However, a second DNA probe, derived from the Shigella flexneri ipaH gene, a multiple-copy element found on the chromosome and invasion plasmid that encodes a 60-kilodalton antigen, was more sensitive in its ability to detect virulent as well as avirulent shigellae and EIEC. Analysis of colony blots and stool blots from pediatric patients with diarrhea indicated that the ipaH probe was more effective in detecting shigellae and EIEC than was either the ipaC or 17-kilobase EcoRI fragment probe.

Genetic determinants of the invasive phenotype of Shigella spp. and enteroinvasive Escherichia coli (EIEC), two common agents of bacillary dysentery, are encoded on large (180- to 210 kilobase), nonconjugative plasmids. Several plasmid-encoded antigens have been implicated as important bacterial ligands that mediate the attachment and invasion of colonic epithelial cells by the bacteria. Selected invasion plasmid antigen (ipa) genes have recently been cloned from Shigella flexneri serotype 5 into the lambda gt11 expression vector. Portions of three ipa genes (ipaB, ipaC, and ipaD) were tested as DNA probes for diagnostic detection of bacillary dysentery. Under stringent DNA hybridization conditions, all three DNA sequences hybridized to a single 4.6-kilobase HindIII fragment of the invasion plasmids of representative virulent Shigella spp. and EIEC strains. No hybridization was detected in isogenic, noninvasive Shigella mutants which had lost the invasion plasmid or had deleted the ipa gene region. Furthermore, these probes did not react with over 300 other enteric and nonenteric gram-negative bacteria tested, including Salmonella, Yersinia, Edwardsiella, Campylobacter, Vibrio, Klebsiella, Aeromonas, Enterobacter, Rickettsia, and Citrobacter spp. and various pathogenic E. coli strains. The use of unique invasion-essential gene segments as probes for the specific detection of invasive dysentery organisms should benefit both epidemiologic and diagnostic analyses of Shigella spp. and EIEC.