Hemolysin-positive enteroaggregative and cell-detaching Escherichia coli strains cause oncosis of human monocyte-derived macrophages and apoptosis of murine J774 cells

Infection of human monocyte-derived macrophages (HMDM) and J774 cells (murine macrophage cell line) with several enteroaggregative and cytodetaching Escherichia coli (EAggEC and CDEC, respectively) strains demonstrated that some strains could induce macrophage cell death accompanied by release of lactate dehydrogenase activity and interleukin 1beta (IL-1beta) into culture supernatants. The mode of cell death differed in the two types of macrophages. Damage to macrophage plasma membrane integrity without changes in nuclear morphology resulted in cytolysis of HMDM. This mechanism of cell death has been previously described for virulent Shigella infection of HMDM and is termed oncosis. In contrast, infection of J774 cells by EAggEC and CDEC strains resulted in apoptosis. The presence of alpha-hemolysin (Hly) in EAggEC and CDEC strains appears to be critical for both oncosis in HMDM and apoptosis in J774 cells. Bacteria lacking Hly, including Hly- EAggEC strains as well as enterotoxigenic, enteropathogenic, and enterohemorrhagic E. coli strains, behaved like avirulent Shigella flexneri in that the macrophage monolayers were intact, with no release of lactate dehydrogenase activity or IL-1beta into the culture supernatants.

A Shigella flexneri invasion plasmid gene, ipgH, with homology to IS629 and sequences encoding bacterial sugar phosphate transport proteins

Sequences representing the 2684 bp flanking the ipaH4.5 gene on the invasion plasmid of Shigella flexneri 5 indicate an unusual fusion gene, designated ipgH, in which the first 27 amino acids (aa) are identical to ORF2 of IS629. The aa sequence encoded by the remainder of ipgH bears significant homology to Escherichia coli and to Salmonella typhimurium GlpT and UhpT proteins and to the S. typhimurium PgtP protein, which are involved in the uptake of high-energy sugar phosphates from an external source.

Evaluation of alkaline phosphatase-labelled ipaH probe for diagnosis of Shigella infections

The presence of many enteropathogens which are not easily detectable by routine stool culture has led to the development of alternative diagnostic methods. One of these techniques, nucleic acid probe hybridization, has been used to identify Shigella spp. and enteroinvasive Escherichia coli (EIEC) in stool specimens through the detection of genetic material encoded by a specific large approximately 200-kbp virulence-related plasmid. In the present study, an alkaline phosphatase-labelled oligonucleotide probe developed to detect the gene for ipaH, a repetitive genetic sequence thought to be present on both the virulence-related plasmid and the chromosomes of all strains of Shigella and EIEC, was tested in a developing-country setting through a prospective clinical trial. In a group of 219 Peruvian adults and children with acute gastroenteritis, the ipaH probe detected 85% of cases of shigellosis and demonstrated a specificity of 95% when compared with simultaneous detection by several stool culture techniques. Additionally, three cases of EIEC infection which could not be diagnosed by culture methods alone were detected with the ipaH probe and were confirmed by plasmid analysis and Sereny testing. These preliminary results suggest that, with further research, the ipaH probe should prove to be a useful and rapid adjunct in the diagnosis of acute gastroenteritis in developing countries.

Prospective study of systemic and mucosal immune responses in dysenteric patients to specific Shigella invasion plasmid antigens and lipopolysaccharides

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.

Virulence phenotype and genetic characteristics of the T32-ISTRATI Shigella flexneri 2a vaccine strain

The T32-ISTRATI strain, which has been used as an oral attenuated Shigella flexneri 2a vaccine, has lost the invasive phenotype due to a spontaneous deletion in the shigella virulence plasmid. This deletion has eliminated three plasmid loci (ipaBCDA, invA and virG) that are necessary for production of a positive Sereny test by Shigella species. Virulence in the Sereny test was reconstituted in the T32-ISTRATI strain by the conjugal transfer of an intact 140 M Da virulence plasmid from S. flexneri 5. The T32-ISTRATI vaccine is safe when given orally in multiple doses of 50-100 x 10(9) organisms, and both homologous and heterologous protection has been reported in large Romanian and Chinese field trials. Although the protective antigen(s) in this vaccine have not been identified, the potential use of non-invasive plasmid deletion mutants as living shigella vaccines is illustrated by the T32-ISTRATI vaccine.