Genetics of virulence in Shigella

Involvement of a 70-kb plasmid of the epidemic Shigella dysenteriae type 1 (Dt66) strain in drug-resistance, lipopolysaccharide synthesis, and virulence

The present work characterizes a 70 kb plasmid of the Indian epidemic isolate Shigella dysenteriae type 1 strain Dt66 in relation to virulence and drug-resistance characters. Curing of the plasmids of Dt66 strain by acriflavine (AF) yielded seven groups of AF-cured derivatives. One group of derivatives containing 5 plasmids (70-2.5 kb) as against 6 (120-2.5 kb) in the wild type showed resistance to chloramphenicol, pivmecillinam, streptomycin, and tetracycline (CmrPivrSmrTcr) but sensitivity for ampicillin and nalidixic acid (AmsNals). This derivative when used as donor in conjugation experiments with a plasmidless E. coli KL318 strain (AmsCmsPivsSmrTcsNalr) as recipient, transferred only its 70 kb plasmid into the recipient with concomitant transfer of the CmrPivr phenotype. Both the donor and the transconjugant exhibited a diffuse pattern of adherence and produced keratoconjunctivitis in guinea pigs. Transconjugants contained a 42 kDa lipopolysaccharide band which was absent in recipient. Moreover, both donor and transconjugants showed Congo red binding ability. The results suggest that the 70 kb plasmid of S. dysenteriae Dt66 strain encodes not only Cmr,Pivr character but also is associated with virulence-related characters such as Congo red binding, LPS-biosynthesis, HeLa cell adherence, and keratoconjunctivitis related to virulence.

Treatment of HeLa cells with bacterial water extracts inhibits Shigella flexneri invasion

Pathogenesis mediated by Shigella flexneri requires invasion of the gastrointestinal epithelium. It has been previously shown that HeLa cells challenged with S. flexneri show alterations in their phosphotyrosine-containing protein profile. In this report, we demonstrated that bacterial water extracts (WE) abrogated the invasion of HeLa cells by S. flexneri in a dose-dependent manner. A proteinaceous component of S. flexneri was shown to be responsible for this inhibitory activity. Proteins encoded on the 140-MDa plasmid were not responsible for the observed inhibition. WE from other Gram-negative bacteria also inhibited Shigella invasion of HeLa cells pretreated with WE showed changes in the profile and the intensity of phosphotyrosine-containing protein bands. These data were consistent with a surface protein component in WE which initiated aberrant host cell signaling at the membrane which may account for the inhibition of bacterial entry.

Shigella flexneri-HeLa cell interactions: a putative role for host cell protein kinases

Epithelial cell invasion has been shown to be a prerequisite for Shigella flexneri virulence. Recently, we have documented the induction of transcription factor DNA binding activities as a result of S. flexneri challenge of HeLa cells. In this report, we show that HeLa cells challenged with S. flexneri display differences in phosphotyrosine-containing proteins. These changes are detected as early as 5 min post-challenge. Challenge with a noninvasive ipaB mutant strain resulted in the induction of a similar, but less intense, profile of phosphotyrosine-containing host cell proteins. Phosphotyrosine-containing proteins could be detected in S. flexneri, but were unique from those detected following HeLa cell challenge. S. flexneri invasion of HeLa cell monolayers was reduced by treatment with protein kinase inhibitors. These data suggest a role for protein kinases in the initial response of host cells to S. flexneri.

Porphyromonas gingivalis invades oral epithelial cells in vitro

The aim of the present study was to analyze the adhesive and invasive potential of a number of P. gingivalis strains, in an in vitro system utilizing cultures of human oral epithelial cells (KB cell line, ATCC CCL 17). P. gingivalis strains W50 and FDC 381 (laboratory strains) and OMGS 1738, 1743 and 1439 (clinical isolates) as well as E. coli strain HB 101 (non-adhering, non-invasive control) were used. Adherence was assessed by means of scintillation counting and light microscopy, after incubation of radiolabelled bacteria with epithelial cells. In the invasion assay, monolayers were infected with the P. gingivalis and E. coli strains and further incubated with an antibiotic mixture (metronidazole 0.1 mg/ml and gentamicin 0.5 mg/ml). Invasion was evaluated by (i) assessing presence of bacteria surviving the antibiotic treatment, and (ii) electron microscopy. All P. gingivalis strains adhered to and entered into the oral epithelial cells. After 3 hours of incubation, bacteria were frequently identified intracellularly by means of electron microscopy. The cellular membranes, encapsulating the microorganisms in early stages of the invasive process, appeared later to disintegrate. The presence of coated pits on the epithelial cell surfaces suggested that internalization of P. gingivalis was associated with receptor-mediated endocytosis (RME). Formation of outer membrane vesicles (blebs) by intracellular bacteria indicated that internalized P. gingivalis was able to retain its viability. E. coli strain HB 101 neither adhered to nor invaded epithelial cells.

Etiology and mechanisms of acute infectious diarrhea in infants in the United States

Infectious diarrhea, caused by a wide variety of viral, bacterial and parasitic pathogens, is a common reason for morbidity and hospitalization for children in the United States. Overall, rotavirus is the most common cause of acute diarrheal disease in infants. Salmonella, Shigella, and Campylobacter are the most frequently isolated bacterial pathogens, and Giardia and Cryptosporidium are the parasites that most commonly produce acute infectious diarrhea. The mechanisms by which these enteropathogens cause diarrhea are highly variable, and include crypt cell proliferation, cellular invasion, elaboration of enterotoxins or cytotoxins, and enteroadhesion. In infants the incidence of diarrheal disease is higher and the severity of the illness is greater than in older children and adults. An increased rate of exposure to enteropathogens, as a result of fecal-oral contamination, may explain some of the increased incidence of diarrhea in infants. However, age-specific differences in host defense mechanisms may also account for the increased susceptibility to and severity of certain enteric infections in infants.

Laboratory investigations on the low pathogenic potential of Plesiomonas shigelloides

The pathogenic properties of 16 Plesiomonas shigelloides strains recovered from humans with extraintestinal and intestinal illnesses, infected animals, and environmental sources were investigated. Most strains possessed a high cell charge and low surface hydrophobicity analogous to those of Shigella spp.; additionally, serogroup O:17 strains reacted with Shigella group D antisera. However, unlike the shigellae, P. shigelloides strains did not universally bind Congo red, were noninvasive in HEp-2 cell assays, and did not produce a Shiga-like toxin on Vero cells. On HEp-2, Y1, and possibly Vero cells, a low-level cytolysin was consistently produced by all 16 P. shigelloides strains when grown in either Evan Casamino Acids-yeast extract or Penassay broth. The median 50% lethal dose for all 16 P. shigelloides strains in outbred Swiss Webster mice was 3.5 x 10(8) CFU (range, 3.2 x 10(7) to greater than 1 x 10(9) CFU). Animal pathogenicity did not correlate with cytolysin expression, possession of a greater than or equal to 120-MDa plasmid, protein profile, or resistance to complement-mediated lysis. No strain analyzed produced siderophores or a heat-stable enterotoxin. The results suggest that members of the genus Plesiomonas have an overall low pathogenic potential, irrespective of the site of isolation or phenotypic, serologic, or surface properties shared with other traditional enteropathogens.

Sequence and molecular characterization of a multicopy invasion plasmid antigen gene, ipaH, of Shigella flexneri

A lambda gt11 expression library of Tn5-tagged invasion plasmid pWR110 (from Shigella flexneri serotype 5, strain M90T-W) contained a set of recombinants encoding a 60-kilodalton protein (designated IpaH) recognized by rabbit antisera raised against S. flexneri invasion plasmid antigens (J. M. Buysse, C. K. Stover, E. V. Oaks, M. M. Venkatesan, and D. J. Kopecko, J. Bacteriol. 169:2561-2569, 1987). Southern blot analysis of wild-type S. flexneri serotype 5 invasion plasmid DNA (pWR100) digested with various combinations of five restriction enzymes and hybridized with defined ipaH probes showed complex hybridization patterns resulting from multiple copies of the ipaH gene on pWR100. DNA sequence analysis of a 2.9-kilobase (kb) EcoRI fragment directing IpaH antigen synthesis in plasmid recombinant pWR390 revealed an open reading frame coding for a 532-amino-acid protein (60.8 kilodaltons); this size matched well with the estimated size of IpaH determined by Western blot analysis of M90T-W cells and maxicell analysis of Escherichia coli HB101(pWR390) transformants. Examination of the amino acid sequence of IpaH revealed a hydrophilic protein with six evenly spaced 14-residue (L-X2-L-P-X-L-P-X2-L-X2-L) repeat motifs in the amino-terminal end of the molecule. Southern blot analysis of HindIII-digested pWR100 DNA probed with defined segments of the pWR390 2.9-kb insert demonstrated that the multiple band hybridization pattern resulted from repeats of a significant portion of the ipaH structural gene in five distinct HindIII fragments (9.8, 7.8, 4.5, 2.5, and 1.4 kb). Affinity-purified IpaH antibody, used to monitor the expression of the antigen in M90T-W cells grown at 30 and 37 degrees C, showed that IpaH synthesis was not regulated by growth temperature.

Expression in Escherichia coli K-12 of the 76,000-dalton iron-regulated outer membrane protein of Shigella flexneri confers sensitivity to cloacin DF13 in the absence of Shigella O antigen

One of the chromosomal segments associated with virulence in Shigella flexneri encodes the production of aerobactin and the synthesis of an iron-regulated 76-kilodalton outer membrane protein believed to be the ferric-aerobactin receptor. However, S. flexneri expressing this putative aerobactin receptor, which is slightly larger than that encoded by pColV, is insensitive to the killing action of cloacin DF13, a bacteriocin which binds to other aerobactin receptor proteins and kills the cells. In this paper we show that the conjugal transfer of DNA encoding the iron-regulated 76-kilodalton protein from S. flexneri to Escherichia coli K-12 conferred cloacin DF13 sensitivity on the recipients. However, E. coli K-12 which had also inherited genes specifying Shigella O-antigen biosynthesis remained cloacin insensitive. The data suggest that it is unwise to use cloacin DF13 sensitivity alone to screen transconjugants or clinical isolates for the expression of aerobactin receptor proteins.

Congo red-mediated regulation of levels of Shigella flexneri 2a membrane proteins

The ability of Shigella spp. to bind Congo red from agar medium is generally correlated with their virulence properties. We used a metabolically active culture of Shigella flexneri 2a to determine the effect of Congo red on its membrane protein profiles. Virulent S. flexneri grown in the presence of Congo red at 37 degrees C showed increased levels of three proteins with Mrs of 43,000, 58,000, and 63,000 (43K, 58K, and 63K proteins) in the Sarkosyl-soluble membrane fractions. The observed phenomenon was temperature dependent. At 30 or 42 degrees C the protein levels remained unaffected by the presence of Congo red. Similar regulation of the levels of the 43K, 58K, and 63K membrane proteins was also observed with Shigella dysenteriae 1 and enteroinvasive Escherichia coli, but not with enteropathogenic E. coli. The cellular uptake of Congo red seemed to be essential, but not sufficient, for regulation. All three proteins reacted with human convalescent-phase sera in immunoblots of S. flexneri 2a Sarkosyl-soluble membrane fractions. Using the 43K-specific antiserum as the primary antibody, by indirect immunofluorescence studies, we detected an increase in the level of the 43K protein in S. flexneri which had invaded epithelial cells. These observations strongly indicate that the 43K, 58K, and 63K proteins are virulence associated. We propose that the observed regulatory effect of Congo red on membrane proteins of S. flexneri is mediated through induction. Since the same regulatory effect was also observed during the invasion of epithelial cells by S. flexneri, it is suggested that Congo red mimics some host tissue factor in vitro.

Human milk immunoglobulin A antibodies to Shigella virulence determinants

Because human milk is thought to protect infants from shigellosis, we evaluated milk for immunoglobulin A to Shigella virulence determinants. Milk was preincubated to remove antibodies unrelated to each locus of interest, using defined Shigella and E. coli hybrids containing known Shigella genetic segments prior to immunoblotting. The milk could not be shown to contain antibodies to chromosomally encoded virulence loci except for the expected antibodies to the products of the histidine locus. However, all the milk samples contained antibodies to antigens encoded by the large virulence plasmid. The finding of these antibodies suggests a possible mechanism by which human milk might protect infants.

Common themes in microbial pathogenicity

A bacterial pathogen is a highly adapted microorganism which has the capacity to cause disease. The mechanisms used by pathogenic bacteria to cause infection and disease usually include an interactive group of virulence determinants, sometimes coregulated, which are suited for the interaction of a particular microorganism with a specific host. Because pathogens must overcome similar host barriers, common themes in microbial pathogenesis have evolved. However, these mechanisms are diverse between species and not necessarily conserved; instead, convergent evolution has developed several different mechanisms to overcome host barriers. The success of a bacterial pathogen can be measured by the degree with which it replicates after entering the host and reaching its specific niche. Successful microbial infection reflects persistence within a host and avoidance or neutralization of the specific and nonspecific defense mechanisms of the host. The degree of success of a pathogen is dependent upon the status of the host. As pathogens pass through a host, they are exposed to new environments. Highly adapted pathogenic organisms have developed biochemical sensors exquisitely designed to measure and respond to such environmental stimuli and accordingly to regulate a cascade of virulence determinants essential for life within the host. The pathogenic state is the product of dynamic selective pressures on microbial populations.

Characterization of invasion plasmid antigen genes (ipaBCD) from Shigella flexneri

The large invasion plasmid of Shigella flexneri M9OT-W was used to generate recombinant plasmids carrying the ipaA, -B, -C, and -D genes, whose products are associated with the entry of the bacteria into colonic epithelial cells. Complete DNA sequences of ipaB, -C, and -D were determined. The proteins predicted (62, 42, and 37 kDa, respectively) from the nucleotide sequences lack a signal-peptide sequence. Hydrophilic segments of the IpaB and IpaC proteins were found to overlap known epitopic domains of these membrane antigens. Analysis of total RNA demonstrated that temperature control of ipa gene expression occurs at the level of transcription. Multiple mRNA bands were detected by using ipa gene fragments as hybridization probes, and a putative transcript map for the ipa genes was constructed. Comparison of this map with the DNA sequence reveals a complex system of ipa gene regulation.

Comparison of the invasion strategies used by Salmonella cholerae-suis, Shigella flexneri and Yersinia enterocolitica to enter cultured animal cells: endosome acidification is not required for bacterial invasion or intracellular replication

Strains of Escherichia, Salmonella, Shigella and Yersinia actively enter eukaryotic cells. Several techniques were used to compare and contrast the invasion mechanisms of Salmonella cholerae-suis, Yersinia enterocolitica and Shigella flexneri. Three animal cell lines (CHO, HEp-2 and MDCK) were examined for susceptibility to bacterial entry by these strains. Levels of intracellular bacteria varied widely between cell lines, but CHO cells were the most susceptible to bacterial invasion, HEp-2 invasion levels were intermediary, whereas polarized MDCK cells were invaded to a lesser extent. This illustrates that tissue culture models can be optimized to study bacterial invasion and intracellular replication. We used these tissue culture models to examine the interactions between host cells and these invasive bacteria. The use of lysosomotropic agents (methylamine and ammonium chloride), cationic ionophores (monensin) and acidification-defective CHO cell lines demonstrated that endosome acidification is not required for bacterial invasion or intracellular replication. Drugs which inhibited microfilament formation (cytochalasins B and D) prevented internalization of S. cholerae-suis, Y. enterocolitica and S. flexneri, indicating that invasion is a microfilament-dependent event. The microtubule inhibitors, colchicine, vincristine and vinblastine, did not affect bacterial internalization.

Development and testing of invasion-associated DNA probes for detection of Shigella spp. and enteroinvasive Escherichia coli

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.