The pathogenesis of Shigella diarrhea. VI. Toxin and antitoxin in Shigella flexneri and Shigella sonnei infections in humans.

Two strains of Shigella flexneri and one of Shigella sonnei were studied for toxin production in vitro. All of the three strains produced a cell-free cytotoxin that showed marked similarity to that produced by Shigella dysenteriae 1. Each toxin eluted in two distinct peaks on chromatography with Sephadex G-150, was destroyed by heating at 90 C for 30 min, and was neutralized by S. dysenteriae 1 antitoxin. Patients with infections due to S. flexneri and S. sonnei developed antibody that neutralized S. dysenteriae 1 toxin in vitro. In three of seven positive sera studied by sucrose density-gradient ultracentrifugation, antibody activity was associated only with the IgM fraction. The time course of the antibody response resembled that found in infections due to S. dysenteriae 1, in which an IgM antitoxin antibody has also been described. Since three species of Shigella have now been shown to be toxigenic, it is possible that bacterial toxin may play a role, along with bacterial invasion, in the pathogenesis of infections due to S. flexneri and S. sonnei, as well as those due to S. dysenteriae 1.     

Shigella infections in the United States, 1974-1980

During the seven-year period 1974-1980, 93,516 Shigella isolates from humans were reported to the Centers for Disease Control, Atlanta, through a nationwide surveillance system. Over the past 30 years, the reported incidence of shigellosis has been declining in contrast to that reported for salmonellosis. Shigella sonnei (group D) now accounts for approximately 70% of the Shigella isolates reported, since most of the decline has been due to the increasingly less frequent infections caused by Shigella flexneri (group B); Shigella dysenteriae infections are now uncommon. The highest reported rates of shigellosis are from the western states and among children from one to five years of age. Among the total population of the United States, the ratio of S. flexneri to S. sonnei isolates was 0.28 among persons from one to 19 years of age and 0.45 among persons greater than or equal to 20 years of age; among American Indians the proportion of S. flexneri was much greater, the corresponding ratios being 2.1 and 2.9, respectively; in both groups the ratios for children younger than one year of age resembled those obtaining for adults. Age-related acquisition of immunity to S. sonnei may explain these observations.     

Shigella and enteroinvasive Escherichia coli infections in households of children with dysentery in Bangkok.

Shigellae and enteroinvasive Escherichia coli (EIEC) were identified in children with dysentery and their household contacts in Bangkok. Shigellae were isolated from 49% and EIEC from 6% of 306 children with dysentery seen at the outpatient department of Children's Hospital on weekdays during January through June 1989 and October 1989 through October 1990. The same serotype infecting the index child was isolated from 21 (4%) of 522 household contacts of 151 index children with Shigella infections and from none of 60 household contacts of 19 index children with EIEC infections. Amplification of DNA sequences coding for the invasion-associated locus (ial) by polymerase chain reaction increased the identification of Shigella and EIEC infections from 57% (111/193) to 68% (132/193). ial sequences were identified in 3 of 20 drinking water specimens from which shigellae or EIEC were not isolated. Amplification of ial sequences identified more shigellae and EIEC than did bacteriologic and colony hybridization methods in children with dysentery and in drinking water in Bangkok.     

Analysis of fecal leukocytes and erythrocytes in Shigella infections in urban Bangladesh

We evaluated the usefulness of enumeration of fecal leukocytes and erythrocytes in making an early diagnosis of Shigella infection, where Shigella is a leading cause of invasive diarrhea. Stool specimens from 561 invasive diarrhea patients were submitted for microscopic examination. A presumptive diagnosis of shigellosis based on microscopic examination was made in 389 of them; 227 had stool cultures positive for Shigella spp (Shigella patients). One hundred sixty-two patients with no detectable Shigella infection (non-Shigella invasive diarrhea cases) served as a comparison group. Two hundred twenty-seven randomly selected Shigella patients and 227 non-Shigella infectious diarrhea cases from the surveillance system database of the hospital constituted another group for comparative evaluation. The stool specimens of the patients were examined under the microscope, and isolation, biochemical characterization and serotyping of Shigella were performed. In comparison with non-Shigella invasive diarrhea cases, the presence of >50 WBC/hpf in association with any number of RBC in the fecal sample had a modest sensitivity of 67%, specificity of 59%, positive predictive value of 70%, negative predictive value of 56%, accuracy of 64%, and positive likelihood ratio of 1.6 in predicting shigellosis. Comparison between Shigella and non-Shigella infectious diarrhea patients revealed the presence of >20 WBC/hpf was a less accurate predictor of shigellosis (sensitivity 51%, specificity 88%, positive predictive value 81%, negative predictive value 64%, accuracy 69%, and positive likelihood ratio 4.1). Direct microscopical examination of stool specimens for the presence of WBC and RBC may facilitate the early diagnosis of shigellosis, and may be a cheap alternative to stool culture in this setting.     

Shigella

Shigella strains capable of producing illness possess a 140 megadalton plasmid that determines its property of epithelial cell penetration. In the classic presentation, there is a descending intestinal tract infection beginning with the passage of small numbers of large-volume stools and progressing to the passage of many small-volume stools that contain blood and mucus. Shigellae are efficiently transmitted to contacts in person-to-person spread and only 200 viable cells are sufficient to produce illness in susceptible persons. Shigellosis should be suspected in persons with severe diarrhea, in those with fever, when bloody stools are passed, or when stool smears contain many leukocytes. The current treatment of choice is TMP-SMX or, in adults, a quinolone (norfloxacin or ciprofloxacin).     

Shigella

Much is known about the molecular effectors of pathogenicity of gram-negative enteric pathogens, among which Shigella can be considered a model. This is due to its capacity to recapitulate the multiple steps required for a pathogenic microbe to survive close to its mucosal target, colonize and then invade its epithelial surface, cause its inflammatory destruction and simultaneously regulate the extent of the elicited innate response to likely survive the encounter and achieve successful subsequent transmission. These various steps of the infectious process represent an array of successive environmental conditions to which the bacteria need to successfully adapt. These conditions represent the selective pressure that triggered the “arms race” in which Shigella acquired the genetic and molecular effectors of its pathogenic armory, including the regulatory hierarchies that regulate the expression and function of these effectors. They also represent cues through which Shigella achieves the temporo-spatial expression and regulation of its virulence effectors. The role of such environmental cues has recently become obvious in the case of the major virulence effector of Shigella, the type three secretion system (T3SS) and its dedicated secreted virulence effectors. It needs to be better defined for other major virulence components such as the LPS and peptidoglycan which are used as examples here, in addition to the T3SS as models of regulation as it relates to the assembly and functional regulation of complex macromolecular systems of the bacterial surface. This review also stresses the need to better define what the true and relevant environmental conditions can be at the various steps of the progression of infection. The “identity” of the pathogen differs depending whether it is cultivated under in vitro or in vivo conditions. Moreover, this “identity” may quickly change during its progression into the infected tissue. Novel concepts and relevant tools are needed to address this challenge in microbial pathogenesis.     

Shigella Hepatitis

A 17-year old girl developed Shigella dysentery. She also developed cholestatic hepatitis which was evident by the sixth day of illness and subsided with the clinical improvement of shigellosis. This may be the first biopsy proven case of hepatitis in endemic shigellosis.