Shigella as a Food borne Pathogen

Abstract

Shigellosis is classically thought of as a waterborne disease; however, public health data suggest that foodborne outbreaks are a significant, if not the major cause of the disease in the United States. The role of Shigella as a foodborne pathogen is reviewed, including discussions of taxonomy, epidemiology, virulence factors, growth and survival in foods and model systems , and methods for detection/identification in food products. Most food microbiologists consider the gastrointestinal disease caused by Shigella species as being mainly water-borne; however, evidence indicates that the foodborne route is quite important in terms of Shigella-induced infections. Black et al. (6) presented data which indicate that for the period 1961-1975, there were 72 outbreaks (10,648 cases) of foodborne shigellosis while there was only 38 outbreaks (5893 cases) for waterborne Shigella infections. For the period 1973-1982, there were 60 outbreaks (4519 cases) of foodborne shigellosis (17-23,64), but only 21 cases of waterborne shigellosis for the period 1971-1978 (24). From the public health data, it would appear that foodborne Shigella infections are more common than waterborne infections. The host range of Shigella species is limited to higher primates including humans. The number of Shigella cells required to initiate an infection appears to be quite low-of the order of 10 1 to 10 4 cells/person (14). The organisms are transmitted through food and/or water contaminated by fecal matter from infected individuals with poor personal hygiene (58). Given the low infective dose, the consumer who eats food prepared by such individuals is at considerable risk of contracting shigellosis. Mossel (71) stated that while the reported incidence of foodborne shigellosis is lower than that of salmonellosis, the problem of foodborne Shigella infections is probably greater than is realized. As the methodology for detecting Shigella species in foods improves, an evaluation of the true incidence of foodborne shigellois can be made. We feel that foodborne shigellois has been a neglected area of study and we wish to acquaint food microbiologists with various aspects of the Shigella organism and with its behavior as a foodborne pathogen. THE MICROORGANISM-TAXONOMY AND GENERAL CHARACTERISTICS The genus Shigella is located in the family Enterobac-teriaceae. Rowe and Gross (99) describe the genus as: "Straight rods similar in morphology to other En-terobacteriaceae. Gram-negative. Nonmotile. Faculta-tively anaerobic, having both a respiratory and a fermen-tative type of metabolism. Catalase-positive (with exception of one species). Oxidase-negative. Chemoorganot-rophic. Ferment sugars without gas production (a few exceptions produce gas). Do not use citrate or malonate as a sole carbon source. Do not grow in KCN or produce H 2 S. Intestinal pathogens of man and other primates, causing bacillary dysentery. The mole% G + C of the DNA is 49-53." In addition, Shigella species lack lysine decarboxylase, phenylalanine deaminase, and urease. They do not use gluconate as a sole source of carbon, do not liquefy gela-tin, and are Voges-Proskauer negative but methyl red-positive. Each of the four species of Shigella-5. dysenteriae, S. flexneri, S. boydii, and S. sonnei-contain serovars (S. flexneri also has subserovars) which are antigenically distinct (99). The species of Shigella may be differentiated by various chemical tests (Table 1.) Colicin and phage-typing have been found useful in studies of the epidemiology of Shigella outbreaks (99). Brenner (10) has pointed out that Shigella and Es-cherichia coli are a single species on the basis of their DNA homology (70-100% related). The two genera can be difficult to separate on the basis of biochemical tests since there are gas-producing shigellae as well as lactose-negative, non-gas producing and nonmotile E. coli strains. Additionally, there are E. coli strains that produce a toxin similar to that produced by Shigella species and a similar disease is casued (72,109). Brenner (10) summed up the Shigella-E. coli problem by stating "Shigellae are actually metabolically inactive biotypes of E. coli."