Gram Negative Short Rods

The heterogeneous group of organisms consisting of gramnegative small rods, when motile, possess peritrichous (all-over-the-cell) flagella. They are facultative anaerobes that are found in water, soil, human and animal environments, and the food chain. Some of the organisms are exceedingly important in food microbiology. Many of them are food pathogens and food-spoilage organisms. The most important family in this group is the Enterobacteri-aceae. According to the newest classification there are 14 genera in this family: Escherichia, Shigella, Salmonella, Citrobacter, Klebsiella, Enterobacter, Erivinia, Serrada, Hafnia, Edwardsiella, Proteus, Providencia, Morganella, and Yersinia.

Escherichia is a true fecal coliform; the type species is the well-known E. coli. This organism has been used as an indicator of fecal contamination. Its presence indicates the potential presence of other, more pathogenic enteric organisms and is highly undesirable. Recently the serotype, E. coli 0157:H7, has been the source of many major outbreaks involving hundreds of people and resulting in deaths. Major recalls of foods (up to 25 million lb of ground beef) were also made because of contamination by this organism. E. coli 0157:H7 is now considered a food adulterant and is the cause of many food-safety regulations. E. coli is the most monitored organism in food and water microbiology.

Salmonella is an organism of great concern to the food industry. It is ubiquitous in the animal population and especially in poultry flocks. Some raw poultry products harbor Salmonella; thus all poultry products should be well cooked before consumption. The organism, when consumed in large number (1 million), can cause a disease called salmonellosis, which includes vomiting, nausea, diarrhea, chills, and fever. Mortality can occur in the very old, the very young, or the immunocompromised. More than 2,000 serotypes of Salmonella are reported. Many new rapid methods are now being developed to detect this organism in about 1 day. The conventional method takes about 5 to 7 days.

All members of the Enterobacteriaceae are potentially pathogenic. A more thorough discussion of bacterial pathogens is recorded in a different section of this volume (see the articles Foodborne diseases and Rapid methods of microbiological analysis). It is important to note that occurrence of members of the Enterobacteriaceae in food is generally undesirable. Proper food handling will prevent or retard their growth and keep food safer for the consumers.

It should be noted that the naming of bacteria is a dynamic process in a constant state of revision and updating as new research demands. Consult Bergey's Manual of Determinative Bacteriology, 9th edition (2) for the current genus and species names of bacteria of interest.

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