"No reported. 'AOAC first action. "Negative predictive value. ^Positive predictive value. "Increases with IMS step. fSIS method.

®Not available in North America. 'Heat labile enterotoxin detection. Source: Ref. 1, used with permission.

veloped that separates Salmonella and Serratia colonies from Yersinia by the development of black colonies. Control of yersiniosis is through proper handling of raw and cooked food of all types, especially pork products, and of water for food processing. There has been no reported outbreaks of Y. enterocolitica between 1988 and 1992.

Listeria monocytogenes

Listeria monocytogenes has developed into a very important food pathogen in the past 10 years from the standpoint of economic and public health impact. The organism is a small, short, Gram-positive non-spore-forming rod. It is motile by a characteristic tumbling motion or slightly rotating fashion. The organism grows on simple laboratory media in the pH range between 5 and 9. On solid agar, the colonies are translucent, dewdrop-like, and bluish when viewed by 45° incident transmitted light (Henry's illumination step). Biochemically, this organism can be confused with such organisms as Lactobacillus, Brochothrix, Erysi-pelohrix, and Kurthia. A variety of biochemical tests have been devised to separate L. monocytogenes from other Listeria species, such as L. innocua, L. welshimeri, and L. murrayi. Serotyping is also important in the identification of this organism, the most important ones being l/2a, l/2b, l/2c, 3a, 3b, 3c, and 4b. Listeria is a psychrotroph capable of growing at temperatures as low as 2.5°C and as high as 44°C. Because dairy products have been implicated in outbreaks of listeriosis, much research has been directed toward cheese and milk products. The organism has been found to survive the processing of cottage cheese, cheddar cheese, and Colby cheese. A question of great concern is whether L. monocytogenes can survive the current pasteurization temperature of milk (ie, 63°C for 30 min or 72°C for 15 s). Data on this issue are still inconclusive, and research on this topic is still ongoing. It is important to note that, at present, the time and temperature regulation for pasteurization of milk has not been affected by the possible heat resistance of L. monocytogenes. The disease starts with infection of the intestine—the infective dose is not known at this point. Patients may develop transitory flulike symptoms such as malaise, diarrhea, and mild fever. In severe cases, virulent strains are capable of multiplying in macrophages and later producing septicemia. When this occurs, the bacteria can affect the central nervous system, the heart, the eyes, and may invade the fetus of pregnant women and result in abortion, stillbirth, or neonatal sepsis.

Several well-documented cases of listeriosis have been reported in Nova Scotia (1981), Massachusetts (1983), and the most well known one involving Mexican-style soft cheese in Southern California (1985). Due to concerted effort by the food industry and government agencies, the outbreaks of L. monocytogenes seemed to have subsided for about 10 years. Between 1998 and 1992 only one outbreak, involving two cases and one death, was recorded for L. monocytogenes, and it appeared that the problem of L. monocytogenes was under control. However, recently in 1998 and 1999 the organism was found in surfaces and equipment of production lines of frankfurters, air condition lines, hot dogs, lunch meat, and turkey breasts and caused many outbreaks and recalls. One company recalled 30 million lb of ready-to-eat products due to L. monocytogenes. Another company recalled 15 million lb of hot dog and deli meat products due to an outbreak of L. monocytogenes that included 20 deaths—14 adults and 6 miscarriages/ stillbirths—and at least 97 illnesses in 22 states. There is a resurgence of concern about this organism due to the pathogenic nature of the illness involving miscarriages and stillbirth of fetuses.

L. monocytogenes has been isolated in a variety of commodities, including poultry carcasses, meat and chopped beef, dry sausages, milk and milk products, cheese, vegetables, and surface water. Control measures include eliminating occurrence of the organism in the raw food materials, transporting vehicles, and food-processing plants (especially in controlling cross-contamination of raw and finished products); in practicing good general sanitation of the entire food-processing environment; in regular monitoring of the occurrence of this organism in the food-processing facilities; and in preventing pregnant females from working in and around an environment that might have the possibility of exposing them to L. monocytogenes. Because the organism is killed by heat and is susceptible to sanitizing agents, proper cooking of food and decontamination of the food preparation environment will also help reduce risks. Much research has been devoted to the rapid isolation, enumeration, and identification of this organism. Many diagnostic kits, immunological systems, and genetic systems have been developed to rapidly screen this organism in the food supply.

Aeromonas hydrophia

Aeromonas hydrophia has been associated with foodborne infection, although the evidence is not conclusive. The organism is a facultative anaerobic, Gram-negative, motile rod. Biochemically, it is similar to E. coli and Klebsiella. The optimal temperature for growth is 28°C and the maximum is 42°C. Many strains can grow at 5°C, which is a temperature usually considered adequate to prevent growth of foodborne pathogens.

Diseases caused by A. hydrophila include gastroenteritis (cholera-like illness and dysentery-like illness) and extraintestinal infections such as septicemia and meningitis. This organism has been isolated from fish, shrimp, crabs, scallops, oysters, red meats, poultry, raw milk, vacuum-packaged pork and beef, and even bottled mineral water.

Because the organism is a psychrotroph, cold storage is not an adequate preventive measure. Proper heating of food offers sufficient protection against this organism. Consumption of undercooked food or raw food such as raw shellfish is discouraged.

Plesiomonas shigelloides

Plesiomonas shigelloides has been a suspect in foodborne disease cases. The organism is Gram-negative, facultative anaerobic, catalase negative, and fermentative. It is oxidase positive and can be differentiated from bacteria in the family Enterobacteriaceae by this test, since the latter is oxidase negative. The organism also resembles Shigella but can be differentiated from Shigella by being motile. It is capable of producing many diseases, ranging from enteritis to meningitis.

Gastroenteritis by P. shigellosides is characterized by diarrhea, abdominal pain, nausea, chills, fever, headache, and vomiting after an incubation time of one to two days. Symptoms last for a week or longer. All reported food involved with cases of gastroenteritis were from aquatic origin (salted fish, crabs, and oysters). The organism can be isolated from a variety of sources, including humans, birds, fish, reptiles, and crustaceans. The true nature of this organism as a foodborne agent is not fully known because the organism has not been well studied to date.

Miscellaneous Bacterial Foodborne Pathogens

Many other microbes are suspected of being foodborne pathogens. However, they are not currently being labeled as true foodborne pathogens owing to a lack of reports of these organisms, as well as a lack of isolation methods and research on these organisms. Many of these organisms may be identified as foodborne pathogens in the future. Among these organisms are the Gram-negative bacteria Citrobacter, Edwardsiella, Enterobacter, Klebsiella, Haf-nia, Kluyvera, Proteus, Providencia, Morganella, Serratia, Vibrios, and Pseudomonas and the Gram-positive bacteria Corynebacterium, Streptococcus, and other species of Bacillus and Clostridium. Miscellaneous organisms include Brucella, Mycobacterium (T, B), Coxiella burnetii (Q-fever), and Leptospirosis, Erysipelas, and Tularemia.

Foodborne Viruses

Foodborne viruses are much less studied by food microbiologists than are bacteria and fungi owing to the difficulty of cultivating these entities, as conventional bacteriologic media will not allow these particles to grow. There are, no doubt, many foodborne outbreaks and cases caused by a variety of viruses, but scientists in many cases are not able to identify the sources of the infection. Viruses that have been incriminated in foodborne diseases include hepatitis A virus (oysters, clams, doughnuts, sandwiches, and salad), Norwalk virus (oysters), polio virus (milk and oysters), ECHO virus (oysters), enteroviruses (oysters), and coxsackievirus (oysters). Much more research needs to be done in the field of food virology to help reduce the incidences of foodborne diseases caused by viruses.

Protozoa and Related Organisms

Protozoans such as Cryptosporidium, Cyclospora, Toxoplasma, Giardiasis, Entamoeba, Balantidium, and others can also cause human foodborne diseases. The most sensational outbreak was the one involving Cryptosporidium parvum, which affected 400,000 people and caused several deaths in Milwaukee in 1993. Cyclospora cayetanensis also was in the news due to imported fruits and caused a foodborne outbreak. These organisms have complex life cycles and are studied by specialists in this area. Recently a organism named Pfiesteria piscicida was responsible for killing million of fish in the Eastern shores of the United States. The organism has 24 life stages ranging from a cyst stage to a toxic zoospore phase to an amoeba stage. People in contact with water infected with this organism com plained of vomiting and liver problems, but no conclusive data are available on the pathogenicity of this organism on human.

Nonmicrobial Foodborne Disease Agents

Consumption of food containing other living organisms can directly and indirectly cause foodborne diseases as well. Among nonmicrobial foodborne disease agents are scombroid fish (associated with high level of histamine), ces-todes (flatworms such as Taenia saginata, T. solium, and Diphyllobothrium latum), nematodes (hookworm such as Trichinella spiralis), trematodes (fluke such as Clonorchis sinensis), shellfish (indirectly toxin from the dinoflagellate Gonyaulax catenella), ciguatera (from eating fish such as barracudas, groupers, and sea basses that feed on toxic algae), and other poisonous fish (such as puffer fish and moray eel).

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