Foodborne Bacterial Infections ◾ 361
member of the genus, but it causes plague in humans transmitted by rodents and their fleas.
Foodborne outbreaks with Yer. pestis is rare, but it has been associated with consumption of camel
meat and can cause severe gastroenteritis 35 Only Yer. enterocolitica and its association in foodborne
illnesses are discussed here.
Foodborne yersiniosis was first confirmed in the United States in 1976 following an outbreak
among a large number of school children from the consumption of chocolate milk contaminated
with Yer. enterocolitica. In many other countries, foodborne yersiniosis was recorded earlier. In
Denmark, yersiniosis is one of the most common forms of gastroenteritis. Following the first outbreak in the United States, Yer. enterocolitica was designated as an emerging foodborne pathogen.
Since the first outbreak in the 1970s, several others were recorded until the early 1980s. In 1982,
three outbreaks were recorded, one from the consumption of contaminated tofu packed with
contaminated spring water and the other two from the consumption of contaminated pasteurized
milk. But yersiniosis is not a frequent cause of foodborne infection in the United States. No incidents were reported between 1982 and 1987, but one was reported in 1988. However, recent data
suggest that an estimated 98,000 cases of human diseases occur as a result of Yer. enterocolitica
infection annually in the United States, and 90% of those are foodborne.
Characteristics
Yer. enterocolitica cells are gram-negative short rods (coccobacilli) that are non-spore-forming,
motile below 37°C, and facultative anaerobic. The strains grow between 0°C and 44°C with optimum growth at 25°C–29°C. Growth occurs in milk and raw meat at 1°C but at a slower rate.
Cells can grow in 5% NaCl and at a pH above 4.6. Cells are sensitive to pasteurization. There
are six major biotypes designated 1A, 1B, 2, 3, 4, and 5, and except biotype 1A, all biotypes are
pathogenic and carry a virulence plasmid, pVY. Certain strains of biovar 1A has been reported to
cause gastroenteritis and several outbreaks 36 The major pathogenic serotypes are O:1,2,3; O:2,3;
O:3; O:8; O:9; O:4,32; O:5,27; O:13a,13b; O:18; O:20; and O:21.
Habitat
Yer. enterocolitica is a normal inhabitant of intestines of food animals (pigs, cattle, sheep, and
goats) and birds, pets (dogs and cats), wild animals (rodents, hares, raccoons, foxes), and humans.
Pigs serve as a major reservoir, and pork products are thought to be the primary source for human
infection. Human carriers do not show any disease symptoms. Different types of food can be
contaminated from these sources.
Virulence Factors
Not all strains can cause yersiniosis. Most strains isolated from the environment are nonpathogenic. Pathogenic strains are predominant in pigs. Both the pathogenic and nonpathogenic strains
produce a heat stable enterotoxin (Yst); thus, toxin production is not directly related to the ability
of a strain to cause yersiniosis. However, toxin production is thought to be temperature dependent,
producing toxins when grown at 25°C but not at 4°C. Pathogenic strains carry several virulence
factors encoded in chromosomes (Invasin, AiL, Yst) and in a 70-kb virulence plasmid (pVY),
which encodes virulence genes for YadA (Yersinia adhesion protein), and Yersinia outer membrane
proteins, such as YopH, YopB, YopD, YopE, etc., required for adhesion, invasion, anti-phagocytic
action, and colonization in intestinal epithelial cells and lymph nodes. Only after colonization
member of the genus, but it causes plague in humans transmitted by rodents and their fleas.
Foodborne outbreaks with Yer. pestis is rare, but it has been associated with consumption of camel
meat and can cause severe gastroenteritis 35 Only Yer. enterocolitica and its association in foodborne
illnesses are discussed here.
Foodborne yersiniosis was first confirmed in the United States in 1976 following an outbreak
among a large number of school children from the consumption of chocolate milk contaminated
with Yer. enterocolitica. In many other countries, foodborne yersiniosis was recorded earlier. In
Denmark, yersiniosis is one of the most common forms of gastroenteritis. Following the first outbreak in the United States, Yer. enterocolitica was designated as an emerging foodborne pathogen.
Since the first outbreak in the 1970s, several others were recorded until the early 1980s. In 1982,
three outbreaks were recorded, one from the consumption of contaminated tofu packed with
contaminated spring water and the other two from the consumption of contaminated pasteurized
milk. But yersiniosis is not a frequent cause of foodborne infection in the United States. No incidents were reported between 1982 and 1987, but one was reported in 1988. However, recent data
suggest that an estimated 98,000 cases of human diseases occur as a result of Yer. enterocolitica
infection annually in the United States, and 90% of those are foodborne.
Characteristics
Yer. enterocolitica cells are gram-negative short rods (coccobacilli) that are non-spore-forming,
motile below 37°C, and facultative anaerobic. The strains grow between 0°C and 44°C with optimum growth at 25°C–29°C. Growth occurs in milk and raw meat at 1°C but at a slower rate.
Cells can grow in 5% NaCl and at a pH above 4.6. Cells are sensitive to pasteurization. There
are six major biotypes designated 1A, 1B, 2, 3, 4, and 5, and except biotype 1A, all biotypes are
pathogenic and carry a virulence plasmid, pVY. Certain strains of biovar 1A has been reported to
cause gastroenteritis and several outbreaks 36 The major pathogenic serotypes are O:1,2,3; O:2,3;
O:3; O:8; O:9; O:4,32; O:5,27; O:13a,13b; O:18; O:20; and O:21.
Habitat
Yer. enterocolitica is a normal inhabitant of intestines of food animals (pigs, cattle, sheep, and
goats) and birds, pets (dogs and cats), wild animals (rodents, hares, raccoons, foxes), and humans.
Pigs serve as a major reservoir, and pork products are thought to be the primary source for human
infection. Human carriers do not show any disease symptoms. Different types of food can be
contaminated from these sources.
Virulence Factors
Not all strains can cause yersiniosis. Most strains isolated from the environment are nonpathogenic. Pathogenic strains are predominant in pigs. Both the pathogenic and nonpathogenic strains
produce a heat stable enterotoxin (Yst); thus, toxin production is not directly related to the ability
of a strain to cause yersiniosis. However, toxin production is thought to be temperature dependent,
producing toxins when grown at 25°C but not at 4°C. Pathogenic strains carry several virulence
factors encoded in chromosomes (Invasin, AiL, Yst) and in a 70-kb virulence plasmid (pVY),
which encodes virulence genes for YadA (Yersinia adhesion protein), and Yersinia outer membrane
proteins, such as YopH, YopB, YopD, YopE, etc., required for adhesion, invasion, anti-phagocytic
action, and colonization in intestinal epithelial cells and lymph nodes. Only after colonization