346 ◾ Fundamental Food Microbiology
may form short chains, but under stress conditions, such as in the presence of high salt (>10%) or
temperature (>45°C), the cells may appear elongated or be present in long chains. It is hemolytic
and produces β-hemolysis on sheep or horse blood agar plates. In the CAMP (Cristie, Atkinson,
Munch, Peterson) test, in the presence of β-lysin positive Sta. aureus, it produces a characteristic
large zone of hemolysis at the junction of two cultures on the blood agar plate, and this property
allows differential identification of Lis. monocytogenes from hemolytic Lis. ivanovii, which does
not produce such a hemolytic zone with Sta. aureus. In contrast, Lis. ivanovii exhibits a positive
CAMP test (zone of hemolysis) at the junction when streaked with Rhodococcus equii.
Lis. monocytogenes is a psychrotroph and grows between 1°C and 44°C with optimum growth
at 30°C–37°C. At 7°C–10°C, it multiplies relatively rapidly. It ferments rhamnose but not xylose
and ferments glucose without producing gas. It can grow in many foods and environments. The
cells are also relatively resistant to freezing, drying, high salt (>10%) and pH 5.0 and above. Lis.
monocytogenes is also acid tolerant, which is essential for survival in stomach acid. The acid tolerance is a result of the presence of a glutamate decarboxylase (GAD) enzyme system. It is sensitive
to pasteurization temperature (71.7°C for 15 seconds or 62.8°C for 30 minutes), but when inside
the white blood cells, a temperature of 76.4°C–77.8°C for 15 seconds is required to kill the cells.
Habitat
Lis. monocytogenes is primarily a saprophyte 9 and is isolated from many environmental samples, such
as soil, sewage, water, and dead vegetation. It is isolated from the intestinal contents of domesticated
animals and birds. Humans can also carry the organisms in the intestine without any symptoms. As
carriers, humans may harbor the organism in the gall bladder. A large proportion of uncooked meat,
milk, egg, seafoods, and fish, as well as leafy vegetables and tubers (potatoes and radishes, in particular), are found to contain Lis. monocytogenes. Many heat-processed foods, such as pasteurized milk and
dairy products, and ready-to-eat meat preparations have also been found to contain the organism. Lis.
monocytogenes is isolated in high frequency in different places of food processing and storage areas and
are responsible for post-processing contamination of RTE products. 10,11
Virulence Factors
Lis. monocytogenes possesses a large number of virulence factors (Table 26.4) that are essential for
pathogenesis. 12,13 The virulence genes are located primarily in a 9.0 kb region in the chromosome
called the pathogenicity island (PAI). A majority of these genes are regulated by two regulatory
factors; protein regulatory factor (PrfA) and sigma B. Carbon sources, temperature, salt, bile salts,
and acidic and anaerobic environments differentially regulate the expression of virulence genes
and thus affect the severity of infection in a host.
Disease and Symptoms
Listeria monocytogenes causes two forms of diseases: (1) febrile gastroenteritis and (2) invasive systemic diseases. 12,13
Febrile Gastroenteritis
The exact mechanism of gastroenteritis is not known; however, epidemiological study suggests
that this form is mostly associated with healthy individuals, and the infectious dose is in the range
may form short chains, but under stress conditions, such as in the presence of high salt (>10%) or
temperature (>45°C), the cells may appear elongated or be present in long chains. It is hemolytic
and produces β-hemolysis on sheep or horse blood agar plates. In the CAMP (Cristie, Atkinson,
Munch, Peterson) test, in the presence of β-lysin positive Sta. aureus, it produces a characteristic
large zone of hemolysis at the junction of two cultures on the blood agar plate, and this property
allows differential identification of Lis. monocytogenes from hemolytic Lis. ivanovii, which does
not produce such a hemolytic zone with Sta. aureus. In contrast, Lis. ivanovii exhibits a positive
CAMP test (zone of hemolysis) at the junction when streaked with Rhodococcus equii.
Lis. monocytogenes is a psychrotroph and grows between 1°C and 44°C with optimum growth
at 30°C–37°C. At 7°C–10°C, it multiplies relatively rapidly. It ferments rhamnose but not xylose
and ferments glucose without producing gas. It can grow in many foods and environments. The
cells are also relatively resistant to freezing, drying, high salt (>10%) and pH 5.0 and above. Lis.
monocytogenes is also acid tolerant, which is essential for survival in stomach acid. The acid tolerance is a result of the presence of a glutamate decarboxylase (GAD) enzyme system. It is sensitive
to pasteurization temperature (71.7°C for 15 seconds or 62.8°C for 30 minutes), but when inside
the white blood cells, a temperature of 76.4°C–77.8°C for 15 seconds is required to kill the cells.
Habitat
Lis. monocytogenes is primarily a saprophyte 9 and is isolated from many environmental samples, such
as soil, sewage, water, and dead vegetation. It is isolated from the intestinal contents of domesticated
animals and birds. Humans can also carry the organisms in the intestine without any symptoms. As
carriers, humans may harbor the organism in the gall bladder. A large proportion of uncooked meat,
milk, egg, seafoods, and fish, as well as leafy vegetables and tubers (potatoes and radishes, in particular), are found to contain Lis. monocytogenes. Many heat-processed foods, such as pasteurized milk and
dairy products, and ready-to-eat meat preparations have also been found to contain the organism. Lis.
monocytogenes is isolated in high frequency in different places of food processing and storage areas and
are responsible for post-processing contamination of RTE products. 10,11
Virulence Factors
Lis. monocytogenes possesses a large number of virulence factors (Table 26.4) that are essential for
pathogenesis. 12,13 The virulence genes are located primarily in a 9.0 kb region in the chromosome
called the pathogenicity island (PAI). A majority of these genes are regulated by two regulatory
factors; protein regulatory factor (PrfA) and sigma B. Carbon sources, temperature, salt, bile salts,
and acidic and anaerobic environments differentially regulate the expression of virulence genes
and thus affect the severity of infection in a host.
Disease and Symptoms
Listeria monocytogenes causes two forms of diseases: (1) febrile gastroenteritis and (2) invasive systemic diseases. 12,13
Febrile Gastroenteritis
The exact mechanism of gastroenteritis is not known; however, epidemiological study suggests
that this form is mostly associated with healthy individuals, and the infectious dose is in the range