Fundamental Food Microbiology, Third Edition - Fuad Fathir

368 FUNDAMENTAL FOOD MICROBIOLOGY
However, an understanding of the type of foods that are mostly involved in listeriosis,
the food processing steps that can contaminate ready-to-eat foods with this pathogen,
and the special groups of people most susceptible to the disease helped regulatory
agencies develop procedures to reduce the number of foodborne listeriosis. These
have been achieved through changes in the processing steps and testing of foods so
that contaminated ready-to-eat foods do not reach consumers, and by educating
susceptible consumer groups about food choices, eating habits, and sanitary practices
in food preparation. As a result, in developed countries, the number of listeriosis
cases has dropped; in the U.S., the number of human listeriosis cases has fallen from
2000 cases in 1986 to ca. 1000 cases in 1991.
The genus Listeria contains several species, of which Lis. monocytogenes is
considered to be pathogenic. The species has several serogroups: 1/2a, 1/2b, 1/2c,
3a, 3b, 3c and 4b. Whereas 1/2a and 1/2b were the predominant serogroups isolated
in foodborne human listeriosis in Europe, 4b was predominant in Canada and the U.S.
B. Characteristics
Lis. monocytogenes is a Gram-positive, psychrotrophic, facultative anaerobic, nonsporulating,
motile, small rod. In fresh culture, the cells may form short chains. It
is hemolytic and ferments rhamnose but not xylose.
Lis. monocytogenes is a psychrotroph and grows between 1 and 44�C, with
optimum growth at 35 to 37�C. At 7 to 10�C, it multiplies relatively rapidly. It
ferments glucose without producing gas. It can grow in many foods and environments.
The cells are relatively resistant to freezing, drying, high salt, and pH 5.0
and above. They are sensitive to pasteurization temperature (71.7�C for 15 s or
62.8�C for 30 min), but when inside the white blood cells, a temperature of 76.4 to
77.8�C for 15 s is required to kill the cells. 9,10,12
C. Habitat
Lis. monocytogenes 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. A large proportion of uncooked meat, milk, egg, seafoods,
and fish, as well as leafy vegetables and tubers (potatoes and radishes, in particular),
contains Lis. monocytogenes. Many heat-processed foods, such as pasteurized milk
and dairy products, and ready-to-eat meat preparations also contain the organism.
Lis. monocytogenes is isolated in high frequency from different places of food
processing and storage areas. 10,14
D. Toxin
The virulence factor of Lis. monocytogenes is a specific type of hemolysin, listeriolysin
O. It is produced during the exponential growth of the cells. The pathogens
invade different body tissues and multiply inside the body cells, releasing the toxin.
The toxin causes death of the cells. 9,10,14