Listeria Monocytogenes
Description
Listeria monocytogenes is a gram-positive motile facultative anaerobe that inhabits a variety of environments. Using selective media it can be readily isolated from soil, water, vegetation and processed products, including ready-to-eat products designated for human consumption (Graves et al., 1992). The bacterium was named monocytogenes because one distinguish characteristic of infection in rabbits, which was the production of monocytosis in blood (Shuin et al., 1982).
L. monocytogenes is phychrophilic, has a temperature for growth of 0°C to 45°C (Barbosa et al., 1994), and enjoys a competitive advantage against other gram-positive and gram-negative microorganisms in cold environments, such as refrigerators. Recent investigations indicate that the organism can initiate growth at pH values as low as 4.4. Also, it grows optimally at water activity of 0.97. For the majority of strains, the minimum water activity needed for growth is 0.93 (Lou et al., 1999); however, some strains may be able to grow at water activity values as low as 0.90 and survive for long periods of time at 0.83 (Shahamat et al., 1980).
L. monocytogenes is able to grow in the presence of 10 to 12% sodium chloride; it grows to high populations in moderate salt concentrations (6.5%). The bacterium survival in high-salt environments is significantly increased by lowering the temperature.
Route of Exposure
L. monocytogenes is particularly interesting as a foodborne pathogen in that it is ubiquitous in nature. The current understanding of human listeriosis epidemiology suggests that the organism is a common contaminant of food products. This contamination usually takes place on the surface of the products, with up to 15% harboring the organism. The presence of L. monocytogenes in the food processing chain is evidence by the widespread distribution of the listeriae in processed products.
Occurrence of sporadic listeriosis appears to be more common in the spring and summer months. This could
Listeria monocytogenes is a gram-positive motile facultative anaerobe that inhabits a variety of environments. Using selective media it can be readily isolated from soil, water, vegetation and processed products, including ready-to-eat products designated for human consumption (Graves et al., 1992). The bacterium was named monocytogenes because one distinguish characteristic of infection in rabbits, which was the production of monocytosis in blood (Shuin et al., 1982).
L. monocytogenes is phychrophilic, has a temperature for growth of 0°C to 45°C (Barbosa et al., 1994), and enjoys a competitive advantage against other gram-positive and gram-negative microorganisms in cold environments, such as refrigerators. Recent investigations indicate that the organism can initiate growth at pH values as low as 4.4. Also, it grows optimally at water activity of 0.97. For the majority of strains, the minimum water activity needed for growth is 0.93 (Lou et al., 1999); however, some strains may be able to grow at water activity values as low as 0.90 and survive for long periods of time at 0.83 (Shahamat et al., 1980).
L. monocytogenes is able to grow in the presence of 10 to 12% sodium chloride; it grows to high populations in moderate salt concentrations (6.5%). The bacterium survival in high-salt environments is significantly increased by lowering the temperature.
Route of Exposure
L. monocytogenes is particularly interesting as a foodborne pathogen in that it is ubiquitous in nature. The current understanding of human listeriosis epidemiology suggests that the organism is a common contaminant of food products. This contamination usually takes place on the surface of the products, with up to 15% harboring the organism. The presence of L. monocytogenes in the food processing chain is evidence by the widespread distribution of the listeriae in processed products.
Occurrence of sporadic listeriosis appears to be more common in the spring and summer months. This could
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