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 be explained by seasonal variations in the type of food products eaten by human populations, with higher-risk products eaten in the warmer months. Some outbreaks investigations suggest that certain ready-to-eat processed foods pose a high risk of contracting listeriosis for susceptible populations. These foods are usually preserved by refrigeration and offer an appropriate environment for the multiplication of L. monocytogenes during manufacture, aging, transportation and storage.
The entry of L. monocytogenes into food processing plants occurs through soil on clothing or equipments, contaminated hides or surfaces, and possibly healthy human carriers. The humidity and presence of nutrients support the growth of Listeria, which is commonly found in moist areas such as processing equipment, drains, etc (Cox et al., 1989). In addition, Listeria can attach to different types of surfaces and biofilm formation has been observed in meat and dairy processing environment (Jeong et al., 1994).
Post-processing contamination is the most likely route of contamination of processed foods. Currently, there is no evidence to indicate that L. monocytogenes can survive heat processing protocols. However, because it is a frequent contaminant of raw material used in food processing plants, there are plenty opportunities for reintroduction of listeriae into food processing facilities by cross contamination (Doyle, 1988). If the product is contaminated during its elaboration, the bacteria can survive and multiply throughout storage causing disease when it is consumed. In conclusion, epidemiologic investigations have repeatedly revealed that the consumption of contaminated food is the primary mode of transmission of listeriosis. Food has been identified as the vehicle of several major outbreaks of listeriosis investigated since 1981.
Some of the food products associated with L. monocytogenes include unpasteurized milk and products prepared from unpasteurized...