Enumeration of Salmonella Bacteria in Food and Feed Samples by PCR

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Enumeration of Salmonella Bacteria in Food and Feed Samples by Real-Time PCR for Quantitative Microbial Risk Assessment

Burkhard Malorny, Charlotta Lofstrom, Martin Wagner, Nadine Kramer, and Jeffrey Hoorfar
Appl. Environ. Microbiol. 2008. 74:1299-1304.

Link to full pdf version of publication: http://aem.asm.org/cgi/reprint/74/5/1299.pdf

Quantitative microbial risk assessment (QMRA) is an important approach for food safety in which risk and factors that influence food safety are identified. The goal is to provide an estimate of the level of illness that a pathogen can cause in a given population (13). For QMRA, there is a need for microbiological methods that generate quantitative data. Furthermore, the sample preparation methods preceding the analytical method itself (e.g., PCR) need to be able to produce quantitative results.

Salmonellosis is one of the most important food-borne disease and causes substantial medical and economic burdens worldwide (9, 30). Food is the main source of infection by Salmonella in humans. Beside eggs, egg products, and poultry meat, pork is one of the most important sources of human salmonellosis (4, 17). Because of this, a number of actions have been taken to reduce the prevalence of Salmonella serovars with public health significance in food-producing animals (3), including a QMRA study of Salmonella in slaughter and breeder pigs (10). QMRA is still hampered by the lack of quantitative data, and often assumptions that generate high degrees of uncertainty have to be included. The generation of appropriate data with high sensitivity is a challenge for microbiologists since currently used bacteriological quantitation methodologies are laborious.

In the past, it was shown that the severity of salmonellosis and the percentage of infected humans after consumption of food are associated with the level of contamination (dose-response relationship [12]). Moreover, the infection dose of Salmonella depends on the food item itself. Salmonellae can enter the food chain at every stage, and the consequences for humans after consumption of the contaminated end product depend on the food-processing conditions. For example, a well-known source of contamination is the lairage environment of slaughterhouses for incoming nonaffected animals (6). Later, salmonellae can multiply to harmful levels due to inappropriate storage conditions. Generally, Salmonella does not grow at temperatures below 6°C for as long as 15 days on chicken meat, while significant growth has been reported at 8°C (27). However, some other reports have indicated that growth at 2 to 7°C might occur (8). Furthermore, quantitative salmonella data for foods associated with severe outbreaks have shown that the type of food plays a major role in the severity of illness. Salmonellae in fatty foods may have an advantage during passage through the acidic environment of the stomach to the intestine, where the cells become invasive regardless of the damage caused by the acids.

Very low numbers of Salmonella cells are typically found in food, feed, and environmental samples (6, 7, 11, 29). Carcasses may be contaminated during transport or slaughtering, resulting in low levels and uneven distribution. However, such contamination may be fatal because of the possibility of multiplication of the cells on the meat, leading to a high risk for consumers. Consequently, to identify critical contamination points and to provide risk modelers with quantitative data for each processing chain, cost-effective methods that can also enumerate low levels of Salmonella are needed.