Laboratory analysis for Salmonella

Salmonella is a rod-shaped gram-negative bacterium of the Enterobacteriaceae family. It was named after Daniel Elmer Salmon who administered the Veterinary Division of the United States Department of Agriculture.

Salmonella causes food poisoning in which young children and older adults are at most risk. Common symptoms are fever, diarrhea, and abdominal cramps. Contamination often results from improper food processing and handling. Foods contaminated with Salmonella usually smell and look normal. Therefore, laboratory testing is important to determine its presence in food samples.

Laboratory analysts test for Salmonella in various food samples such as milk powders, pasteurized milk, pasteurized cream, yogurt, cheese and cheese products, margarine, ice cream, spices, meat and meat by-products, and chocolates.

Sample preparation is generally done by weighing a specified amount (usually 25 g) and addition of lactose broth (or other medium depending on the type of sample). Adjustment of pH might be necessary depending on the medium and samples chosen.

Incubation is usually done at 350C for 24 hours. After the incubation, 1 mL mixture from the suspected samples will be transferred to a broth depending on the sample type. Incubation at 350C for 24 hours is required here before the next step.

After the incubation, you do a streaking (a loopful) from the previous broth into the agar medium such as hektoen enteric (HE) agar or xylose lysine desoxycholate (XLD) agar. The plates will then be incubated for 24 hours at 350C. After the incubation, the analysts will examine the plates for the presence of Salmonella colonies.

The appearance of the Salmonella colonies will differ depending on the agar medium you chose earlier. On HE agar, the colonies will appear blue to blue-green. On XLD agar, there might be pink colonies if Salmonella is present in the sample.

Additional biochemical tests will be required to confirm the presence of Salmonella. After performing careful confirmation tests, you can now refer to standards and requirements set by international agencies.

The reference criteria for the presence of Salmonella in food samples are strict. The number of samples might also be increased depending on the risk to the population. For example, 30 is the required minimum number of samples in the testing of milk powders for high-risk population.

Out of five 25-gram samples, all of them should test negative for the presence of Salmonella. That’s why it’s important to be careful from the first step up to the confirmatory stage in the analysis of Salmonella in food samples.