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From: Food Quality & Safety magazine, October/November 2007

DIY Staph Testing

Monitor for Staphylococcus aureus so managing the results can be easily accomplished in house

by Jody L. Huemoeller

In-house testing for microorganisms such as Escherichia coli is routine for most food manufacturers. An increasing number of companies, however, are also performing in-house testing for Staphylococcus aureus. In fact, 62% of the 1,400 manufacturers surveyed in 2000 were already doing their own evaluations.

One of those manufacturers is The Schwan Food Company (Marshall, Minn.), a producer of frozen foods that serves approximately 50 countries through home delivery, grocery stores, online shopping, and sales to food service. The company has multiple brands and offers pizza, entrees, convenience foods, fruit, vegetables, bakery items, and desserts.

Some manufacturers may have concerns about in-house testing for S. aureus since it is classified as a pathogen. “However, there is a difference between Staph and Listeria or Salmonella,” says Anne Sherod, director of Food Safety, Microbiology, at Schwan’s Research and Development, Inc.

“Illness is not caused directly by the Staph organism. Any illness would be due to enterotoxin, which is produced by Staph aureus, and a large number of Staph aureus organisms would be required to produce sufficient enterotoxin to cause illness,” Sherod adds. She insists that if well-trained personnel employ appropriate methodologies and good laboratory practices, there should not be any more or any less risk involved in testing for S. aureus in house than in testing for any other organism. Sherod identifies four key steps to follow in setting up an in-house testing program for S. aureus: establishing, monitoring, evaluating, and correcting.

Before testing begins, determine acceptable microbiological criteria for sampling of S. aureus, along with limits on where enterotoxin testing should be performed. “Base those levels on literature, past experience in your plant, ingredients, finished products and processes, and the potential for toxin production in your setting,” says Sherod. “Staph aureus is generally found on animals, but humans can also carry Staph aureus.” She adds that an effective testing program can provide verification of a hazard analysis and critical control point (HACCP) system.

Monitoring Manufacturing

“Manufacturers must understand their processes as well as the potential for Staph aureus growth and cross-contamination,” Sherod says. “Consider where Staph aureus could be found or grow to levels that would produce enterotoxin.”


She recommends scheduled monitoring of ingredients; components such as batters, mixes, or fillings; work in progress; and finished products and manufacturing equipment, including dipping bays, waterfalls, or recirculating systems. “Look for equipment that is aging or no longer fully cleanable, and determine if the equipment needs repair. For instance, do small parts like O-rings need to be replaced to avoid hosting microorganisms?”

Monitoring employee behavior is also necessary to guarantee that good practices are being followed. For instance, make sure that employees are wearing gloves appropriately and changing them often enough.

Evaluating Samples

The Schwan Food Company uses a two-part approach to test for S. aureus. Primary testing identifies levels of S. aureus present. Depending on those results, secondary testing may be done to determine if enterotoxin has been produced.

Schwan uses Petrifilm Staph Express Count Plates from 3M (St. Paul, Minn.) for their primary testing; the plates are approved as an official method of analysis by AOAC International, an organization that provides and facilitates validated analytical methods and laboratory quality assurance programs. The sample-ready plates are coated with rehydratable nutrients, a cold-water-soluble gelling agent, and chromogenic, modified Baird-Parker medium, which is selective and differential for S. aureus. Only one incubation temperature is required.

While equivalent to the bacteriological analytical manual, three plate, Baird-Parker agar and tube coagulase method, Petrifilm Staph Express Count Plates produce results more quickly. Distinctive, red-violet colonies are counted as S. aureus and appear within 22 to 29 hours instead of 69 to 78 hours.

Sherod says The Schwan Food Company chose the 3M plates because they are accurate, easy to use, and require much less space than conventional plates. She adds that they also eliminate the time-consuming and labor-intensive media preparation required with traditional pour plates. “Testing with Petrifilm plates also gives us critical data we need to maintain our HACCP programs.”

The results of testing with Petrifilm plates determine the next steps taken, Sherod says. “We have internal specifications that govern our procedures. If Staph aureus organisms are present at certain levels in a particular ingredient, work in process, or finished product, we do secondary testing to determine whether enterotoxin is present.”

Schwan uses the Staph enterotoxin 2 or SET 2 method for secondary testing. At this stage, some companies may seek support from an outside laboratory to provide secondary testing or confirm their in-house results.

Making Necessary Corrections

A predetermined corrective action plan is another important aspect of an effective in-house testing program. Sherod recommends having a written plan ready before beginning the testing. If S. aureus is detected at unacceptable levels, then appropriate action can be taken.

“If our in-house testing indicates conditions are out of specification, we do a root cause analysis to find the ingredient, work in process, or finished product of concern. If the problem were in an ingredient, we would go back to the supplier to address the situation with them. If the problem were internal, we would revisit our good manufacturing processes with personnel who were working in the area where the problem occurred or take other appropriate corrective actions based on the root cause investigation,” Sherod says.

By following these steps, testing for S. aureus and managing the results can be done easily in house. Using proven equipment and count plates can facilitate the process by providing valuable time and labor savings.

Huemoeller is a market development supervisor at 3M; reach her at


  1. Downes FP, Ito K, eds. Compendium of Methods for the Microbiological Examination of Foods. 4th ed. Washington, D.C.: American Public Health Association; 2001.
  2. Jay JM. Modern Food Microbiology. 6th ed. Gaithersburg, Md: Aspen Publishers; 2000.



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