Microbiological testing

Every aspect of the world — including humans, animals and plants — contains bacteria.  Bacteria serve a critical role in ecology of our planet and the day-to-day activities of our lives.  Some bacteria — like those used to produce food products such as yogurt, cheese and fermented sausages — are considered “good bacteria.”  These “good bacteria” provide a positive purpose like flavor and color development.  Other, much rarer types of bacteria can be harmful to health.  These bacteria are called “pathogens.”   Since bacteria are not visible to the naked eye, detection and measurement of the levels of bacteria is a sophisticated scientific process involving a variety of technologies to analyze and interpret the test results.

Reducing pathogens on products is the top priority for the meat and poultry industry, but it is no simple task.  A century ago, the major food safety challenge facing the meat industry was the threat of animal diseases, which could be diagnosed through physical examination.  But today’s food safety challenges are not that simple;  eliminating invisible pathogens is far more complex.

Food safety experts, including the National Academy of Sciences and the National Advisory Committee on Microbiological Criteria for Foods, agree that reducing pathogens – especially on raw, unprocessed foods – requires a farm-to-table approach.  When everyone in the production, processing, distribution and preparation chain understands their respective roles in ensuring safety, the safest possible meat and poultry supply is possible.

HACCP approach

The cornerstone of the meat and poultry industry’s preventive food safety approach is a system called HACCP (Hazard Analysis and Critical Control Point).  HACCP was developed for the food industry by Pillsbury in the 1960s to make food safe for the emerging space program.  The system has been adopted by many food processors during the past 50 years and is considered the “gold standard” food safety process management system.

Using HACCP, meat and poultry companies carefully analyze processes used for each product they make and identify critical control points (CCP) – steps in the production process where potential biological, physical, and/or chemical hazards can be controlled.  A CCP could include ensuring the proper cooling of meat to control bacterial growth, or thorough cooking of ready-to-eat products like deli meats to kill bacteria.  Once these CCPs are identified, companies implement and monitor CCPs to control and document their process – a key component of ensuring their food safety system is working correctly.  This type of approach identifies the most high risk parts of the food manufacturing process and allows companies to focus resources accordingly.

Another component of verifying a company’s food safety process management system is effective is by utilizing microbiological testing.  Scientific research indicates that all raw agricultural products – including meat and poultry — naturally may contain bacteria, even pathogenic bacteria.  A meat and poultry company’s HACCP plans must reflect this fact and address the potential hazards. While a meat and poultry plant’s goal is to reduce all bacteria, it is not possible to determine with 100 percent certainty the presence or absence of pathogenic bacteria.  Why?  Because each microbiological test destroys the sample that is tested.  And results only apply to the tested sample.  It is possible to pull a 300 gram sample of ground beef, test it and receive a negative result despite pathogen’s presence elsewhere in the larger batch.    Therefore, pathogen testing of raw meat and poultry should not be a measure of plant’s success or failure, but used as a tool within a well-designed HACCP plan to verify the food safety system is working correctly.

What does HACCP suggest about microbial testing of raw meat and poultry?  Under HACCP programs, much of the microbial testing is done to track amounts of commonly found generic bacteria — the harmless bacteria that naturally exist in measurable quantities on raw meat and poultry.  The levels of these generic bacteria on meat and poultry are used as an indicator of how well a plant is succeeding in eliminating the much rarer and harder to find pathogenic strains.

For example, a meat company might routinely test its raw ground beef for total aerobic bacteria in a test commonly called “total plate count,” to make sure the quantity of microbes falls within the company’s normal range.  The “total plate count” is then compared to historical data from that facility and can demonstrate if the facility is operating under control.  For instance, if the company averages 1,000 organisms per sample, and several samples in a row fall significantly higher outside the normal range, the plant would implement its action plan to identify potential causes for the increasing microbial counts.  The increase does not necessarily mean that the product is unsafe, since cooking a raw product destroys bacteria, but it may indicate that perhaps something in the process had changed and should be examined to ensure the food safety process management system is still effective.

Testing for bacteria levels in a food product can be compared to polling.  Much like voter polls leading up to an election are based on a small sample of the voting public, bacterial test results are merely a representative  estimate of the amount of bacteria that may be present in the product.  Just like with the voting polls, bacterial tests are subject to error based on many factors such as the size of the sample, how representative the sample is of the total population, and most importantly, what specific bacteria are being detected.  If the test is designed to detect a broad class of bacteria, the results are likely to yield larger numbers and will provide information that is more meaningful.  On the other hand, if the test is designed to detect one specific type of bacteria that is rare, the results are likely to yield inconclusive and less reliable results.  Many of the test results from this type of testing will simply provide “zero” or “negative” data.

The challenge of microbiological testing

First, one must recognize the greatest complication to microbial testing is the fact that the testing process destroys the sample.  Therefore, to know with 100 percent certainty whether the container of ground beef or poultry contains a pathogen, the entire container must be tested and thereby destroyed.

It is also important to understand the limitations of microbiological testing of meat and poultry.  For example, let’s say a tablespoon sample is taken from a 100 pound container of ground beef and tested for a pathogenic bacterium.  The test result may be negative for the pathogen, but that test result only applies to the tablespoon of ground beef.  The pathogen may actually be in other places in the container, though the sample that was selected simply did not contain it.

In fact, sampling probabilities indicate that in order to find one percent of Escherichia coli O157:H7 in a “lot” of ground beef with a 95 percent confidence rate, 299 samples would need to be taken and tested.  This is compared to 2,995 samples that would need to be taken to detect 0.1 percent of Escherichia coli O157:H7 with a 95 percent confidence rate.  Currently, the U.S. Department of Agriculture’s Food Safety and Inspection Service (FSIS) data reports the prevalence level of E. coli O157:H7 in 2010 was 0.24 percent, which means thousands of tests would be needed to detect that level.  Again, each test would destroy the sample.  For these reasons, pathogen testing and “pass/fail” standards do little to enhance meat safety while destroying large quantities of safe product that could have been consumed safely and purchased cheaper.

Different ecology

It is also important to understand that different types of bacteria originate from different sources.  For instance, E. coli O157:H7 and other shiga toxin-producing E. coli originate from the live animal and are introduced into a meat packing plant either on the hides of cattle or from the digestive tract of the animal, which can sometimes inadvertently contaminate carcasses during processing.  Other bacteria, like Listeria, are environmental bacteria that are found in soil and water and have been shown to establish niches in floor drains, cracks and crevices in food processing facilities and can thrive in cold, moist environments like meat packing plants and even home refrigerators.  Because each bacterium has a unique origin and ecology, each requires a different strategy to prevent it from contaminating meat and poultry.

Tests commonly performed

The meat and poultry industry is regulated by the U.S. Department of Agriculture (USDA).  USDA requires several different tests, some of which are conducted by plants and some of which are collected by USDA inspectors and run in USDA laboratories.

Tests and testing programs that may be run in federally inspected meat and poultry plants can include,

  • Baseline data collection.  USDA’s FSIS established a series of tests to acquire microbiological profiles of the meat and poultry for selected microorganisms, such as E. coli O157:H7 and Salmonella.  These baseline studies are used to develop new prevention programs and develop new pathogen reduction performance standards for plants to meet. Over time, these profiles will help measure the effects of changes in slaughtering and processing on the levels of pathogens on raw products.
  • Generic E. coli for carcasses. All federally inspected plants that slaughter livestock and poultry are required to test for generic E. coli to verify that their process control systems work as intended to prevent contamination.  Test results that reveal marginal or unacceptable levels of E. coli that deviate from a plant’s average E. coli counts can indicate the plant needs to review process controls and take corrective actions
  • Salmonella for carcasses and raw ground products. To verify that a plant’s HACCP systems are effectively controlling contamination in raw products, the Pathogen Reduction/HACCP final rule instituted a Salmonella performance standard that plants must meet.  Products covered by the standard include carcasses of cattle, swine and broilers; and ground beef, ground chicken and ground turkey.  The pathogen reduction standard is based on the national prevalence of Salmonella in each of these products.  Plants must conduct a series of tests for Salmonella (the numbers and intervals vary for each product).  When a positive sample is found, plants must take corrective actions to prevent Salmonella contamination.  Progress reports indicate that industry continues to reduce Salmonella prevalence on meat and poultry products.
  • E. coli O157:H7 for ground beef. USDA collects roughly 10,000 samples of ground beef and raw material used to make ground beef per year in plants, retail stores, import facilities, and tests these samples for the presence of E. coli O157:H7.  When E. coli O157:H7 is found in raw ground beef, the product is deemed to be “adulterated” or unfit for consumption.  If the pathogen is found on a whole muscle cut like a steak, USDA does not consider it adulterated.  USDA treats these products differently because the inside of muscle meat is sterile, and external bacteria on cuts like steaks are destroyed when the steak is heated – even if eaten rare.  The process of grinding or tenderizing beef, however, distributes bacteria through the meat, making it essential that ground or tenderized beef products be cooked thoroughly.  Given these distinctions, USDA treats the presence of E. coli O157:H7 on ground or “non-intact” (e.g., tenderized) beef and whole muscle beef differently.  In addition to government tests, many companies conduct their own tests for E. coli O157:H7 voluntarily, sometimes in an effort to meet customer specifications which require such testing, or to verify CCPs.  The meat and poultry industry takes greater than one million E. coli O157:H7 tests per year.  FSIS also collects samples from cooked, ready-to-eat meat patties and dry fermented sausage in federally inspected plants.  When E. coli O157:H7 is found on these products, they also are considered adulterated.
  • Generic Listeria in the environment of a ready-to-eat plant.
  • Listeria monocytogenes and Salmonella testing in ready-to-eat products and poultry products.  Ready-to-eat products or fully cooked are much as they sound:  intended to be eaten right out of the package.  For this reason, in the U.S., there is a “zero tolerance policy” in effect for pathogens on ready-to-eat meat and poultry products because they can pose a risk to certain populations, like the elderly, pregnant women and those who are immunocompromised.  USDA began testing these products for Salmonella in 1983 and for Listeria monocytogenes in 1987.  Since 2002, FSIS published numerous directives, which have increased the focus on Listeria control programs at establishments, including environmental sampling and testing programs. Should a product test positive for either pathogen, FSIS will recommend an immediate product recall and conduct follow up testing at the plant.  On the other hand, because fresh meats like raw ground beef are intended to be cooked, the presence of Listeria monocytogenes or Salmonella does not constitute a violation of federal rules since cooking destroys these pathogens.  In addition to USDA testing, many companies also voluntarily test their ready-to-eat meat and poultry products for the presence of pathogens.
  • Food safety assessments and in-depth verification testing by USDA regulatory personnel.

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