« back to Insight

Antimicrobial susceptibility testing of food and feed bacteria

Biosafe's ISO 17025 laboratory accreditation covers antimicrobial susceptibility testing of selected bacteria performed as broth microdilution on microtiter plates according to Clinical & Laboratory Standards Institute (CLSI) standards. Why did we choose this test to be accredited and what does it mean?

MIC tests reveal the bug and the treatment

In order to treat a pathogenic infection correctly, health care professionals must know what causes the infection and what antimicrobial is effective. To this end, an antimicrobial susceptibility test called the Minimum Inhibitory Concentration (MIC) test was developed. MIC tests are needed when a species is expected to be resistant to the most common antibiotics. Thus, the tests have been validated and verified mostly for clinical, pathogenic species.

AMR, a global menace

Antimicrobial resistance (AMR) is a widespread problem in the clinical setting. Humans and animals may be infected by bacteria resistant to antibiotics, which can lead to a situation where a selected treatment may not be effective. Any microbe may become resistant through horizontal gene transfer. It means that a bacterium obtains genetic material from another organism, which confers the bacterium resistant to an antibiotic. This often involves bacteriophages or plasmids.

Testing for AMR in food and feed

As a basic principle, microorganisms intentionally introduced in the food chain must not add to the pool of antimicrobial resistance. Consequently, for bacteria, antimicrobial susceptibility must be analysed, either based on the genome sequence and/or by standardised MIC tests. This applies to both viable bacteria (e.g. probiotics) and production strains. We have accreditation for the most commonly used microbe species in the food chain and for most antimicrobials, which EFSA requires tests for. We have been granted accreditation for testing for a large combination of microbe species and antimicrobials.

Microbes and antimicrobials don't necessarily fit a standard

We routinely test for microbial species and antimicrobials for which standardised protocols are not available. There are no standardised methods for novel microbe species used in food or feed or some anaerobic species. This has had some practical implications for the test, which we have had to take into account when preparing our standard operating procedures (SOPs) for these situations.

For example, the tested bacteria might grow much faster or slower compared to the ones described in the standard documents. Some bacteria might grow in another medium or in non-standard conditions, or the cell counts may be off from the requirements. In the first case, the quality control strains, which are always included in the test, can be tested in the conditions used for the test organism, but there might be difficulties ahead when interpreting the results. In the latter case, we always verify the cell counts before testing.

Our knowledge is the backup system

Sometimes we must test the susceptibility of a strain against antimicrobial substances for which no cut-off values are available for the quality control strain. The lack of a quality control strain is significant because it is not possible to verify that the test has been successful for a particular antimicrobial. In this case, we rely on our best knowledge and data. We collect data on all our tests to be able to evaluate how certain species normally behave, and we learn from experience. We generally also rely on literature and the whole genome analysis to draw conclusions.

We strive to continuously improve our quality and that applies to this test as well. There is a notable lack of knowledge in respect to the antimicrobial susceptibility of "safe" species. We work and collaborate with several partners to share knowledge and fill this gap.

To find out more about MIC tests and Antimicrobial susceptibility testing, contact Jouni Heikkinen or Meeri Sutinen.