Antimicrobial susceptibility test (MIC)

Microbial feed additives, or other microorganisms used intentionally in the food chain, should not add to the pool of antimicrobial resistance (AMR) genes already present in the gut microbial population, or otherwise increase the spread of AMR (EFSA FEEDAP Panel, 2018).

Phenotypic testing of antimicrobial susceptibility should be made using internationally recognised and standardised methods. Such standards are published by e.g. the Clinical and Laboratory Standards Institute (CLSI) and the International Organization for Standardization (ISO). EFSA accepts agar and broth dilution methods, but not agar diffusion methods (disc diffusion, E-test).

MIC is the lowest antimicrobial concentration, which prevents the growth of target microorganism. To distinguish between susceptible and resistant strains, breakpoint values are available for antimicrobials against clinical isolates (e.g. EUCAST or CLSI). EFSA provides cut-off values to a set of representative antimicrobials listed by WHO as critically or highly important. In addition to these antimicrobials, it is necessary to test the target bacterium against antimicrobials, for which putative resistance genes are found from its genome.

Biosafe performs over 100 MIC tests each year for both aerobic and anaerobic bacteria.

Challenges include the lack of cut-off values for non-clinical, safe industrial bacteria. Furthermore, atypical growth of the test bacterium and non-conventional antimicrobial agent need special attention. Reference bacteria are always included in the test, and are run parallel to the test bacterium under the same conditions besides the ones defined by the Standard.

Genus/species tested by Biosafe include: Escherichia coli, Lactobacillus sp., Pediococcus sp., Bacillus sp., Corynebacterium sp., Enterococcus faecium, Megasphaera sp., Clostridium sp., Burkholderia sp., Streptomyces sp.

Antimicrobials tested