By: Dr. Meera Surendan-Nair

At Antech Diagnostics, we use a fully automated system (Sensititre System) to generate true  Minimum Inhibitory Concentration (MIC) values, alongside a manual disk diffusion method (Kirby Bauer).

1. MIC

The Sensititre system uses a gold standard method – broth microdilution – to provide accurate and reproducible MIC values.

At Antech, we custom design our test plates to allow us to include the most relevant antibiotics for each bacterial organism, at the most appropriate concentrations.
Antibiotics are first added to our test plates in halving dilutions e.g. pradofloxacin is added in the following concentrations: 2 g/mL; 1 g/mL; 0.5 g/mL; 0.25 g/mL and 0.12 g/mL. A fixed concentration of the test organism is then added, and the test plates are incubated at the appropriate temperature for the optimal time.

The MIC value is then read as the lowest concentration of antibiotic on the test plate that can inhibit bacterial growth. The lower the MIC value, the less antibiotic that is needed to inhibit the growth of the test organism, and the more susceptible it is.

2. Disk Diffusion (Kirby-Bauer Method)

Disk diffusion is also used in our laboratory where indicated e.g. for topical preparations. This method uses disks impregnated with a set concentration of antibiotic. The disks are added to a culture plate inoculated with a known concentration of the test bacterial organism. The antibiotic diffuses into the plate medium during incubation. Where the concentration of antibiotic is sufficiently high to prevent the growth of susceptible bacteria, a clear zone appears, this is the zone of inhibition. The diameter of this zone is measured and interpreted against the appropriate clinical breakpoint.

What is the MIC value, and how useful is it?

The MIC value is the lowest concentration of an antibiotic that inhibits the growth of the test organism.
When considering which antibiotic might be the most effective, MIC values should not be compared between antibiotics since different concentrations of each antibiotic are used in the MIC test process, depending on the clinical breakpoints. It can be useful, however, to compare the MIC value to the breakpoint for that antibiotic-organism combination. The further the MIC value is away from the breakpoint (efficacy ratio), the more likely it is to be effective. If the MIC value in the report is prefixed with ≤ , this means that the MIC value may be lower than the lowest antibiotic concentration tested. In the example above, the MIC of nitrofurantoin is reported as ≤ 32 – this means that no growth was detected at this concentration, but as this was the lowest concentration on the plate, the actual MIC value may be lower.

How are the MIC and zone diameter values interpreted?

MIC and zone diameter values are interpreted as Susceptible (S) intermediate (I) Resistant (R) and Nonsusceptible (NS) based on cut-off values or clinical breakpoints established by the Clinical and Laboratory Standards Institute (CLSI) and the European Committee on Antimicrobial Susceptibility Testing (EUCAST).

At this time, only the CLSI provides standards specifically for use in veterinary medicine.

  • Susceptible implies that the test organism is inhibited by the usually achievable concentration of the antibiotic at the infection site.
  • Intermediate implies that the test organism may be susceptible to the antibiotic at increased exposure levels e.g. in sites where the antibiotic is physiologically concentrated or where a higher-than-normal dosage of an antibiotic can be used.
  • Resistant implies that the test organism is resistant to the usually achievable concentration of the antibiotic at the infection site.
  • Nonsusceptible can only be applied to a test organism with only a susceptible breakpoint because resistant strains are rare or absent. If the MIC of the test isolate is above the susceptible value, these are reported as nonsusceptible.

Clinical breakpoints are not fixed values, and vary with the test organism, the antibiotic, the animal species and infection site. Breakpoints can also change in time, and it is important that testing laboratories are using up to date information to interpret the MIC and zone diameter values.

Caution: Clinical breakpoints are not available for all bacterial organism-animal species-site combinations.
When breakpoints are applied to different bacterial species, or different animal species, or different infection sites than the approved breakpoint, the reliability of the interpretation result is reduced.

Caution: Ensure that the infection site and animal species information is listed on the submission form, as this information is used to determine the breakpoints.

How can I make an informed choice on antibiotic selection?

Caution: The MIC value alone should not be used to make a decision on clinical effectiveness of a given antibiotic treatment.

There are many other factors to consider, including the following:

  • Site of Infection
    Maximise pathogen exposure to an effective antibiotic by considering the physiological concentration of the antibiotic at the infection site. Is topical administration appropriate?
  • Patient factors
    Consider the underlying patient health, and expected owner-patient compliance
  • Treatment Guidelines
    Review your practice guidelines for first- and second-line therapy (e.g. BSAVA ProtectMe and the expert consensus on appropriate treatment (e.g. ISCAID)
  • MIC value and Breakpoint
    How far is the MIC from the breakpoint for that antibiotic-organism combination? Is the breakpoint available for that antibiotic-organism-animal species-infection site? If not, the reliability of the interpretation is reduced