99 problems...part 3: sensitivity, MIC, and the antibiogram

 

How can the laboratory help me make better choices with antimicrobials?

Sensitivity Analysis: What does it mean?

Sensitivity analysis determines how well that organism can grow in the presence of a particular antimicrobial at varying concentrations in vitro (on the culture plate). But this is not reality: the bacteria has been removed from the site of infection (e.g. uterus, lung, skin etc) and conditions have changed.  Even if an organism is sensitive to the antimicrobial tested on the culture plate inside the horse it be unaffected. The goal of sensitivity analysis is to assess at what concentration of antimicrobial we would reasonably expect treatment success, and with knowledge of the conditions at the site of infection, whether is it reasonable to assume we can achieve that concentration.  This is a judgment call based on our experience and knowledge of biology.  The conditions at the site of infection can render the antimicrobial ineffective, or where a barrier has formed to exposure e.g. a thick-walled abscess, the antimicrobial may not penetrate to the site of infection at a sufficient concentration to be effective even though the organism is deemed sensitive.  By considering how antimicrobials work and how they will distribute, veterinarians can make the most appropriate treatment choices.

Sensitivity analysis does not consider the ability of the bacteria to counter the antimicrobial and promote persistence.  Some bacteria are able to produce enzymes which break down antimicrobials they are tested against.  These enzymes are not produced in vitro on a culture plate.  Therefore, although an organism is sensitive to a particular drug on the culture plate, the organism is resistant at the site of infection.  Identification of the organism will allow suspicion of this ability, and advanced testing for resistance genes will confirm this ability to resist.

 

What is MIC?

The Minimum Inhibitory Concentration (MIC) is the lowest concentration of antimicrobial that inhibits visible growth of a specific organism in vitro.  Three results are given: susceptible (S), resistant (R) and intermediate (I) which can be considered a grey zone in response making knowledge of the actual conditions that may exist where the organism resides in the animal very important.  It is not possible to compare the MIC of one agent to another for a particular bacteria, that is, an MIC for one antimicrobial of 2ug/ml and another of 1ug/ml for that bacteria does not mean one is twice as effective the other.  This just means that the concentration that will inhibit the growth of that particular organism is different.  You must achieve that concentration at the site of infection to be effective, which is where knowing both your patient and the intended treatment is so important.

 

Using an Antibiogram

This information is gained from collating antimicrobial sensitivity information of cultured bacteria derived from submitted diagnostic samples over a period of time from a laboratory.  Results are regional and/or practice-area specific and should be regularly updated to maintain relevance.  This information is useful when making treatment decisions once the organism has been identified, but sensitivity analysis is not yet available.  Veterinarians can assess the likelihood of sensitivity of cultured microorganisms to a particular antimicrobial using local knowledge derived from organisms cultured from infected sites in horses within the catchment of the laboratory.  Remember this is an in vitro system and as such may not reflect the ability of the organism to produce resistance compounds.

In the ongoing war between the biology of survival and control of disease you can never have too many friends or know too much.

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