Antimicrobial Susceptibility Testing
Laboratory testing for antibiotic resistance is performed for several reasons.
- The main goal of antibiotic susceptibility testing is to predict the clinical success or failure of the infection treatment with this antibiotic.
- Furthermore, susceptibility testing provides drug resistance monitoring in the population. It makes it possible to determine the resistance of the microbial species to various groups of antibiotics.
- In clinical practice, susceptibility testing is especially valuable, if the isolated strain is supposed to be initially resistant, or it can cause rapid severe health disorder in case of delay of specific treatment.
- Also, it is useful for monitoring of specific treatment course for chronic infections that require the long course of antibiotic therapy.
MIC and MBC
- The relationships between a certain microbial strain and a particular antibiotic drug are characterized by minimum inhibitory concentration (MIC) of antibiotic. It is equal to the lowest concentration of the antibiotic that inhibits visible growth of tested microorganism in standard experimental conditions.
- Another widely used index of microbial susceptibility is the minimum bactericidal concentration (MBC) of antibiotic, which provide complete death of all bacteria in standard experimental conditions.
- MBC can be equal or greater than MIC and cause the death of at least 99,9% of tested bacteria.
- Having determined MIC of antibiotic, the conclusion about the resistance of investigated bacterial culture is made. Depending on MIC value, the isolated bacterial strain is categorized as “susceptible”, “intermediate” or “resistant” to this antibiotic.
- The interpretive criteria for MIC values are worked out by the Clinical and Laboratory Standards Institute (CLSI, USA) and European Committee on Antimicrobial Susceptibility Testing (EUCAST).
According to the current ISO standards, the clinical grades of resistance, based on MIC determination, are characterized as follows:
- “Bacterial strain is susceptible if inhibited in vitro by a concentration of an antimicrobial agent that is associated with a high likelihood of therapeutic success“;
- “bacterial strain is intermediate if inhibited in vitro by a concentration of an antimicrobial agent that is associated with uncertain therapeutic effect”;
- “bacterial strain is resistant if inhibited in vitro by a concentration of an antimicrobial agent that is associated with a high likelihood of therapeutic failure”.
In the practice of clinical laboratories, microbial susceptibility to antibiotics is assessed by agar diffusion tests and by serial dilution methods in liquid or solid media.