Determination of minimum inhibitory concentration

Serial twofold dilutions of antimicrobial agents in a suitable growth medium, such as Mueller-Hinton broth, are prepared in test tubes (macrodilution method) or microdilution wells (microdilution method), with an inoculum of 105 to 106 organisms per milliliter. MICs can also be performed by the agar dilution method; multiple inocula of 104 organisms per milliliter are placed on antimicrobial-containing plates with a replicating device. After overnight incubation at 35 °C, the lowest antimicrobial concentration completely inhibiting growth is the MIC. Minimum bactericidal concentrations (MBCs) can be determined by quantitative subculture; the MBC is the lowest concentration of antimicrobial agent producing at least a 99.9 per cent reduction in the original inoculum. However, MBCs are rarely determined because of a lack of reproducibility of results and absence of quality control strains.

As the macrodilution method is time consuming and expensive, the microdilution method is preferred and is commercially available in the form of frozen or lyophilized trays. A new method for MIC determination is the E-test, which is performed with a plastic carrier strip that releases an antimicrobial gradient when placed on an agar plate and combines the simplicity of disk diffusion with the accuracy of MIC determination. The E-test is particularly useful for susceptibility testing of fastidious organisms such as Strep. pneumoniae, H. influenzae, and anaerobes.

MICs are expressed in micrograms per milliliter and should be at least two- to fourfold lower than the mean achievable drug levels to produce an adequate therapeutic response. MICs can also be interpreted in categories based on achievable serum drug concentrations (.NatioQaLCom^

An organism is susceptible if it is inhibited by antimicrobial levels readily attained in the blood on usual dosage, including oral when applicable, moderately susceptible or intermediate if inhibited only by blood levels achieved with high dosage or when the drug is concentrated (e.g. in urine), and resistant if inhibitory levels are above clinically achievable levels. These general categories are influenced by the patient's condition, drug pharmacokinetics, renal and hepatic disease, the patient's age, presence of shock, and effects of other drugs. They may need to be modified for infections in sites of poor drug penetration such as cerebrospinal fluid.

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