Final answer:
Microbes resist antimicrobial drugs through target modification, drug inactivation, reduced uptake, or increased efflux. The MIC is the lowest drug concentration inhibiting visible growth in a dilution series, while the MBC is determined through additional agar tests. Assays like Kirby-Bauer can determine resistance but not the bactericidal or bacteriostatic nature of a drug.
Step-by-step explanation:
Microbes can resist antimicrobial drugs through various mechanisms, such as modifying the drug target, inactivating the drug, reducing drug uptake, or increasing drug efflux. The minimum inhibitory concentration (MIC) is determined by locating the lowest concentration in a dilution series that inhibits visible bacterial growth.
A subsequent test on agar media with no antibiotic can determine the minimal bactericidal concentration (MBC), which is the concentration that kills ≥99.9% of the bacteria. To ascertain if a drug is bacteriostatic (inhibits bacterial growth) or bactericidal (kills bacteria), further evaluation after the MIC determination is necessary.
Resistance in a disc diffusion assay is determined by comparing the diameter of zones of inhibition around antimicrobial-impregnated disks to standardized charts to assess if the bacterium is susceptible or resistant to the drug.
A disc diffusion assay, such as the Kirby-Bauer test, assesses the effectiveness of antimicrobials by establishing zones of inhibition around the disks. These zones are compared to standard sizes to determine if the bacteria is resistant or susceptible to the drug.
These assays, although useful for initial susceptibility screening, do not provide information on the bacteriostatic or bactericidal nature, nor do they allow for the direct comparison of drug potencies. These tests are also influenced by how well the drug can diffuse in the agar and do not necessarily equate to in vivo efficacy.