Phenotypic testing of bacterial antimicrobial resistance has been widely used in clinical and diagnostic microbiology laboratories. These methods have been well studied and standardized. They have the advantages of being low cost, easy to perform (automated systems), and interpretation criteria readily available for commonly encountered organisms. These assays also are essential for new resistance discovery.

Direct testing of clinical isolates against antimicrobial agents in vitro is the most practical way to assess the in vivo activity of drugs routinely in the clinical setting (Greenwood, 1981). In the United States, dilution and disk diffusion tests are two basic methodologies that have been standardized by the Clinical and Laboratory Standards Institute (CLSI), formerly known as the National Committee for Clinical Laboratory Standards (NCCLS). Dilution tests are performed by detecting bacterial growth in broth or agar containing antimicrobial agents in a series of twofold dilutions. The lowest concentration that inhibits the visible growth of an organism is the MIC (minimum inhibitory concentration) value. MICs provide a quantitative evaluation of bacterial growth inhibition by antimicrobial agents. In the disk diffusion method, the drug concentrations are created by diffusion of the testing drug through the agar from filter paper disk containing a single concentration (Barry, 1991). The size of the growth inhibition zone is used to determine the susceptibility of the organism to the drug qualitatively. Based on the pharmacoki-netic and pharmacodynamic properties of the drug, the clinical and bacteriological response rates of organisms to the drug, and the population distributions of MICs, the CLSI provides guidelines for interpretative criteria that give the values of MICs or growth inhibition zone sizes to determine the categories of susceptible, intermediate, and resistant (NCCLS, 2001). The susceptible category is defined as that when infection due to the strain tested may be appropriately treated with the dose of antimicrobial agents recommended for that type of infection. The intermediate category indicates that the strain tested can be effectively inhibited if the drugs are physiologically concentrated at the infected body sites or when a high dosage of a drug can be safely administered. Resistant strains are not inhibited by the usually achievable systemic concentrations of the agent with the normal dosage schedules and/or treatment failures are likely caused by specific microbial resistant mechanisms (CLSI, 2005). The interpretative criteria are specific for each organism/antimicrobial combination along with the specimen type. To achieve the best possible correlation between the in vitro test results and clinical outcome, the test procedures and quality controls suggested by CLSI must be closely followed.

The CLSI guidelines offer standardized methods and interpretative standards for antimicrobial susceptibility testing for organisms commonly encountered in clinical microbiology laboratories, including members of the Enterobacteriaceae, Gram-negative bacilli that are not members of the Enterobacteriaceae such as Acinetobacter spp., Stenotrophomonas maltophilia, Pseudomonas spp., and other nonfastidious, glucose-nonfermenters, Staphylococcus spp., Enterococcus spp., Streptococcus spp., Haemophilus spp., Neisseria gonorrhoeae, Vibrio cholerae, Helicobacter pylori, Listeria monocytogenes, and four potential agents of bioter-rorism: Bacillus anthracis, Yersinia pestis, Burkholderia mallei, and Burkholderia pseudomallei (CLSI, 2005). For the other clinical isolated organisms that are not described, susceptibility testing is not routinely performed in most diagnostic laboratories due to lack of standardized testing methods, or lack of resistance to the drugs of choice for the treatment and interpretation criteria, or lack of the correlation between in vitro susceptibility tests and clinical response.

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