sensitive, intermediate, or resistant to an antibiotic by comparing the diameter of the zone of inhibition to defined thresholds which correlate with MICs. Mueller–Hinton agar is frequently used in the disc diffusion test. The Clinical and Laboratory Standards Institute (CLSI) and European Committee on Antimicrobial Susceptibility Testing (EUCAST) provide standards for the type and depth of agar, temperature of incubation, and method of analysing results. Disc diffusion is considered the cheapest and most simple of the methods used to test for susceptibility, and is easily adapted to testing newly available antibiotics or formulations. Some slow-growing and fastidious bacteria cannot be accurately tested by this method, while others, such as Streptococcus species and Haemophilus influenzae, can be tested but require specialized growth media and incubation conditions. Gradient methods, such as Etest, use a plastic strip placed on agar. A plastic strip impregnated with different concentrations of antibiotics is placed on a growth medium, and the growth medium is viewed after a period of incubation. The minimum inhibitory concentration can be identified based on the intersection of the teardrop-shaped zone of inhibition with the marking on the strip. Multiple strips for different antibiotics may be used. This type of test is considered a diffusion test. In agar and broth dilution methods, bacteria are placed in multiple small tubes with different concentrations of antibiotics. Whether a bacterium is sensitive or not is determined by visual inspection or automatic optical methods, after a period of incubation. Broth dilution is considered the gold standard for phenotypic testing. The lowest concentration of antibiotics that inhibits growth is considered the MIC. Automated Automated systems exist that replicate manual processes, for example, by using imaging and software analysis to report the zone of inhibition in diffusion testing, or dispensing samples and determining results in dilutional testing. Automated instruments, such as the VITEK 2, BD Phoenix, and Microscan systems, are the most common methodology for AST. The specifications of each instrument vary, but the basic principle involves the introduction of a bacterial suspension into pre-formulated panels of antibiotics. The panels are incubated and the inhibition of bacterial growth by the antibiotic is automatically measured using methodologies such as turbidimetry, spectrophotometry or fluorescence detection. An expert system correlates the MICs with susceptibility results, and the results are automatically transmitted into the laboratory information system for validation and reporting. While such automated testing is less labour-intensive and more standardized than manual