the infection. Antibiotic sensitivity testing is also conducted at a population level in some countries as a form of screening. This is to assess the background rates of resistance to antibiotics (for example with methicillin-resistant Staphylococcus aureus), and may influence guidelines and public health measures. Methods Once a bacterium has been identified following microbiological culture, antibiotics are selected for susceptibility testing. Susceptibility testing methods are based on exposing bacteria to antibiotics and observing the effect on the growth of the bacteria (phenotypic testing), or identifying specific genetic markers (genetic testing). Methods used may be qualitative, meaning that a result indicates resistance is or is not present; or quantitative, using a minimum inhibitory concentration (MIC) to describe the concentration of antibiotic to which a bacterium is sensitive. There are many factors that can affect the results of antibiotic sensitivity testing, including failure of the instrument, temperature, moisture, and potency of the antimicrobial agent. Quality control (QC) testing helps to ensure the accuracy of test results. Organizations such as the American Type Culture Collection and National Collection of Type Cultures provide strains of bacteria with known resistance phenotypes that can be used for quality control. Phenotypic methods Testing based on exposing bacteria to antibiotics uses agar plates or dilution in agar or broth. The selection of antibiotics will depend on the organism grown, and the antibiotics that are available locally. To ensure that the results are accurate, the concentration of bacteria that is added to the agar or broth (the inoculum) must be standardized. This is accomplished by comparing the turbidity of bacteria suspended in saline or broth to McFarland standards—solutions whose turbidity is equivalent to that of a suspension containing a given concentration of bacteria. Once an appropriate concentration (most commonly an 0.5 McFarland standard) has been reached, which can be determined by visual inspection or by photometry, the inoculum is added to the growth medium. Manual The disc diffusion method involves selecting a strain of bacteria, placing it on an agar plate, and observing bacterial growth near antibiotic-impregnated discs. This is also called the Kirby-Bauer method, although modified methods are also used. In some cases, urine samples or positive blood culture samples are applied directly to the test medium, bypassing the preliminary step of isolating the organism. If the antibiotic inhibits microbial growth, a clear ring, or zone of inhibition, is seen around the disc. The bacteria are classified as