13. Containment of Microorganisms: Antibiotic Sensitivity Testing Introduction Antibiotics are chemical compounds produced by microorganisms that inhibit growth or kill other microorganisms. Antibiotics are more specific and limited in the types of organisms that they kill than are antiseptics & disinfectants. For example, a given antibiotic will generally be effective in killing only some bacterial species, whereas antiseptics/disinfectants are lethal to a wide range of microbes. KIRBY-BAUER DISK DIFFUSION METHOD CULTURES To compare how effective one antibiotic is to another, or to measure the degree of antibiotic resistance in a bacterium, a procedure called the Kirby-Bauer test can be done. To do this, a pure culture of bacteria is isolated from an infected person. This pure culture is then spread over the surface of a special medium, called Mueller-Hinton agar, to create a lawn of bacteria. Mueller Hinton is a very rich medium that allows the growth of a large number of bacteria and was adopted as a standard medium for hospitals and research laboratory for comparison purposes. Small filter paper disks, impregnated with standardized amounts of antibiotic, are gently pressed on to the surface of the agar (your group will use a multi-disk dispenser). The plates are incubated overnight while the antibiotic diffuses from the disk into the agar. After incubation, the plates are examined for the presence of zones of inhibition (clear rings around the antibiotic disk). If the chemical agent being tested inhibits the test microbe there will be a clear zone of inhibition (ZI) surrounding the disk where no microbial growth has occurred due to the presence of the agent. If there is no inhibition, growth extends up to the rim of the disks on all sides, and the organism is reported as resistant (R). In general the larger the diameter of the ZI, the more effective the test chemical is. This procedure has been standardized for antibiotics.
Staphylococcus aureus culture tested with several antibiotics http://phil.cdc.gov/phil/home.asp
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