The susceptibility of a given bacterial species to an antibiotic is usually determined either from laboratory tests or from the result of treating an establish ed infection.

In laboratory tests, bacteria isolated from an infected patient are inoculated into tubes containing a liquid culture medium enriched with graded concentration of an antibiotic agent. The lowest concentration of antibiotic that inhibit microbial growth is termed the minimal inhibitory concentration (MIC). When this is compared with concentrations attainable in the body, one can judge whether the bacteria are sensitive, partly sensitive, or resistant to the antibiotic

Alternatively, the surface of a semisolid growth medium can be inoculated with bacteria, and antibiotic-impregnated filter-paper disks placed on that surface. The antibiotic leaves clear zones around habited. The diameter of the inhibitory zones can be measured accurately and the results precisely interpreted.
Occasionally such tests do not correlate with clinical results, particularly when a person has a malignant disease, immune-system impairment, or severe injury, or is receiving drugs that reduce resistance to infection. Outcome may also be unpredictable because of failure to absorb the antibiotic, deterioration of the antibiotic during storage, or its inactivation by simultaneously administered drugs.


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