5.3 Use of control strains

One important quality procedure for AST is the use of control strains.

Control strains are bacterial isolates that have been widely used and have a known, well characterised antibiotic susceptibility profile. They are usually obtained from a reputable source, such as the American Type Culture Collection (ATCC). Both susceptible and resistant control strains are usually used, and the results are compared to standard tables that ensure that they are within acceptable ranges.

Using control strains means that laboratory staff can ensure that:

• the medium performs satisfactorily
• the strength of the antibiotic used in the disk diffusion test is correct
• the incubation conditions are appropriate, for example the correct temperature is maintained
• tests are performed correctly.