METHODS FOR DETERMINING SUSCEPTIBILITY OF FASTIDIOUS AND UNUSUAL PATHOGENS TO SELECTED ANTIMICROBIAL AGENTS

Not all clinically significant bacteria can be tested by the standardized disk diffusion method (Bauer et al., 1966; Gavan et al., 1981; NCCLS, 1984; NCCLS, 1986). They may grow too slowly, require special nutrients, require special atmospheres, or simply may not have been tested adequately to show that they can be tested accurately and reproducibly by the standard NCCLS method (NCCLS, 1984; NCCLS, 1985b). We call these bacteria "fastidious" and/or "unusual" isolates. For susceptibility testing purposes, the "fastidious" organism will not grow on Mueller-Hinton medium without supplementation, and the "unusual" organism has not been studied to show that it can be reliably tested by standard methods, even though it may grow well on Mueller-Hinton media, or else it presents special problems in testing its susceptibility to antimicrobial agents. We describe here some antimicrobial susceptibility testing methods that can be used for some of these fastidious or unusual organisms and, for the most part, are tests that we have used or are using in our laboratory at this time. We have included only selected references for the methods since this is not intended to be a review of methods for these organisms. The methods described here should not be considered standard, but for some of the organisms intensive study of methodology is now ongoing, and standard recommendations may soon be available.