If the organisms are clinically significant, what are the chances they could be resistant to the antimicrobial agents commonly used to eradicate them? Unfortunately, the increasing dissemination of resistance among clinically relevant bacteria has diminished the number of bacteria for which antimicrobial susceptibility can be confidently predicted based on identification without the need to perform testing. Table 1 categorizes many of the commonly encountered bacteria according to the need to perform testing to detect resistance.
Table1. Categorization of Bacteria According to Need for Routine Performance of Antimicrobial Susceptibility Testing *
Acquired resistance to various antimicrobial agents dictates that susceptibility testing be performed on all clinically relevant isolates of several bacterial groups, genera, and species. For other organisms, such as H. influenzae and N. gonorrhoeae, resistance to the original drugs of choice (ampicillin, penicillin, and recently ceftriaxone) has become widespread, and more potent anti biotics (e.g., ceftriaxone), for which no resistance has been described, have become the drugs of choice. Therefore, although testing used to be routinely indicated to detect ampicillin and penicillin resistance, testing for resistance to currently recommended antimicrobials for these organisms is not routinely necessary. The possible exception to this is the relatively recent emergence of fluoroquinolone resistance in N. gonorrhoeae that may warrant testing of clinical isolates.
One notable exception to the widespread emergence of resistance has been the absence of penicillin resistance among beta-hemolytic streptococci. Because susceptibility to penicillin is extremely predictable among these organisms, testing against penicillin provides little, if any, information that is not already provided by accurate organism identification. However, if the patient cannot tolerate penicillin, alternative agents, such as erythromycin, may be considered. Erythromycin resistance among beta-hemolytic streptococci has been well documented, and susceptibility testing in this instance would be indicated.
The recommendations outlined in Table 1 are guidelines. In any clinical setting, exceptions will arise that must be considered in consultation with the physician. Also, these guidelines are for providing data used for the management of a single patient’s infection. When susceptibility testing is performed as a means of gathering surveillance data for the monitoring of emerging resistance, the guidelines may not necessarily apply.
