The ESP culture system for drug susceptibilities of Mycobacterium avium complex

Objective To validate the non-radiometric, broth-based ESP system for determining Mycobacterium avium complex (MAC) susceptibilities. Methods MAC isolates from sterile body sites of 20 adult HIV-infected patients who were failing their present MAC regimen were identified. Susceptibilities were determined and comparisons made between the agar proportion method and the ESP system for clarithromycin, ethambutol, sparfloxacin and cycloserine. Results Ninety-nine percent of the MICs generated by the ESP system were identical to or lower than the MICs determined by the agar proportion method. In vitro resistance was documented by the ESP system for 86% of the drugs that patients were taking at the time of breakthrough, and no resistance was seen to cycloserine, a drug that no patient was taking. Conclusions The ESP system, a fast and reliable method for determining MAC susceptibilities, could be used to optimize MAC regimens in a timely fashion, avoid the use of ineffective drugs, minimize emerging resistance and ultimately improve outcome.