Determination of in vitro susceptibility of Mycobacterium avium complex isolates to antimycobacterial agents by various methods

Abstract

Various methods were used to determine the in vitro susceptibility of Mycobacterium avium complex strains isolated from patients with acquired immunodeficiency syndrome. Our results confirm the noted resistance of the M. avium complex to conventional antituberculosis agents. The procedures used were both agar dilution and broth dilution, including a commercially available radiometric system (BACTEC; Johnston Laboratories, Towson, Md.). In general, all strains were more resistant by an agar dilution procedure than by a broth dilution procedure. Radiometric data were analyzed by defining a value, termed T100, which provides a discrete MIC. The broth (radiometric) procedure is reproducible and convenient for screening antimicrobial agents for in vitro activity and assessing potential therapeutic efficacy. Nevertheless, there is no standard procedure for determining the in vitro susceptibility of the M. avium complex, and appropriate clinical correlation studies are needed to accurately assess the clinical relevance of any in vitro result.Disseminated infections caused by the Mycobacterium avium complex commonly occur in patients with acquired immunodeficiency syndrome (AIDS) and are increasingly recognized as a cause of serious disease in other immunocompromised patients (7, 22; R. C. Good, Clin. Microbiol. Newsl., 1:1-4, 1979). Treatment of M. avium complex infections in patients with AIDS has been particularly frustrating. M. avium complex clinical isolates are commonly resistant to isoniazid (INH) and variably susceptible to rifamycins (RIF), ethambutol (EMB), and aminoglycosides (19; R. C. Good, V. A. Silcox, J. C. Kilburn, and B. D. Plikaytis, Clin. Microbiol. Newsl. 7:133-136, 1986). There is recent evidence that M. avium complex isolates from patients with AIDS may be more resistant to antimicrobial agents than are isolates from non-AIDS patients (10). Treatment regimens including four to six different antimycobacterial agents are often used, but there is little evidence that treatment can eradicate the infection and prolong life in these severely immunocompromised patients (2, 3). The situation is complicated not only by the lack of clearly effective therapeutic agents but by the lack of a standard method for determining the in vitro susceptibility of M. avium complex isolates to either conventional or entirely novel antimycobacterial agents. Interpretation of results determined by any in vitro method is mitigated by a lack of correlation with clinical efficacy. Our strategy was to identify a convenient and reproducible procedure for in vitro susceptibility testing of M. avium complex isolates. Such a procedure should provide a discrete MIC and a reasonably rapid result, be convenient for screening large numbers of agents, and correlate with in vivo studies of single drugs in animals and, eventually, with clinical efficacy in humans. Angeles, Los Angeles. In each case, the patient was diagnosed as having disseminated M. avium complex disease, and the sites of isolation included blood, bone marrow, lung, liv...

A bone marrow-derived murine macrophage model for evaluating efficacy of antimycobacterial drugs under relevant physiological conditions

“…In a previous study this compound was identified as having excellent MICs for a large panel of M. avium isolates (21), and the findings of the present study now confirm that the activity of this compound is indeed bactericidal within the infected host cell.…”

“…Existing treatment regimens are primarily based on early results in the field in which multiple clinical isolates were tested for their in vitro susceptibilities to a wide variety of drugs, with the two most widely used methods being the broth microdilution method (43) and the radiometric BACTEC method (15,21). In both of these methods, the antimicrobial agent is added directly to broth cultures, and hence is in direct contact with the bacilli.…”

Epidemiology of Infection by Nontuberculous Mycobacteria

“…This has led to studies comparing different methods of susceptibility testing (244) and demonstration that the growth stage of mycobacteria influences antibiotic susceptibility (558). However, methods of susceptibility testing may not be entirely at fault.…”

EDP-420, a Bicyclolide (Bridged Bicyclic Macrolide), Is Active against Mycobacterium avium

“…To confirm the initial observation in vivo, the bactericidal activity of EDP-420 was determined, as previously reported (17,24), and the concentration was used in vitro to determine the frequency of natural resistance to EDP-420 in a population of MAC. Inocula of 10 7 to 10 11 bacteria were incubated with 32 g/ml of EDP-420 in 7H9 broth for one week to select for resistant bacteria, and then bacteria were plated onto 7H10 agar with or without 32 g/ml of EDP-420 to isolate resistant mutants.…”

“…MICs were determined by a radiometric broth nanodilution method and the T100 method of data analysis (17). The inoculum for the susceptibility testing was prepared by obtaining 5 to 10 colonies from a 7H11 agar plate and placing them into 7H9 broth; the samples were then tested directly or frozen at Ϫ70°C.…”