Comparison of methods for susceptibility testing of mycobacterium avium complex to clarithromycin resistance

Abstract

Mycobacterium avium complex (MAC) has become a major human opportunistic pathogen, especially in HIV infected patients and resistance of MAC to clarithromycin remains a therapeutic challenge in these patients. Information of susceptibility testing result provides information for the management of MAC-infected patients. This requires a rapid and standardized method. The BACTEC MGIT 960 system (Becton Dickinson, U.S.A) and Epsilometer test (E test; AB Biodisk, Solna, Sweden), new susceptibility methods, were compared to NCCLS recommended method, broth microdilution, for the antimicrobial susceptibility testing of Mycobacetium avium complex (MAC). One hundred MAC isolates from hemoculture were tested against clarithromycin. The turnaround time for identification of resistance was 15, 12 to 14 (median, 13), and 13 days for the broth microdilution method, BACTEC MGIT 960 and E test, respectively. One hundred percent agreement was demonstrated for all MAC clinical isolates, 97 isolates (97%) were susceptible and 3 isolates (3%) were resistant to clarithromycin. Agreement between the E test MICs and the broth microdilution MICs, within ±1 log2 dilution was 95% and within ±2 log2 dilution was increased to 100%. Nucleotide sequence of domain V of the 23S rRNA gene was determined in clarithromycin resistant isolates (MIC>256(µg/ml). All of the 3 resistant isolates contained mutation of the domain V in 23S rRNA gene at position adenine (A) 2058 ( cognate with Escherichia coli base). One isolate contained point mutation at position A2058C and 2 isolates contained point mutation at position A2058G. The BACTEC MGIT 960 system and E test are accurate for determining susceptibility of MAC strains to clarithromycin. However, the high cost of BACTEC MGIT 960 system limited its use to only determine the cut-off MIC for resistance. This study concludes that the E test is a promising alternative to the broth microdilution in diagnostic mycobacteriology laboratory because it is reliable, easy, rapid and cost-effective.