Susceptibility testing of Mycobacterium abscessus by isothermal microcalorimetry

We evaluated a new method for susceptibility testing of a rapidly growing mycobacterium using real-time measurement of heat (microcalorimetry). MICs of 2 clinical Mycobacterium abscessus isolates were determined by microbroth dilution and E-test. For microcalorimetry, Middlebrook-7H10 agar + 10% oleic acid-albumin-dextrose-catalase, containing amikacin, clarithromycin, linezolid, and ciprofloxacin was inoculated with ~105 CFU/mL. Heat production was measured at 37 °C for 72 h. Minimal heat inhibition concentration (MHIC) was defined as the lowest antibiotic concentration inhibiting growth-related heat production. Growth of M. abscessus was detected after a median of 16.5 h (range, 8.5-26.9 h). Heat detection was proportionally delayed with increasing concentration of antibiotics. MHICs for the tested strains were 16 to >16 mg/L for amikacin, >8 mg/L for clarithromycin, 4 to >16 mg/L for ciprofloxacin, 24 to >32 mg/L for linezolid. MHICs were in agreement within two 2-fold dilutions with conventional MICs. Microcalorimetry may accelerate antimicrobial susceptibility testing in mycobacteria and provide additional real-time information on the drug effect.