Drug Susceptibility Testing of the Nontuberculous Mycobacteria (NTM)

Due to their rapid growth, some mycobacteria, such as M. fortuitium, M. chelonae and M. abscessus, as well as some other aerobic actinomyces are tested using a microdilution MIC method. This provides quantitative susceptibility results that are more useful than qualitative methods such as disk diffusion. A microtiter plate is inoculated with a small amount of the cultured organism and then incubated for 3 - 5 days and evaluated by the technician using a bright light source and a view box. The purity of the culture is confirmed by assessing agar plates inoculated with the culture. Results are reported after purity is confirmed and susceptibility pattern is reviewed for consistency with species.

The following test panels are offered:

• Drugs tested for rapidly growing mycobacteria
  Amikacin, Kanamycin, Tobramycin, Cefoxitin, Imipenem, Ciprofloxacin, Doxycycline, Clofazamine, Moxifloxacin, Tigecycline, Clarithromycin, Azithromycin, Augmentin, Trimethoprim/Sulfamethoxazole (Bactrim), and Linezolid
• Drugs tested for aerobic actinomyces, Nocardia, Rhodococcus, Gordonia, and Tsukamurella
  Amikacin, Kanamycin, Tobramycin, Gentamycin, Ceftriaxone, Cefepime, Cefotaxime, Imipenem, Ciprofloxacin, Minocycline, Clarithromycin, Azithromycin, Augmentin, Trimethoprim/Sulfamethoxazole (Bactrim), and Linezolid
• Drugs tested for veterinary samples
  Same as for rapidly growing mycobacteria plus Gentamycin, Ceftriaxone, Cefepime, Cefotaxime, and Minocycline.

M. Avium Complex and Other Slowly Growing Nontuberculous Mycobacteria (NTM)

Infections with NTM cause significant morbidity in vulnerable populations. Assessment of in vitro susceptibility of NTM isolates to various drugs may guide the physician to the appropriate selection of therapeutic agents for each patient. There are many debates in the literature about the predictive value of in vitro susceptibility testing for some NTM species. As in many other fields of clinical microbiology, the laboratory report of "susceptible" to a certain drug for a particular isolate is only a suggestion. The physician should make a decision regarding the patient's response to therapy with this agent based on other factors, such as drug absorption, drug penetration to the site of inflammation, pharmacokinetics, and other co-existing issues in the particular patient. On the other hand, a report that the isolate is "resistant" in vitro to drug concentrations exceeding those attainable in vivo, the likelihood that the drug would be therapeutic in a patient is very low. Therefore, drug susceptibility testing of NTM isolates may help in diminishing unnecessary toxic effects of inactive drugs by exclusion from the treatment regiment.

The laboratory uses two methods for susceptibility testing slowly growing mycobacteria:

• Agar Proportion Method
  Determines percent resistant in the agar medium. Drugs tested by the agar proportion method include isoniazid, rifampin, ethambutol, streptomycin, amikacin, kanamycin, capreomycin, ethionamide, cycloserine, and PAS. Results are reported within 3 weeks after obtaining a culture. This test is used for some NTM, such as M. kansasii, M.szulgai, M. marinum, and M.celatum, but it is not recommended for M. avium complex.
• Radiometric MIC Determination in liquid medium (7H12 broth, Bactec-460)
  Multiple drug concentrations (3-4) are tested to determine the MIC. The MIC is defined as the lowest drug concentration that inhibits greater than 99% of the bacteria in the culture. The MIC is reported with a tentative interpretation of "susceptible", "intermediate", or "resistant". The 12-drug panel is recommended for M. avium complex and most of the other slowly growing mycobacteria.
  
  Drugs tested in this panel include ciprofloxacin, moxifloxacin, rifampin, rifabutin, ethambutol, streptomycin, amikacin, kanamycin, capreomycin, ethionamide, cycloserine, and clarithromycin. Additional drug choices include other macrolides and other quinilones, linezolid, and experimental drugs. Results are reported within 2 weeks after obtaining a culture.

Combination testing of rifampin and ethambutol is included in the 12-drug panel. The effect of using combination therapy with rifampin and ethambutol is reported as "synergistic", "additive", or "no effect". Results are reported within 10 days after the single drug MICs are reported.