Proceedings of the 10th International Colloquium on Paratuberculosis

#69 In vitro susceptibility <strong>of</strong> Mycobacterium avium paratuberculosis to antibacterial drugs
individually and in combination with <strong>the</strong> immunosuppressive drug 6-mercaptopurine
Manju Y Krishnan, Elizabeth JB Manning, Michael T Collins
Department <strong>of</strong> Pathobiological Sciences, School <strong>of</strong> Veterinary Medicine, University <strong>of</strong> Wisconsin-Madison, USA
Objectives: To determine 1) <strong>the</strong> in vitro antibiotic susceptibility <strong>of</strong> M. avium ss. paratuberculosis (MAP) and
compare to those <strong>of</strong> M. a. hominisuiss (MAH) and M. a. avium (MAA) and 2) <strong>the</strong> effect <strong>of</strong> <strong>the</strong> immunosuppressive
drug 6-mercaptopurine–antibiotic interactions on in vitro growth <strong>of</strong> MAP.
Materials and Methods: Clinical isolates <strong>of</strong> MAP (n=18, human and animal origin), MAH (n=10) and
MAA (n=5) were used. The MICs <strong>of</strong> 11 antibiotics were determined by <strong>the</strong> proportion method adapted for <strong>the</strong>
BACTECTM MGITTM 960 culture system. Drug interactions were tested by <strong>the</strong> chequer board method adapted
for <strong>the</strong> MGITTM 960 system and interpreted based on fractional inhibitory concentration (FIC) indices.
Results: In <strong>the</strong> absence <strong>of</strong> interpretative criteria specific to MAP, those for M. avium complex were used
to define susceptibility. Azithromycin and clarithromycin were <strong>the</strong> most effective (100% isolates) against MAP,
followed by amikacin (83.3%), cipr<strong>of</strong>loxacin (55.5%), lev<strong>of</strong>loxacin (50%), rifampicin (44.4%) and rifabutin
(38.8%). Twenty two percent <strong>of</strong> <strong>the</strong> isolates were ei<strong>the</strong>r susceptible or moderately susceptible (≤0.3 µg/mL) to
cl<strong>of</strong>azimine. Cl<strong>of</strong>azimine MICs for <strong>the</strong> resistant isolates ranged between 0.6 or 1.2 µg/mL, which may indicate
in vivo susceptibility, since <strong>the</strong> drug is known to concentrate in tissues. Ethambutol was effective against 28%
<strong>of</strong> <strong>the</strong> isolates, but isoniazid and dapsone were completely ineffective. MAP was more susceptible to amikacin
than MAH and MAA, less susceptible to rifamycins than MAH, and more susceptible to ethambutol than MAA.
Synergism was observed when 6-mercaptopurine was combined with azithromycin/clarithromycin/rifamycin /
ethambutol for multiple MAP human isolates. No antagonism was observed with any <strong>of</strong> <strong>the</strong> drugs tested.
Conclusions:
- MAP strains are susceptible to macrolides>amikacin>fluroquinolones=rifamycins>
cl<strong>of</strong>azimine=ethambutol;
- MAP strains are more susceptible to amikacin than are MAH and MAA, but relatively less susceptible to
rifamycins than MAH;
- 6-mercaptopurine may be synergistic with macrolides and rifamycins against MAP.
251