Abstract
Smooth (Sm) and rough (Rg) colonial types of Mycobacterium paratuberculosis ATCC 19698 and two clinical isolates were tested to examine their growth responses in medium containing antimicrobial agents. Susceptibility tests were done in Middlebrook 7H12B medium with and without Tween 80 and one of the following antimicrobial agents: streptomycin, isoniazid, rifampin, ethambutol, ciprofloxacin, and penicillin G. Growth responses in the presence of antimicrobial agents led to the following observations. (i) In the absence of Tween, Rg colony types were more resistant than Sm colony types; (ii) the addition of Tween 80 significantly increased the susceptibility of both Sm and Rg colony types; however, the increase was greater with the Sm colony types. These studies showed that the antimicrobial susceptibility of M. paratuberculosis was significantly affected when Tween 80 was present in either the primary culture medium or the drug susceptibility test medium. In the absence of the perturbing influence of Tween 80, M. paratuberculosis was resistant to the antimicrobial agents tested.