Antimicrobial susceptibility profile of soil isolates of Nocardia asteroides from Kuwait

Objectives To find the antimicrobial susceptibility profile of 42 soil isol ates of Nocardia asteroides against 14 antimicrobial agents representing be ta-lactams, aminoglycosides, ciprofloxacin, minocycline, erythromycin and t hird generation cephalosporins. Methods The antimicrobial susceptibility was determined by the disk diffusi on method using Mueller-Hinton agar medium. A homogeneous suspension giving an inoculum of 10(6)-10(8) CFU/mL was used to streak the plates. The zone of inhibition was read after 36-48 h of incubation at 37 degrees C. Results All the soil isolates of N. asteroides were susceptible to amikacin , imipenem and tobramycin. Susceptibility to cephalosporins was quite varia ble; 86% of the isolates were susceptible to cefotaxime, 57% to ceftriaxone and 40% to cefamandole. Fifty-seven per cent of the isolates showed interm ediate susceptibility to cefamandole, 33% to ceftriaxone and 5% to cefotaxi me. Ninety-three per cent of the isolates were resistant to sulfamethoxazol e alone or in combination with trimethoprim. Conclusions The study rt ports a wide variation in the antimicrobial susceptibility profile of soil isola tes of N. asteroides originating from a single geographical area. Of intere st is the finding that over 90% of N. asteroides isolates were resistant to sulfamethoxazole without any previous exposure to this drug. This may have serious therapeutic implications as sulphonamides or the combination of tr imethoprim-sulfamethoxazole is the therapy of choice for nocardiosis. Demon stration of resistance to beta-lactam antibiotics may be attributed to the presence of beta-lactamases which was detectable in > 90% of the soil strai ns of N. asteroides. The study underscores the importance of antimicrobial susceptibility testing for clinical isolates of Nocardia since individual s trains show considerable differences in their susceptibility patterns neces sitating therapeutic adjustments.