Testing of the susceptibility to 12 antimicrobial agents, including P-lactams, aminoglycosides, ciprofloxacin, and erythromycin, was performed by broth microdilution on 78 consecutive clinical isolates of Nocardia asteroides. Surprisingly, a limited number of patterns of susceptibility were identified that included all drug classes, with 95% of isolates exhibiting one of five patterns. One group (17%) exhibited resistance to the broad-spectrum cephalosporins, one group (18%) was susceptible to both ampicillin and erythromycin, one group (17%) was susceptible to ampicillin and carbenicillin but intermediate in susceptibility to imipenem, and the most common group (35%) was resistant to ampicillin but susceptible to the broad-spectrum cephalosporins and imipenem. The most active parenteral agents were amikacin (95%), imipenem (88%), ceftriaxone (82%), and cefotaxime (82%), while the most active oral agents were the sulfonamides (100%), minocycline (100%), and ampicillin (40%). Additional studies are needed to determine whether differences in ,l-lactamases relate to varying Il-lactam resistance and whether taxonomic differences that correlate with the different susceptibility groups can be identified.There have been a number of recent reports of testing of the susceptibility of Nocardia asteroides to newer antimicrobial agents, including the fluorinated quinolones, the carbapenems, broad-spectrum cephalosporins, and various clavulanic acid combinations (1, 3-8, 10, 12, 21). In general, these studies have shown variable degrees of susceptibility to each of these groups of agents. The reason for this variability is unknown, since drug resistance in nocardiae presumably reflects primary (intrinsic) resistance as most infections are acquired from the environment outside the hospital. Whether there are unlimited patterns of resistance or just a few patterns has never been addressed.While performing susceptibility testing of clinical isolates of nocardia, we noted that most isolates of N. asteroides fell into one of four patterns of susceptibility that involved all classes of drugs. Details of these drug patterns and their taxonomic implications are presented. MATERIALS AND METHODSOrganisms. Seventy-eight consecutive isolates of N. asteroides submitted for susceptibility testing to our laboratory were studied. Isolates were identified to species by standard methods (11); the majority of strains were identified by the Mycology Section of the Texas Department of Health, Austin.Susceptibility testing. Susceptibility testing was performed by broth microdilution, using cation-supplemented MuellerHinton broth and 96-well plates that contained 0.1 ml of drug per well. Ampicillin, carbenicillin, cefotaxime, ceftriaxone, imipenem, minocycline, erythromycin, ciprofloxacin, sulfamethoxazole, amikacin, kanamycin, and gentamicin were prepared in twofold dilutions from diagnostic powders and added to the microdilution wells by using the Mini Quick Spense II System (Bellco Glass, Inc., Vineland, N.J.). The plates were stored at -70°C u...
Previous studies of Mycobacterium fortuitum identified isolates that did not fit its two recognized biovariants. Eighty-five clinical isolates of this group, the "third biovariant complex", were evaluated. They represented 16% of 410 isolates of M. fortuitum submitted to a Texas laboratory and 22% of 45 isolates in Queensland, Australia. Most infections (76%) involved skin, soft tissue, or bone and occurred after metal puncture wounds or open fractures. Isolates differed from biovar fortuitum in resistance to pipemidic acid and use of mannitol and inositol as carbon sources. Two subgroups were present, and examples were deposited in the American Type Culture Collection. Isolates were resistant to doxycycline and one-third were resistant to cefoxitin. All were susceptible to amikacin, ciprofloxacin, sulfamethoxazole, and imipenem. Surgical debridement combined with drug therapy based on in vitro susceptibilities resulted in cures of cutaneous disease or osteomyelitis. DNA homology studies are needed to determine the taxonomic status of these organisms.
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