Comparison of a novel trimethoprim-sulfamethoxazole-containing medium (XT80) with kanamycin agar for isolation of antibiotic-resistant organisms from stool and rectal cultures of marrow transplant patients.

A new medium (XT80) containing trimethoprim-sulfamethoxazole (TMP-SMZ) was characterized and compared with kanamycin-containing tryptic soy agar (KA) for the recovery of multiply resistant organisms (MRO) in rectal and stool cultures. Cultures from 151 patients hospitalized for bone marrow transplantation were screened for MRO. A total of 366 MRO were recovered from 702 cultures on 94 patients during a 6-month period. XT80 detected more gram-negative bacilli and Corynebacterium spp. than KA. Detection of Staphylococcus spp. was equivalent for the two media. Multiple-antibiotic resistance, defined as resistance to three or more classes of antibiotics, was confirmed by standard agar disk diffusion susceptibility testing. Growth on XT80 correctly identified heteroresistant strains of methicillin-resistant Staphylococcus spp. XT80 more rapidly detected thymidine-dependent mutants of Staphylococcus spp. and members of the family Enterobacteriaceae. Lipophilic Corynebacterium spp., including Corynebacterium group JK, also were more readily detected with XT80. TMP-SMZ given as prophylaxis against Pneumocystis carinii infection exerts a selective pressure on organisms that colonize immunocompromised patients and appears to select for colonization with MRO. Colonization with MRO preceded infection for 94% of 36 patients who developed bacteremia. XT80 is a useful screening tool; growth on this medium correlates closely with resistance to TMP-SMZ and is as accurate a predictor as KA for the carriage of MRO.