Abstract
The case of a mentally ill man inadvertently treated with trimethoprim-sulfamethoxazole (TMP-SMX) for 2 y is presented. Quantitative stool cultures revealed a substantially suppressed Gram-negative aerobic flora, while Enterococcus spp. and anaerobes were not affected. Yeasts were moderately increased. TMP-SMX represents an attractive antimicrobial for immunocompromized patients who need the integrity of their intestinal anaerobic flora for colonization resistance.