Emergence of High-Level Trimethoprim Resistance in FecalEscherichia coliduring Oral Administration of Trimethoprim or Trimethoprim-Sulfamethoxazole

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Abstract
The effect of daily administration of trimethoprim (TMP), trimethoprim–sulfamethoxazole (TMP-SMX), or placebo on aerobically grown fecal gram-negative bacteria was monitored in 136 students from the United States during a two-week diarrhea-prevention study in Mexico. Unlike patients in other studies with these agents, who had urinary-tract infection or granulocytopenia, most persons in this study had no change in total fecal Enterobacteriaceae and had high-level TMP and SMX resistance in virtually all these strains. Escherichia coli was the predominant TMP-resistant organism isolated; 96 per cent of 165 TMP-resistant Esch. coli isolates were resistant to at least four antimicrobial agents, and 25 per cent were resistant to seven. TMP resistance was transferable in 40 of 100 strains tested.