Antimicrobial activities of synthetic bismuth compounds against Clostridium difficile

Clostridium difficile is a major nosocomial pathogen responsible for pseudo membranous colitis and many cases of antibiotic-associated diarrhea. Becaus e of potential relapse of disease with current antimicrobial therapy protoc ols, there is a need for additional and/or alternative antimicrobial agents for the treatment of disease caused by C. difficile. We have synthesized a systematic series of 14 structurally simple bismuth compounds and assessed their biological activities against C, difficile and four other gastrointe stinal species, including Helicobacter pylori, Were, we report on Me activi ties of six compounds that exhibit antibacterial activities against C. diff icile, and some of the compounds have MICs of less than 1 mu g/ml. Also tes ted, for comparison, were the activities of bismuth subcitrate and ranitidi ne bismuth citrate obtained from commercial sources. C. difficile and H. py lori were more sensitive both to the synthetic bismuth compounds and to the commercial products than were Escherichia coli, Pseudomonas aeruginosa, an d Protcus mirabilis, and the last three species were markedly resistant to the commercial bismuth salts. Testing with human foreskin fibroblast cells revealed that some of the Synthetic compounds were more cytotoxic than othe rs. Killing curves for C. difficile treated with the more active compounds revealed rapid death, and electron microscopy showed that the bismuth of th ese compounds was rapidly incorporated bye, difficile, Energy dispersive sp ectroscopy X-ray microanalysis of C, difficile cells containing electron-de nse material confirmed the presence of internalized bismuth. Internalized b ismuth was not observed in C, difficile treated with synthetic bismuth comp ounds that lacked antimicrobial activity, which suggests that the uptake of the metal is required for killing activity. The nature of the carrier woul d seem to determine whether bismuth is transported into susceptible bacteri a like C. difficile.