The antimicrobial activity of urine of paraplegic patients receiving methenamine mandelate.

The antimicrobial activity of urine collected from adult male paraplegics ingesting methenamine mandelate (MM) was evaluated. The in vitro bacterial growth in urine from these patients was inhibited when the free formaldehyde (HCHO) concentration was 10 to 22 microgram per ml. When the HCHO concentration was in the region of 28 microgram per ml or greater, bactericidal effect became apparent. Urine containing 1323 microgram of MM per ml with a pH of 5.9 when freshly voided had sufficient HCHO to be bacteriostatic. Urine containing at least 1740 microgram of MM per ml with a pH of 5.1 or less when freshly voided was bactericidal. The latter concentration of MM in urine was usually achieved when the patient ingested 4 g of MM per day in divided doses and the intake of fluid was not excessive. Under some circumstances an individual receiving MM without an additional acidifying agent may produce urine with a pH low enough to release sufficient HCHO to exert a useful antibacterial effect. However, supplementary acidification with ammonium chloride produced marked lowering of the urinary pH in all patients receiving MM, resulting in bactericidal levels of HCHO.