Quantitative colorimetric measurement of residual antimicrobials in the urine of patients with suspected urinary tract infection

A simple microtitre plate assay is used to detect antimicrobial activity in clinical urine specimens and its potential as a screening tool is assessed. The assay is based on a colorimetric substrate, p-nitrophenyl-β-D-glucopyranoside, in combination with a Bacillus subtilis strain to detect antimicrobial residues. The assay identified antimicrobial activity in 31% of the 527 clinical urine samples tested. The majority of the samples (65%) came from the community, with the rest comprising hospital in-patients (19%) and out-patients (16%). The results demonstrated that there is an association between gender and the presence of inhibitory substances, as 40% of males and 27% of females tested positive. Just over two-fifths of hospital patients (46%) tested positive for inhibitory substances, compared to 26% of samples from community patients. Of the 306 samples that were culture-negative (