Low-count organisms concealed by dominant uropathogenic organisms in urine of patients with asymptomatic bacteriuria

We compared organisms isolated by selective culture media with those isolated by routine bacteriological methods using the same urine specimens. Urine was obtained by suprapubic bladder aspiration from 84 patients suspected of having asymptomatic bacteriuria, including 31 catheterized patients. The selective media included mannitol salt agar, Enterococcus faecalis agar, nalidixic acid cetrimide agar, and desoxycholate hydrogen sulfide lactose agar with polymyxin B or cefsulodin. Colony counts on the selective media approximated to those on heart infusion agar, except for desoxycholate hydrogen sulfide agar with polymyxin B, where the number of organisms was underestimated. In 24 patients the selective media detected a total of 32 strains not reported by the hospital laboratory. The unreported strains included coagulase-negative staphylococci (41%), other cocci (38%), enterobacteriaceae (9%) and glucose-nonfermenting gram-negative bacilli (9%) and fungi (3%), with the number of organisms being mostly < 10(5)/ml. The majority of the unreported strains coexisted with dominant gram-negative bacilli or enterococci. These results indicate that low-count organisms coexisting with dominant organisms tend to be missed by the routine bacteriological methods.