Automated detection of microbial growth in blood cultures by using stainless-steel electrodes

Sterile stainless-steel electrodes implanted in blood culture bottles and monitored electronically were used to detect growth of microorganisms. Each blood culture bottle contained 100 ml of medium and was inoculated with 10 ml of blood seeded with either 300 or 50 colony-forming units of one of several bacterial or yeast species that are commonly isolated from clinical blood cultures. Growth was indicated by a voltage change of at least 0.1 mV/min with an increasing slope over at least three consecutive 15-min intervals. This method was compared to the conventional visual method for detecting microbial growth in broth. Growth detection by both techniques was confirmed by subculture to solid media. Of the 163 cultures seeded with the high inoculum (300 colony-forming units) and confirmed as being positive, 148 (90.8%) were positive by the electronic detection system (EDS). At the lower inoculum (50 colony-forming units), 47 of 53 (88.7%) positive cultures were detected by EDS. Twelve of the 21 false-negatives by the EDS were cultures seeded with Cryptococcus neoformans. Excluding C. neoformans, the rate of detection of growth was 96.0%. Microbial growth was detected an average of 18 h earlier by EDS than by the conventional system in 176 (90.2%) of the cultures. Also examined were 156 patient blood cultures: 13 were positive both by EDS and by conventional methods.