Failure of an automated blood culture system to detect nonfermentative gram-negative bacteria

During a 1-year study we observed that both aerobic and anaerobic blood cul ture bottles from patients were negative by the BacT/Alert system during a 7-day incubation period, However, upon subcultivation of negative bottles, growth of Pseudomonas aeruginosa was detectable, In an attempt to explain t his observation, aerobic BacT/Alert Fan bottles were seeded with a defined inoculum (0.5 McFarland standard; 1 ml) of Escherichia coli, Klebsiella pne umoniae, Serratia marcescens, P. aeruginosa, Stenotrophomonas maltophilia, or Acinetobacter baumannii, Half of the inoculated bottles were loaded into the BacT/Alert system immediately, and the remainder were preincubated for 4, 8, 16, and 24 h at 36 degrees C. With preincubation all bottles seeded with the Enterobacteriaceae signaled positive during the next 1.5 h, Organi sms in bottles seeded with the nonfermentative species P. aeruginosa and A. baumannii remained undetected by the BacT/Alert system for 7 days. S. malt ophilia was detected if the preincubation time was equal or less than 8 h. Without preincubation all bottles seeded,vith the Enterobacteriaceae or non fermentative species signaled positive. Since nonfermentative species seem to enter a state of bacteriostasis within the preincubation period, we reas oned that an unknown factor is consumed. Accordingly, a smaller inoculum sh ould allow the detection of nonfermentative species, even after preincubati on, and serial dilutions of P. aeruginosa were detected in preincubated bot tles. In this case preincubated bottles signaled positive faster than bottl es without preincubation. We conclude that all bottles from clinical settin gs should be subcultured prior to loading to avoid false negatives. An alte rnative may be preincubation at room temperature.