Comparison of the BACTEC™ blood culture system with conventional culture of cerebrospinal fluid samples.

Objective – Many invasive andlife-threatening infections are diagnosed by microbial culture of sterile bodyfluid specimens. The standard procedures used for conventional cultivation ofbacteria and yeasts from sterile body fluids other than blood involveinoculation onto a solid medium. The aim of this study was to evaluate theBactec blood culture system (Becton Dickinson,Sparks, MD) in comparison with the conventional culturemethod for microbial isolation from cerebrospinal fluid (CSF) samples. Methods - From January 2012 to November2013, 788 samples of CSF collected from 390 patients with the clinical suspicion of centralnervous system infection (meningitis, encephalitis) or polytrauma involvingcentral nervous system were routinely sent to the Bacteriology laboratory of the Unit ofClinical Microbiology at the University Hospital of Parma for the diagnosisof infections by bacteria and fungi. Conventional cultures were performed on chocolate agar,blood agar, MacConkey agar plates for aerobic bacteria, and when possibledepending on the volume of sample available on bile-esculine agar and Schaedleragar plates for anaerobic bacteria (KIMA, Piove di Sacco-PD, Italy) accordingto standard procedures; in parallel, an aliquot (1ml) of each sample wasinoculated directly into an aerobic Bactec Ped Plus/F bottle added withFastidious Organism Supplement (Becton Dickinson) which improves theopportunity for growth of fastidious organisms. When a bottle signaled apositive result by the Bactec FX instrument a Gram stain was performed and analiquot was subcultured onto conventional media. Results – Among the 87 positive samplesbelonging to 51 patients analyzed, clinically significant microorganisms(mainly bacteria and in three cases fungi) were identified from 45 samples(51.72%) by both methods. For the remaining 42 specimens (48.28%), growth wasdetected by the Bactec system, while there was no growth onto solid mediainoculated directly with the samples. No microorganism which went undetected bythe Bactec system was detected by conventional cultures. The most frequentlymicroorganisms recovered only by the Bactec system were gram-positive cocci (21coagulase-negative staphylococci, 5 Streptococcus spp., 1 Kocuriakristinae and 1 Rothia mucilaginosa) while theGram-negative bacilli and Gram-positive bacteria were equally recovered by bothculture methods. In two samples belonging to different patients the Bactecsystem alone allowed the isolation of Staphylococcus aureus and in eightsamples (belonging 4 patients) of Gram-negative bacilli. Conclusion – The Bactec system was shownto enhance the detection of microorganisms in CSF versus conventionalmethods of 48.28%. Although part of these additional positive results could belikely referred as due to contamination during sample collection, we canconclude that when applied and evaluated on a wider group of samples, theBactec blood culture system might be in the future routinely used to improvethe yield of clinically significant microorganisms from cerebrospinal-fluid.