Rapid Identification and Susceptibility Testing of Gram-Negative Bacilli from Blood Cultures Using the Vitek 2 System

Providing rapid results for blood culture isolates is a critical function of clinical microbiology laboratories. This study evaluated the accuracy and turnaround time for identification and susceptibility testing of Gram-negative bacilli inoculated directly from positive blood cultures into the Vitekr 2 system. Direct inoculation was compared to conventional methods; which included biochemical tests; commercial identification systems and disc diffusion susceptibility testing. Two hundred and ninety-one of 327 isolates (89) were correctly identified to at least genus level by the direct Vitekr method. Susceptibility test results were compared for 3;925 organism antibiotic combinations. The overall rate of categorical agreement of direct and conventional antimicrobial susceptibility testing was 92with less than 3very major and major errors combined. The mean turnaround time for identification and susceptibility testing was 7.5 hours (SD 3.0 hours) compared to a mean of 32.3 hours (SD 14.7 hours) for the conventional method. These results suggest that direct inoculation of the Vitekr 2 system from blood cultures provides accurate; reliable identification and antimicrobial susceptibility results for the majority of commonly occurring Gram-negative pathogens; while the significantly reduced turnaround time should benefit patients and permit earlier rationalisation of antibiotic therapy; with a reduction in the use of broad spectrum antibiotics. A suggested protocol for routine use is included

Superior Detection of Pathogens in Synovial Fluid by the Bactec 9240 Peds Plus/F System Compared to the Conventional Agar-based Culture Method

To improve culture yield in cases of possible septic arthritis; we compared culture of joint fluid aspirates on conventional agar-based media to culture in Bactec 9240 Peds/Plus F blood culture bottles with and without the addition of fastidious organism supplement (FOS). Over a period of 21 months; we analysed 123 synovial fluid samples and isolated 20 pathogens. The Bactec methods proved superior by yielding more pathogens than the conventional culture method (p=0.074). However; this method also yielded more contaminants within the first three days of incubation (p=0.027). All contaminants detected after three days of incubation were the result of overgrowth on conventional method agar plates. The Bactec methods provided clinicians with a positive pathogen result one day earlier than the conventional counterpart (p=0.001). Four isolates of Neisseria gonorrhoeae were only cultured with the Bactec method. No significant benefit was demonstrated by supplementing blood culture bottles with FOS. We recommend that whenever infection by fastidious organisms is suspected; synovial fluid aspirates should be cultured using automated blood culture systems to increase the culture yield and to decrease the time to detection