Rapid identification and antimicrobial susceptibility profiling of Gram-positive cocci in blood cultures with the Vitek 2 system

Abstract

Rapid identification and antimicrobial susceptibility profiling of the bacteria in blood cultures can result in clinical and financial benefits. Addition of saponin to the fluid from blood culture bottles promotes the recovery of the bacteria and thus may shorten the turnaround time of the microbiological analyses. In this study we compared the identification and susceptibility profiles of saponin-treated and untreated (standard method) blood cultures monomicrobial for Gram-positive cocci using Vitek 2. We concordantly identified 49 (89%) of 55 monobacterial cultures using the results with the standard method as reference. Complete categorical agreement between the susceptibility profiles with the new and the standard method was found for 26 (53%) of 49 isolates, while discrepancies were seen for 23 (47%) cultures. E-tests indicated that the new method resulted in a correct susceptibility profile for 8 (35%) of these 23 blood cultures. Therefore, 34 (69%) of 49 cultures showed a concordant/correct susceptibility profile for all antimicrobials with an overall error rate of 2.3%. Thus, addition of saponin to the fluid from blood culture bottles of the Bactec 9240 leads to the rapid (results available ≥12 hours earlier) and reliable identification and susceptibility profiling of Gram-positive cocci in blood cultures with Vitek 2.

Accuracy of the VITEK 2 system for a rapid and direct identification and susceptibility testing of Gram-negative rods and Gram-positive cocci in blood samples

“…Nonetheless, occurrence of very major errors seem to be recurrent when testing the susceptibility to some antimicrobials such as ampicillin for Gram-negative bacilli (8,19). On the other hand, the absence or very low rate of errors for the susceptibility testing of Gram-positive cocci toward vancomycin is common (8,14,19). Using the VITEK 2 system some bacteria in our study were not identified or were misidentified and various levels of disagreement in the susceptibility testing were found.…”

“…The VITEK system used in the present study was developed by bio Mérieux as an automated system for identification and antimicrobial susceptibility testing and was later improved into the VITEK 2 system. The improved version automates all mandatory steps for identification and antimicrobial susceptibility testing after a standardized inoculum has been loaded into the system (13 Various studies have evaluated the performance of the VITEK 2 system for identification of Gram-negative and -positive bacteria associated with bacteraemia (13,14), but the results vary across studies. This variability does not allow clear and definite conclusions about the performance of the system for both identification and susceptibility testing.…”

Diagnosis of Bloodstream Infections by Mass Spectrometry: Present and Future

“…To shorten the turnaround time required for diagnosis of BSI, the fluid from a positive blood culture bottle may be directly inoculated into an automated system for ID and AST of bacteria, further referred to as direct methods (Figure 1). Several studies have compared the results of the current blood culture method with those by direct methods [10][11][12][13][14][15][16][17][18]. The direct method has been shown to yield reliable results very stable and with a strong signal to noise ratio.…”

“…Treatment with detergents, such as saponin, before inoculation into the appropriate ID and AST cards improves the recovery of Gram-positive cocci as well as P. aeruginosa from positive blood culture, probably by releasing intracellular bacteria from human blood cells [18]. Although some isolates may be missed or misidentified by the direct method, a good categorical agreement between the direct and standard methods is usually observed for AST results, with relatively low overall error rates for the direct method [13,14,17,18]. The main sources of errors by direct methods are mixed cultures and a too small inoculum size.…”

“…The main sources of errors by direct methods are mixed cultures and a too small inoculum size. As a small (6-10%), but significant, percentage of specimens appearing monomicrobial at the Gram staining results to be polymicrobial after subculture [11,13,14,19] the results obtained with the direct method should be considered preliminary until the inoculum has been confirmed to be monomicrobial by subculture. This does not delay the report of results as the mono/polymicrobial nature of specimens can be assessed on subcultures inoculated the previous day.…”