A simple blood culture bacterial pellet preparation for accurate direct bacterial identification and antibiotic susceptibility testing with the Vitek2 system

Abstract number: P1884

Prod'hom G., Durussel C., Bille J., Greub G.

Objectives: Blood cultures are the best approach to establish the etiology of bloodstream infections. Rapid identification and antimicrobial susceptibility tests (AST) are pivotal to guide antibiotic treatment. This may be accurately done using the Vitek 2 system inoculated with subcultures. However, direct testing of bacterial pellets from positive blood cultures do not provide consistent results. Thus, we developed a specific procedure to prepare pellets from positive blood culture that may be directly used to identify both Gram-positive and Gram-negative bacteria with Vitek cards.

Methods: Pellets from positive blood culture vials (Bactec 9240) were treated with ammonium chloride lysing solution and centrifuged. GN and GP Vitek cards were used for the identification of Gram-positive cocci and Gram-negative rods, respectively. The AST-P586 and AST-GN26 Vitek cards were used for AST of staphylococci and Gram-negative rods, respectively. Identification and AST were repeated using subcultures (gold standard). Identification was classified as correct, low-level of discrimination, misidentification, and non-identification. For AST, discordance results were classified as: very major error when the result of direct method was susceptible and that of the reference method was resistant; a major error was the opposite (resistant vs susceptible). All other errors were minor errors.

Results: Among a total of 122 blood cultures, 90 (74%) gave correct identification by Vitek direct inoculation. Among 38 Gram-negative rods, 36 (95%) were correct and 2 (5%) gave low discrimination. Among 84 Gram-positive cocci, 54 (64%) gave a correct identification. The number of low discrimination, misidentification and non-identification were 6 (7.1%), 19 (22.6%)(12/19 S. intermedius), and 5 (5.9%), respectively.

AST results were analysed for 86 isolates with congruent identification. For 34 Gram-negative rods, the overall categorical agreement was 96.9%. The number of very major, major, minor was 1 (0.15%), 5 (0.7%), 15 (2.2%), respectively. 8/21 errors occurred with one isolate (K. pneumoniae). For 52 staphylococci, the overall categorical agreement was 97%. The number of very major, major, minor was 0 (0%), 2 (0.2%), 27 (2.7%), respectively.

Conclusion: Bacterial pellets from positive blood cultures prepared with an ammonium chloride-driven hemolysis allow direct inoculation of Vitek cards used for identification and antimicrobial susceptibility testing with relatively good accuracy.

Session Details

Date: 10/04/2010
Time: 00:00-00:00
Session name: Abstracts 20th European Congress of Clinical Microbiology and Infectious Diseases
Location: Vienna, Austria, 10 - 13 April 2010
Presentation type:
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