Background: Matrix-assisted laser desorption-ionization time-of-flight mass spectrometry (MALDI-TOF MS) is a powerful technique for the rapid identification of bacteria from growing colonies in routine cultures. In this study, we evaluated the feasibility of a 5-hour incubation on solid medium after sub-cultivation of positive blood culture broth without any preparation steps in order to speed up the identification of bacteria.
Methods: In addition to standard laboratory protocols, a Columbia agar plate with 5% sheep blood was inoculated with 1 drop from the blood culture broth. After a 5-hour incubation period, a colony from the culture plate was submitted to MALDI-TOF MS.
Results: A total of 1351 positive blood cultures (1299 monomicrobial and 51 polymicrobial) were analyzed. When compared to routine identification procedure results for positive blood cultures, 79.3% of isolates were correctly identified to the species level. When manufacturer-recommended score values were taken into account, MALDITOF MS correctly identified 98.4% of the isolates to the species level with a score of > 2.0, 89.1% with a score between 1.7 and 2.0, and 75.4% with a score of < 1.7.
Conclusions: ln our evaluation, a large majority of the S. aureus (91.5%) and Enterobacteriaceae (87.6%) were correctly identified at the species level. A 5-hour incubation period was found to be associated with moderate identification results for CoNS, Enterococcus spp., and nonfermentative gram negative bacilli, with failure being mostly observed with Streptococcus spp., Candida spp., and other gram positive bacteria. We believe that the performance of MALDI-TOF MS identification after short-term culture is directly related to the sufficient growth of microorganisms at 5 hours.