Short Incubation of Positive Blood Cultures on Solid Media for Species Identification by MALDI-TOF MS: Which Agar Is the Fastest?

Short incubation of positive blood cultures on solid media is now increasingly applied to speed up species identification by matrix assisted laser desorption ionization-time of flight mass spectrometry (MALDI-TOF MS). Although Columbia blood agar (CBA) and chocolate agar (Choc) are widely used, a direct comparison of standard agars is lacking. We therefore compared the time to species identification of blood cultures incubated on CBA, Choc, and MacConkey agar (MAC, for Gram-negative rods). Positive aerobic/anaerobic blood cultures (2 drops = 50 μl) were incubated on CBA, Choc, MAC, and the required time of incubation to low-confidence identification (score of ≥1.7 to <2) and high-confidence identification (score of ≥2) by MALDI-TOF MS was measured. Exclusion criteria were (i) false-positive blood cultures, (ii) mixed cultures with different species, (iii) growth of anaerobes/fungi, and (iv) a total number of isolates of one group (i.e., Gram-positive/-negative cocci/rods) of <30. A total of 187 blood cultures with Gram-positive cocci (n = 124) and Gram-negative rods (n = 63) were included in the final analysis. The shortest median time to high-confidence identification (score of ≥2) was achieved on MAC for Gram-negative rods (2.0 h; range, 1.9 to 4.2 h) and on CBA for Gram-positive cocci (4.0 h; range, 1.9 to 25.0 h). However, the difference from results obtained with Choc was not statistically significant. When only one agar plate is used for short incubation of positive blood cultures, Choc may represent a compromise in terms of time to high-confidence identification by MALDI-TOF MS and the bacterial spectrum that is covered. However, using only Choc is disadvantageous when the shortest incubation times to identification are strived for. IMPORTANCE When blood cultures are flagged as positive, they are incubated on solid media to produce enough biomass of the bacterium for identification and susceptibility testing. Rapid turnaround times for laboratory results could save lives, and we wanted to assess which solid medium is best to shorten the time to species identification using MALDI-TOF mass spectrometry. For that purpose, we used positive blood cultures from routine diagnostics and compared Columbia blood agar (CBA), Chocolate agar (Choc), and MacConkey agar (MAC, for Gram-negative rods). We found that MAC performed best for Gram-negative rods and CBA was quickest for Gram-positive cocci. However, Choc may represent a compromise if fastidious species should be covered.