Viability of organisms held in the isolator blood culture system for 15 h and their rapid detection by acridine orange staining

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Abstract
Sets of three Isolator blood culture tubes were seeded with low numbers of 96 strains of 26 bacterial species (fresh and stock clinical isolates). One tube was processed immediately, and the other two were held at 22 and 34 degrees C for 15 h before processing. Organism recovery was 99, 99, and 98%, respectively. Organism numbers increased at both 22 degrees C (60% of strains) and 34 degrees C (79% of strains). Especially notable was that the increases were seen with most strains of Staphylococcus aureus, streptococcal species, Pseudomonas, and all of the Enterobacteriaceae tested. Seven strains, including Streptococcus pneumoniae and Haemophilus influenzae, although viable, were recovered with a decreased number of organisms at each temperature. Acridine orange staining detected organisms in 53% of those Isolator tubes being held and 71% of those demonstrating a numerical increase, after incubation at 34 degrees C. In addition, it was noted that after processing, 48% of the strains that had increased in number while being held at 34 degrees C resulted in visible growth on agar media in 6 h. The results suggest that up to a 15-h delay in processing Isolator tubes may be possible and that acridine orange staining for the rapid detection of positive cultures may be useful in such a circumstance.