Investigating the current blood culture system for possible improvements in isolation of fungal pathogens

Abstract

Invasive fungal infections are the cause of morbidity and mortality in developed countries. Unfortunately blood culture systems possess several limitations in the isolation of fungi. The lack of ability to culture Aspergillus species from blood in invasive aspergillosis is probably the most important limitation in the diagnosis of fungal infections. The aim of this project was to evaluate the blood culture system (BacT/Alert) for possible improvements in the isolation of fungi. In the first part of this project, the growth of four Aspergillus species was evaluated in the Standard Aerobic and FAN Aerobic blood culture media. Terminal subcultures were then prepared using a 'novel' subculture technique. In the second part of the project false negatives were tested by preparing terminal subcultures from automated negative blood cultures, which contained the blood of neutropenic and immunocompromised patients. All four species were detected visually in both bottles between 24 and 48 hours, at the highest conidial concentration. Growth was less successful in the bottles inoculated with the hyphal suspensions due to serious limitations in the technique used for their preparation. The novel subculture technique was also successful in the recovery of Aspergillus on solid media, even at the lowest inocula concentrations. After performing terminal subcultures, Candida tropicalis was recovered from one automated negative bottle, thus identifying a false negative. The results obtained were compared with similar studies which utilised the BACTEC blood culture system. The latter produced better results. This study has shown that blood culture media support the growth of Aspergillus. This is concordant with similar studies performed on other blood culture systems. The challenge is therefore finding the cause for the lack of positive blood cultures in patients with invasive aspergillosis. This study has also shown the importance of terminal subcultures in cases of suspected Candidaemia.