Aspergillus and Pseudomonas are frequently found in culture together, especially in patients with cystic fibrosis and lung transplant recipients. Co-culturing of AF conidia & PA results in growth inhibition and death of AF cells. The killing of AF cells by PA is elicited by a heat labile molecule(s) released into the culture medium. In contrast, PA had no fungicidal effect in co-cultures on AF hyphae pregrown for > 12 h. The lack of effect of PA on AF hyphae could be due to poor activity of the fungicidal factor against hyphae or the release of an antibacterial factor by AF hyphae. Since AF is known to produce many small molecules to safeguard its survival, we studied the antibacterial activity of AF culture filtrates (CF).
AF cultures were grown for 24 h & 48 h in Sabouraud’s dextrose broth at 35 oC using 6-well tissue culture plates. The CF was collected and filter-sterilized. For growth inhibition studies, 1 ml aliquots of the CF was inoculated with 1 x 103 to 1 x 107 cells in 6-ml culture tubes, incubated at 35oC for 24 h. Growth was determined by measuring the absorbance at 490 nm and by colony forming unit (CFU) assay.
CF collected from stationary phase (48 h) cultures inhibited the growth of PA, but not CF obtained from 24 h cultures. CFU assays of CF-treated PA cells showed that the effect was bacteriostatic with no decrease in CFUs for the CF-treated group compared to the initial inoculum. CF boiled for 10 min retained its antibacterial activity compared to that of the unboiled CF. In addition to PA, we examined the effect of CF on the growth of S. aureus and C. albicans. CF had no effect on the growth of C. albicans, whereas it inhibited the growth of S. aureus.
Stationary liquid cultures of AF produce a heat stable bacteriostatic compound released into the culture medium that inhibits PA and this antibacterial factor may be at least partly responsible for the protection of AF hyphae from the fungicidal effect of PA in co-cultures.
Full conference title:
- ASM 111th (2011)