Pseudomonas aeruginosa and filamentous fungi colonize airways of cystic fibrosis (CF) patients. P. aeruginosa has direct/indirect inhibitory effects on fungal growth in vitro. Here we assessed P. aeruginosa inhibitory effects on Scedosporium growth.
Twenty-five P. aeruginosa strains (isolate PA14, 9 mucoid and 15 non-mucoid isolates) and 1 strain each of S .aurantiacum and S. prolificans were studied. Strains were co-cultured on Saboraud’s dextrose agar. The zone of inhibition index (Zx) was calculated. The fluorescent stain FUN-1 was used to qualitate degree of hyphal damage after co-culturing with selected P. aeruginosa strains. Growth inhibition of Scedosporium in liquid media was studied using the XTT metabolic assay.
Nineteen (76%) and 20 (80%) P. aeruginosa strains demonstrated Zx values of <1 for S. aurantiacum and S. prolificans, respectively. 5/9 (55.5%) mucoid strains had a Zx <1 for both Scedosporium species with 14/16(87.5%) and 15/16 (93.7%) non-mucoid strains showing similar Zx for S. aurantiacum and S. prolificans, respectively. Overall, similar mean inhibition (Zx=0.76) was observed for both Scedosporium species. However, mean Zx was 0.89-0.88 for mucoid strains and 0.68-0.69 for non-mucoid strains respectively. On FUN-1 staining, S. aurantiacum and S. prolificans co-cultured with live bacteria appeared stunted in growth, damaged or dead. Co-cultivation with dead bacteria revealed actively growing fungi. XTT experiments showed a 55-60% decrease in metabolic activity at 24 hours for both Scedosporium species. When co-cultivated with strain PA14, the OD readings were 0.87 for S. aurantiacum, and 0.95 for S. prolificans (vs. ODs of >2 without PA14 present). Similar results were obtained using a mucoid Pseudomonas clinical strain SP-01.
We found that P. aeruginosa inhibited the growth of S. aurantiacum and S. prolificans on solid media, in liquid media and by fluorescent microscopy staining. Moderate differences in degrees of inhibition were seen with mucoid vs. non-mucoid strains.