Antimicrobial susceptibility testing of mixed biofilms formed by Candida spp. and Staphylococcus aureus co-isolated from the sputum of cystic fibrosis patients in Northwest Algeria.

Cystic fibrosis (CF) is associated with recurring lung infections, which are commonly caused by Staphylococcus aureus and Candida species. These pathogens often form polymicrobial biofilms that are resistant to conventional treatments. Studying their interactions and testing effective therapies is essential for better CF infection control. The current study aimed to examine the antifungal and antibacterial activity of each alone against Candida species and S. aureus co-isolated in both states, planktonic and biofilm (single and polymicrobial biofilm), then in combination for the mixed biofilm of C. tropicalis-S. aureus24. Both Candida spp. and S. aureus isolates were sensitive to the tested antifungals and antibiotics in planktonic mode. However, in biofilm mode, SMICs were up to 32 times higher than SMICs for both species. Thus, all the strains tested are capable of forming biofilms. Treatment of multi-species biofilms (C. tropicalis1/S. aureus24) with combinations of antifungals and antibiotics (AmB/VAN, AmB/AK, AmB/GEN, AmB/CIP) and (Vrc/VAN, Vrc/AK, Vrc/GEN, Vrc/CIP) showed a significant reduction in SMICs compared with those obtained with the two antimicrobials separately. Calculation of the FICIs highlighted two types of interactions, synergistic and indifferent, between the antifungals and the antibiotics tested. The study shows that antifungal-antibiotic combinations can effectively reduce biofilm resistance in CF infections. These synergistic effects may improve treatment outcomes while lowering drug doses and resistance risk.