Synergistic Effects of Candida strains and Staphylococcus aureus in Promoting Biofilm Formation and Therapeutic Resistance

Abstract

Background: Polymicrobial biofilms are often the source of chronic infections. Crucially, biofilms resistance to antimicrobial therapy is one of the main reasons that treating these illnesses is so challenging.

Objective: The purpose of the present study was to evaluate the resistance of biofilms to antibiotics and antifungal drugs formed by clinical Candida strains, once in monomicrobial biofilms and another in compatible with Staphylococcus aureus strains via co-culture biofilm formation.

Methodology: This study was designed as a laboratory-based cross-sectional investigation. The clinical samples were collected from patients at Al-Hussein Teaching Hospital in Al-Muthanna Governorate, southern Iraq, from October 2024 to January 2025. All microbiological assays, including isolation, identification, and biofilm assays for both Staphylococcus aureus and Candida species, were performed in the microbiology laboratory of the biology department, College of Science, University of Thi-Qar, Iraq. Staphylococcus aureus (29 isolates) and Candida species (26 isolates) were obtained from several body sites, including burns, urine, blood, vagina, oral swabs, diabetic foot, pus, and sputum. Biofilm formation was evaluated using flat-bottom microtiter plate assay with crystal violet. The anti-biofilm assay was achieved by using the microdilution method in 96-well flat-bottomed microplate, according to the Clinical and Laboratory Standards Institute (CLSI) guidelines.

Results: According to the anti-biofilm assay, 29 isolates of Staphylococcus aureus developed biofilms even with presence of antibiotic. However, strains of Staphylococcus aureus were more resistant to ampicillin and tetracycline (13.8% biofilm formation) than vancomycin (10.3%) at high concentrations. Thirteen Candida tropicalis, seven Candida krusei and six Candida albicans strains were able to form biofilms even in the presence of antifungal drugs. At 256 µg/ml, itraconazole was more effective than fluconazole (19.2%) and voriconazole (50.0%) at inhibiting the Candida species (0.0% biofilm). Compared to monomicrobial biofilms, polymicrobial biofilms of Staphylococcus aureus and Candida species exhibited 100% biofilm formation with greater resistance to antimicrobial treatments.

Conclusion: In conclusion, biofilms composed of compatible strains of both Staphylococcus aureus and Candida species enhanced antibiotic resistance, compared to monomicrobial biofilms.

Bangladesh Journal of Infectious Diseases, December 2025;12(2):205-214