Predominance of SCCmec type III, multidrug resistance, and evidence of ica-independent biofilm formation among clinical Staphylococcus aureus isolates in Riyadh, Saudi Arabia

Staphylococcus aureus remains one of the most common causes of infections, with its clinical treatment complicated by the emergence of multidrug-resistant (MDR) strains and the bacterial ability to form biofilms. This study aimed to determine biofilm formation, MDR, and staphylococcal cassette chromosome mec (SCC mec ) typing of S. aureus isolated from a private hospital in Riyadh, Saudi Arabia. A descriptive cross-sectional study was conducted over a one-year period (February 2025 – February 2026). A total of 1,604 clinical samples were collected from patients (> 18 years), and isolates were identified using matrix-assisted laser desorption/ionization time of flight mass spectrometry (MALDI-TOF MS). Antibiotic susceptibility was determined by the Kirby-Bauer disk diffusion method. Biofilm formation was assessed via crystal violet assay, quantified by OD 492 , and classified by Stepanović’s method. The presence of mecA , icaA , and SCC mec types were detected via polymerase chain reactions (PCR). From 1,604 samples, 107 (6.67%) non-duplicate S. aureus isolates were obtained, primarily from urine (34.6%; 37/107). Isolates were more frequent among females (61.7%; 66/107) and patients aged 41–60 (58.8%; 63/107). Among the isolates, 62.6% (67/107) were identified as methicillin resistant S. aureus (MRSA) and 71.9% (77/107) were MDR. Biofilm formation was observed in 72.9% (78/107) of isolates (25.2% strong, 17.8% moderate, and 29.9% weak). Notably, only 15.9% (17/107) of isolates carried the icaA gene, suggesting that the majority of biofilm formation occurs via ica -independent mechanisms. Biofilm-forming isolates exhibited significantly higher resistance to cefoxitin ( P < 0.001) and ciprofloxacin ( P = 0.028) compared to non-biofilm isolates. MRSA, MDR, and the mecA and icaA genes were likewise significantly more prevalent in biofilm-formers ( P < 0.05). SCC mec type III was the most frequent among the MRSA isolates (46.3%; 31/67). This study highlights a strong correlation of biofilm formation with increased antibiotic resistance among our S. aureus clinical isolates. The high prevalence of MRSA, MDR, and SCC mec type III suggest the need for continuous surveillance and robust infection control strategies to manage clinical pathogens.