Prevalence of exotoxin and type III secretion system genes in multidrug-resistant Pseudomonas aeruginosa isolates from patients with burn injuries in Southern Iran

Pseudomonas aeruginosa is a leading cause of burn wound infections, with pathogenicity driven by exotoxins and the Type III Secretion System (T3SS). Among its components, ExoA, ExoT, and the popB gene (T3SS translocon) are particularly associated with acute tissue damage and treatment failure. In this cross-sectional study (March 2021–March 2022), 108 non-duplicate, drug-resistant P. aeruginosa isolates were collected from burn patients at Amir-Al-Momenin Hospital, Shiraz. PCR assays detected exoA, exoT, exoS, exoU, exoY, and T3SS (popB). Antimicrobial susceptibility testing was performed according to CLSI 2022 guidelines, with MIC determination for colistin and imipenem. MDR was defined as resistance to ≥ 3 antibiotic classes. Among the detected genes, exoA (81.5%) and exoT (77.8%) were the most prevalent, followed by exoY (68.5%) and T3SS (popB) (66.7%), while exoS (63.9%) and exoU (46.3%) were less frequent. Isolates carrying T3SS (popB) showed significantly higher resistance to multiple antibiotics compared to T3SS (popB) negative isolates (p < 0.05). MIC analysis demonstrated preserved susceptibility to colistin (MIC50 = 0.25 µg/mL, MIC90 = 0.5 µg/mL) but high imipenem resistance (MIC50 = 32 µg/mL, MIC90 = 64 µg/mL). ExoA and ExoT were the most prevalent exotoxin genes, while the T3SS (popB) gene, although slightly less frequent, showed a strong association with multidrug resistance. These findings highlight the clinical relevance of these virulence determinants, particularly popB, and support further investigation into T3SS-targeted therapies.