Type III Secretion System Genotypes and Biofilm-Forming Capacity of Pseudomonas aeruginosa Bloodstream Isolates: An Exploratory Analysis of the exoU–Fluoroquinolone Association

Abstract

The pathogenicity of Pseudomonas aeruginosa in bloodstream infection is shaped by biofilm formation and the type III secretion system (T3SS). We characterised the biofilm-associated and T3SS genotypes of 100 non-duplicate bloodstream isolates and tested whether either trait co-segregates with antimicrobial resistance. Biofilm was assessed by Congo red agar (CRA) and the quantitative microtiter plate (MTP) assay; biofilm-associated (pslA, pelA, ppyR) and T3SS effector (exoS, exoY, exoT, exoU) genes by conventional PCR; and susceptibility by an automated system interpreted with EUCAST criteria. Biofilm was detected in 84% of isolates by MTP and 25% by CRA (Cohen’s κ = 0.16). pslA was present in 100%, pelA and ppyR in 99%; all isolates carried exoY and exoT, exoS was found in 88% and exoU in 27%. Multidrug resistance was 15%. Biofilm phenotype was unrelated to resistance. In contrast, levofloxacin resistance was more frequent among exoU-positive isolates (29.6% vs. 9.6%; p = 0.024), and this association survived adjustment for biofilm phenotype and hospital unit in multivariable analysis (adjusted OR 3.90, 95% CI 1.20–12.67). However, the association did not remain significant after correction for multiple comparisons or after excluding exoS/exoU co-positive isolates. The exoU–fluoroquinolone relationship is therefore presented as an exploratory, hypothesis-generating finding that requires confirmation by reference susceptibility testing and clonal characterisation.