Biofilm Formation and Quorum-Sensing-Molecule Production by Clinical Isolates of Serratia liquefaciens

Serratia spp. are opportunistic human pathogens responsible for an increasing number of nosocomial infections. However, little is known about the virulence factors and regulatory circuits that may enhance the establishment and long-term survival of Serratia liquefaciens in the hospital environment. In this study, two reporter strains, Chromobacterium violaceum CV026 and VIR24, and high-resolution triple-quadrupole liquid chromatography-mass spectrometry (LC-MS) were used to detect and to quantify N-acyl-homoserine lactone (AHL) quorum-sensing signals in 20 S. liquefaciens strains isolated from clinical samples. Only four of the strains produced sufficient amounts of AHLs to activate the sensors. Investigation of two of the positive strains by high-performance liquid chromatography (HPLC)-MS confirmed the presence of significant amounts of short-acyl-chain AHLs (N-butyryl-L-homoserine lactone [C-4-HSL] and N-hexanoyl-L-homoserine lactone [C-6-HSL]) in both strains, which exhibited a complex and strain-specific signal profile that included minor amounts of other short-acyl-chain AHLs (N-octanoyl-L-homoserine lactone [C-8-HSL] and N-3-oxohexanoyl-L-homoserine lactone [OC6-HSL]) and long-acyl-chain (C-10, C-12, and C-14) AHLs. No correlation between biofilm formation and the production of large amounts of AHLs could be established. Fimbria-like structures were observed by transmission electron microscopy, and the presence of the type 1 fimbrial adhesin gene fimH in all strains was confirmed by PCR. The ability of S. liquefaciens to adhere to abiotic surfaces and to form biofilms likely contributes to its persistence in the hospital environment, increasing the probability of causing nosocomial infections. Therefore, a better understanding of the adherence properties of this species will provide greater insights into the diseases it causes.