Detection of virulence factors in Serratia strains isolated from contact lens-associated corneal ulcers

Purpose: This paper reports on the microbiological findings pertaining to three Serratia isolates from soft contact lens-related corneal ulcers, which represent a complication of contact lens wear reported with increasing frequency. Methods: Bacterial identification and antibiotic susceptibility testing were performed using the Vitek system. Serratia’s ability to form biofilm, produce gelatinase, elastase and alkaline protease, and invade human corneal epithelial (HCE) cells was investigated. The isolates’ susceptibility to the following disinfectants was tested: (a) ReNu MultiPlus, containing polyaminopropyl biguanide 0.0001%; (b) Opti-Free Express, containing polyquaternium-1 0.001% and myristamidopropyldimethylamine 0.0005%; (c) Opti-Free Replenish, containing polyquaternium-1 0.001% and myristamidopropyldimethylamine 0.0005%, and (d) Oxysept Comfort, a one-step 3% hydrogen peroxide-catalase system. Results: Two Serratia marcescens and one Serratia liquefaciens were identified. All the strains were susceptible to aminoglycosides and fluoroquinolones. No isolate formed biofilm or significantly invaded HCE cells; all produced alkaline protease and gelatinase, but not elastase. Opti-Free Express and Opti-Free Replenish were active against S. liquefaciens, but failed to kill the S. marcescens isolates within the minimum recommended time (6 hours). ReNu MultiPlus needed 6 hours to kill one strain of S. marcescens, which is 2 hours more than recommended. Conversely, Oxysept Comfort was always effective within the minimum recommended time (6 hours). Conclusions: The ability to produce alkaline protease and gelatinase may play a major role in the pathogenesis of contact lens-related Serratia keratitis. Several types of contact lens solutions may be ineffective in eradicating Serratia from contaminated contact lens cases within the minimum recommended time. Only exposure to Oxysept Comfort was always effective against Serratia in this study.