PO33 | EFFECT OF CITRIC ACID-BASED COMPOUNDS WITH SILVER IONS AGAINST BIOFILMS FORMED BY LISTERIA MONOCYTOGENES AND PSEUDOMONAS FLUORESCENS ISOLATED FROM THE FOOD INDUSTRY: A COMPARISON BETWEEN STATIC AND DYNAMIC CULTURING CONDITIONS

Cristina Vono, Andrea Ferrier, Giacomo Di Giacinto, Muhammad Tauseef Ahmad, Felice Panebianco, Pierluigi Di Ciccio | Dipartimento Scienze Veterinarie, Torino, Italy.

Aim. Two of the most frequently isolated microorganisms in food processing plants are Listeria monocytogenes, a major foodborne pathogen, and Pseudomonas fluorescens, a well-known spoilage microorganism. Both can coexist in biofilms and persist for long periods in food processing environments, growing on surfaces under static and dynamic conditions. The interactions between the two species can increase the biofilm’s resilience and tolerance to sanitation treatments. In addition to conventional biocides new biofilm control strategies include citric acid activated by silver ions, which appear to have promising antimicrobial effects. The aim of this study is to investigate the biofilms of L. monocytogenes and P. fluorescens under static and dynamic conditions on stainless steel (SS) and polycarbonate (PC) surfaces, and to compare the effect of a silver ion-activated, citric acid-based molecule on the growth of these biofilms under such conditions.

Methods. Two wild-type strains, isolated from the dairy (P. fluorescens) and meat (L. monocytogenes) processing industries, were selected for this study. Reference strains were included. All strains were categorized for their biofilm-forming capacity following a standard colorimetric assay. Two formulations of citric acid-silver ion, SC30 and SC2400, were tested. The minimum biofilm eradication concentration (MBEC) had been assessed in accordance with Innovotech Inc. MBEC protocol. The obtained MBEC values were tested on 24-hour biofilms formed on SS and PC coupons under static and dynamic conditions (by CDC Bioreactor) using the macro-method assay and a 20-minute contact time, in order to quantify the reduction of viable biofilm cells.

Results. The ATCC strain of P. fluorescens was sensitive to a high concentration of SC30 (MBEC=50%) and a low concentration of SC2400 (MBEC=3.12%); no MBEC was found for L. monocytogenes to SC30, but the pathogen was sensitive to high concentrations of SC2400 (MBEC from 100% to 75%). The maximum logarithmic reduction (MLR) in viable detached cells was observed for L. monocytogenes, reaching values of ≥6.69 Log CFU/cm^2 under static conditions and ≥5.22 Log CFU/cm^2 under dynamic conditions on both surfaces. For P. fluorescens ATCC, the MLR was observed following treatment with SC30 under static conditions (≥7.75 Log CFU/cm^2 for both SS and PC) and dynamic conditions (≥5.91 Log CFU/cm^2 for both SS and PC), similar to that observed after treatment with SC2400 under dynamic conditions. Only for this strain, under static conditions, the MLR was not achieved with SC2400 treatment, which instead produced logarithmic reduction values between 4.95 and 6.20 Log CFU/cm^2 on SS and PC respectively.

Conclusions. Citric acid-silver ion compounds can be effective for the treatment of biofilms formed under both static and dynamic conditions, although their efficacy was influenced by both the bacterial species and the formulation. Additional trials at concentrations below the MBEC should be performed, including evaluation against biofilms growth under controlled flow conditions. This is crucial to assess any further potential differences in the compound's efficacy against biofilms formed in static, dynamic and variable-flow conditions that are relevant to food-processing environments.