The capability of bacteria to colonize food processing surfaces and to form biofilm has become an emerging concern for food industry. The presence and persistence of biofilm on food processing surfaces may pose a risk of food spoilage or food poisoning. A better understanding of bacterial adhesion and resistance of biofilms is needed to ensure quality and safety of food products. This review focuses on microscopic approaches incorporated to explore biofilm mode of existence in food processing environments. An application of antimicrobial agents for the biofilm control, in particular for bacteria connected with food processing environments, is also highlighted. In addition, some aspects of biofilm resistance, especially the phenomenon of persister cells, are discussed.
Aims: The assessment of the bacterial viability of chlorine-and quaternary ammonium compounds (QACs)-treated Lactobacillus cells by culture-dependent and -independent methods. Methods and Results: Lactobacillus isolates (Lactobacillus plantarum G1, Lactobacillus plantarum B1, Lactobacillus brevis S1 and Lactobacillus paracasei W1) in biofilm and planktonic cell suspensions were treated with chlorine-based (0Á018 and 0Á18%) and QACs-based (0Á2 and 2Á0%) disinfectants for 5 min and then analysed by plate counting, flow cytometry (FCM) and fluorescence activated cell sorting (FACS). The reaction of sessile cells to disinfectants was assessed with the confocal laser scanning microscopy (CLSM). Plate counts revealed L. paracasei W1 to be substantially inactivated by both disinfectants, while counts of the other isolates to be significantly reduced only by QACs, with L. plantarum B1 and L. brevis S1 showing a greater difference between QACs concentrations and cell types. In several cases, the disinfectants caused slightly higher inactivation of planktonic than biofilm cells, with L. plantarum B1 being significantly less sensitive to QACs in biofilm cells (P < 0Á05). Following FCM with a Syto â 9/PI assay, which addresses cell membrane integrity, the emergence of damaged (Syto â 9 À PI + ) and injured (Syto â 9 + PI + ) subpopulations was often observed in cells when they were treated with QACs, whereas intact (Syto â 9 + PI À ) and unstained (Syto â 9 À PI À ) subpopulations were mostly encountered in chlorine-treated cells. Except Syto â 9 À PI + , all subpopulations were recovered on agar plates following FACS, with biofilm cells showing higher culturability irrespective of conditions, probably because of the residues of the biofilm matrix which serve as a protective cover for the bacteria. The CLSM revealed a substantial cell membrane damage within the QACs-treated biofilms, however, some cells deep in the biofilm were still intact and thus remained protected against this disinfectant. Conclusion: We found that FCM/FACS proved useful in the analysis of lactobacilli membrane integrity in disinfection experiments as well as in recovery evaluation of planktonic-biofilm cell subpopulations. In turn, CLSM was particularly useful in investigating the resistance mechanism when Lactobacillus cells were embedded in biofilms. Significance and Impact of the Study: This study highlights the need for treatment optimization on a case-by-case basis to avoid the emergence of cells in intermediate states with recovery potential and to reach and, thus, kill all bacteria in already developed lactobacilli biofilms.
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