Quantifying the combined effects of pronase and benzalkonium chloride in removing late-stage Listeria monocytogenes-Escherichia coli dual-species biofilms

Efficacy of disinfectant and bacteriophage mixture against planktonic and biofilm state of Listeria monocytogenes to control in the food industry

Fresh produce and animal-based products contaminated with Listeria monocytogenes have been the main cause of listeriosis outbreaks for many years. The present investigation explored the potential of combination treatment of disinfectants with a bacteriophage cocktail to control L. monocytogenes contamination in the food industry. A mixture of 1 minimal inhibitory concentration (MIC) of disinfectants (sodium hypochlorite [NaOCl], hydrogen peroxide [H2O2], and lactic acid [LA]) and multiplicity of infection (MOI) 100 of phage cocktail was applied to both planktonic cells in vitro and already-formed biofilm cells on food contact materials (FCMs; polyethylene, polypropylene, and stainless steel) and foods (celery and chicken meat). All the combinations significantly lowered the population, biofilm-forming ability, and the expression of flaA, motB, hlyA, prfA, actA, and sigB genes of L. monocytogenes. Additionally, in the antibiofilm test, approximately 4 log CFU/cm2 was eradicated by 6 h treatment on FCMs, and 3 log CFU/g was eradicated within 3 days on celery. However, <2 log CFU/g was eradicated in chicken meat, and regrowth of L. monocytogenes was observed on foods after 5 days. The biofilm eradication efficacy of the combination treatment was proven through visualization using scanning electron microscopy (SEM) and confocal microscopy. In the SEM images, the unusual behavior of L. monocytogenes invading from the surface to the inside was observed after treating celery with NaOCl+P or H2O2 + P. These results suggested that combination of disinfectants (NaOCl, H2O2, and LA) with Listeria-specific phage cocktail can be employed in the food industry as a novel antimicrobial and antibiofilm approach, and further research of L. monocytogenes behavior after disinfection is needed.

Recent advances on the formation, detection, resistance mechanism, and control technology of Listeria monocytogenes biofilm in food industry

Listeria monocytogenes is an important foodborne pathogen that causes listeriosis, a severe and fatal condition. Biofilms are communities of microorganisms nested within a self-secreted extracellular polymeric substance, and they protect L. monocytogenes from environmental stresses. Biofilms, once formed, can lead to the persistence of L. monocytogenes in processing equipment and are therefore considered to be a major concern for the food industry. This paper briefly introduces the recent advancements on biofilm formation characteristics and detection methods, and focuses on analysis of the mechanism of L. monocytogenes biofilm resistance; Moreover, this paper also summarizes and discusses the existing different techniques of L. monocytogenes biofilm control according to the physical, chemical, biological, and combined strategies, to provide a theoretical reference to aid the choice of effective control technology in the food industry.

Mechanisms of Listeria monocytogenes Disinfection with Benzalkonium Chloride: From Molecular Dynamics to Kinetics of Time-Kill Curves

Unravelling the mechanisms of action of disinfectants is essential to optimise dosing regimes and minimise the emergence of antimicrobial resistance. In this work, we examined the mechanisms of action of a commonly used disinfectant-benzalkonium chloride (BAC)-over a significant pathogen-L. monocytogenes-in the food industry. For that purpose, we used modelling at multiple scales, from the cell membrane to cell population inactivation. Molecular modelling revealed that the integration of the BAC into the membrane requires three phases: (1) the approaching of BAC to the cellular membrane, (2) the absorption of BAC to its surface, and (3) the integration of the compound into the lipid bilayer, where it remains at least for several nanoseconds, probably destabilising the membrane. We hypothesised that the equilibrium of adsorption, although fast, was limiting for sufficiently large BAC concentrations, and a kinetic model was derived to describe time-kill curves of a large population of cells. The model was tested and validated with time series data of free BAC decay and time-kill curves of L. monocytogenes at different inocula and BAC dose concentrations. The knowledge gained from the molecular simulation plus the proposed kinetic model offers the means to design novel disinfection processes rationally.

Pepsin and Trypsin Treatment Combined with Carvacrol: An Efficient Strategy to Fight Pseudomonas aeruginosa and Enterococcus faecalis Biofilms

Biofilms consist of microbial communities enclosed in a self-produced extracellular matrix which is mainly responsible of biofilm virulence. Targeting this matrix could be an effective strategy to control biofilms. In this work, we examined the efficacy of two proteolytic enzymes, pepsin and trypsin, to degrade P. aeruginosa and E. faecalis biofilms and their synergistic effect when combined with carvacrol. The minimum dispersive concentrations (MDCs) and the contact times of enzymes, as well as the minimal inhibitory concentrations (MICs) and contact times of carvacrol, were determined against biofilms grown on polystyrene surfaces. For biofilms grown on stainless steel surfaces, the combined pepsin or trypsin with carvacrol treatment showed more significant reduction of both biofilms compared with carvacrol treatment alone. This reduction was more substantial after sequential treatment of both enzymes, followed by carvacrol with the greatest reduction of 4.7 log CFU mL−1 (p < 0.05) for P. aeruginosa biofilm and 3.3 log CFU mL−1 (p < 0.05) for E. faecalis biofilm. Such improved efficiency was also obvious in the epifluorescence microscopy analysis. These findings demonstrate that the combined effect of the protease-dispersing activity and the carvacrol antimicrobial activity could be a prospective approach for controlling P. aeruginosa and E. faecalis biofilms.

Hurdle technology based on the use of microencapsulated pepsin, trypsin and carvacrol to eradicate Pseudomonas aeruginosa and Enterococcus faecalis biofilms

The biofilm lifestyle plays a major role in the resistance and virulence of Pseudomonas aeruginosa and Enterococcus faecalis. In this study, two microencapsulated proteases (pepsin ME-PEP and trypsin ME-TRYP) were evaluated for their biofilm dispersal activity and their synergistic effect with microencapsulated carvacrol (ME-CARV). Spray-drying was used to protect enzymes and essential oil and enhance their activities. Cell count analysis proved the synergistic activity of enzymes and carvacrol treatment as biofilms were further reduced after combined treatment in comparison to ME-CARV or enzymes alone. Furthermore, results showed that sequential treatment in the order ME-TRYP - ME-PEP - ME-CARV resulted in more efficient biofilm removal with a maximum reduction of 5 log CFU mL^-1 for P. aeruginosa and 4 log CFU mL^-1 for E. faecalis. This study proposes that the combination of microencapsulated proteases with ME-CARV could be useful for the effective control of P. aeruginosa and E. faecalis biofilms.

Architectural Features and Resistance to Food-Grade Disinfectants in Listeria monocytogenes-Pseudomonas spp. Dual-Species Biofilms

Listeria monocytogenes is considered a foodborne pathogen of serious concern capable of forming multispecies biofilms with other bacterial species, such as Pseudomonas spp., adhered onto stainless steel (SS) surfaces. In an attempt to link the biofilms' morphology and resistance to biocides, dual-species biofilms of L. monocytogenes, in co-culture with either Pseudomonas aeruginosa, Pseudomonas fluorescens, or Pseudomonas putida, were assayed to ascertain their morphological characteristics and resistance toward benzalkonium chloride (BAC) and neutral electrolyzed water (NEW). Epifluorescence microscopy analysis revealed that each dual-species biofilm was distributed differently over the SS surface and that these differences were attributable to the presence of Pseudomonas spp. Confocal laser scanning microscopy (CLSM) assays demonstrated that despite these differences in distribution, all biofilms had similar maximum thicknesses. Along with this, colocalization analyses showed a strong trend of L. monocytogenes to share location within the biofilm with all Pseudomonas assayed whilst the latter distributed throughout the surface independently of the presence of L. monocytogenes, a fact that was especially evident in those biofilms in which cell clusters were present. Finally, a modified Gompertz equation was used to fit biofilms' BAC and NEW dose-response data. Outcomes demonstrated that L. monocytogenes was less susceptible to BAC when co-cultured with P. aeruginosa or P. fluorescens, whereas susceptibility to NEW was reduced in all three dual-species biofilms, which can be attributable to both the mechanism of action of the biocide and the architectural features of each biofilm. Therefore, the results herein provided can be used to optimize already existing and develop novel target-specific sanitation treatments based on the mechanism of action of the biocide and the biofilms' species composition and structure.

Hurdle technology using encapsulated enzymes and essential oils to fight bacterial biofilms

Biofilm formation on abiotic surfaces has become a major public health concern because of the serious problems they can cause in various fields. Biofilm cells are extremely resistant to stressful conditions, because of their complex structure impedes antimicrobial penetration to deep-seated cells. The increased resistance of biofilm to currently applied control strategies underscores the urgent need for new alternative and/or supplemental eradication approaches. The combination of two or more methods, known as Hurdle technology, offers an excellent option for the highly effective control of biofilms. In this perspective, the use of functional enzymes combined with biosourced antimicrobial such as essential oil (EO) is a promising alternative anti-biofilm approach. However, these natural antibiofilm agents can be damaged by severe environmental conditions and lose their activity. The microencapsulation of enzymes and EOs is a promising new technology for enhancing their stability and improving their biological activity. This review article highlights the problems related to biofilm in various fields, and the use of encapsulated enzymes with essential oils as antibiofilm agents. KEY POINTS: • Problems associated with biofilms in the food and medical sectors and their subsequent risks on health and food quality. • Hurdle technology using enzymes and essential oils is a promising strategy for an efficient biofilms control. • The microencapsulation of enzymes and essential oils ensures their stability and improves their biological activities.

Efficacy of flavourzyme against Salmonella Typhimurium, Escherichia coli, and Pseudomonas aeruginosa biofilms on food-contact surfaces

Food contamination is a major public health concern, with Salmonella Typhimurium, Escherichia coli, and Pseudomonas aeruginosa being the prominent causal agents. They often produce resistant shields in food through biofilm formation and are difficult to remove from food-contact surfaces using conventional cleaning agents. In the current study, we investigated the efficacy of flavourzyme, an industrial peptidase, in biofilm removal from ultra-high molecular weight polyethylene (UHMWPE) and rubber surfaces and compared the corresponding efficacies with those of the commonly used DNase I. We noticed a significant reduction of young (24-h-old) and mature (72-h-old) biofilms on both surfaces after treatment with flavourzyme. The overall reduction potentiality of flavourzyme was higher than that of DNase I. The flavourzyme-mediated removal of biofilms appears to be caused by the gradual disruption of amide (NH) and polysaccharide (C-O-C) stretching bands of the extracellular polymeric substances (EPS) released by the microbes. EPS elimination and the cell-friendly behavior of flavourzyme were further confirmed by field emission scanning electron microscopy. Based on these findings, we suggest that flavourzyme can reduce microbial EPS formation, thus possibly controlling microbial food contamination. This finding reveals a new opportunity for the development of a novel method for controlling foodborne illness as well as food spoilage.

New approach for the removal of mature biofilms formed by wild strains of Listeria monocytogenes isolated from food contact surfaces in an Iberian pig processing plant

One of the main objectives of the food industry is to guarantee food safety by providing innocuous food products. Therefore, this sector must consider all the possible biotic or abiotic contamination routes from the entry of raw materials to the release of the final product. Currently, one important problem in this regard is the presence of biofilms on food contact surfaces which can transmit pathogens such as L. monocytogenes. In industrial conditions biofilms are found in a mature state, so it is essential that when carrying out removal effectiveness studies in vitro the tests are realized with models that produce these structures in a similarly mature state. The main objective of this study was to evaluate the effectiveness of an alternative treatment (i.e. enzymatic detergent that include natural antimicrobial agents) and a conventional treatment (i.e. chlorinated alkaline) for the elimination of mature L. monocytogenes biofilms. The results showed a cell detachment from the formed mature biofilms with an effectivity of between 74.75%-97.73% and 53.94%-94.02% for the enzymatic treatment and the chlorinated alkaline detergent, respectively. On a qualitative level, it was observed that the dispersion in the structure was much higher for the enzymatic treatment than for the chlorinated alkaline, which continued to show obvious structure integrity. All this leads to the conclusion that treatments with an enzymatic detergent have a significantly greater impact on the removal of mature L. monocytogenes biofilms, although a further disinfection process would be needed, enhancing even more the treatment effectivity. This may imply that the industrial approach to addressing this problem should be modified to include new perspectives that are more effective than traditional ones.

Efficacy of Synthetic Furanones on Listeria monocytogenes Biofilm Formation

Furanones are analogues of acylated homoserine lactones with proven antifouling activity in both Gram-positive and Gram-negative bacteria though the interference of various quorum sensing pathways. In an attempt to find new strategies to prevent and control Listeria monocytogenes biofilm formation on stainless steel (SS) surfaces, different concentrations of six synthetic furanones were applied on biofilms formed by strains isolated from food, environmental, and clinical sources grown onto AISI 316 SS coupons. Among the furanones tested, (Z-)-4-Bromo-5-(bromomethylene)-2(5H)-furanone and 3,4-Dichloro-2(5H)-furanone significantly (p < 0.05) reduced the adhesion capacity (>1 log CFU cm-2) in 24 h treated biofilms. Moreover, individually conducted experiments demonstrated that (Z-)-4-Bromo-5-(bromomethylene)-2(5H)-furanone was able to not only significantly (p < 0.05) prevent L. monocytogenes adhesion but also to reduce the growth rate of planktonic cells up to 48 h in a dose-dependent manner. LIVE/DEAD staining followed by epifluorescence microscopy visualisation confirmed these results show an alteration of the structure of the biofilm in furanone-treated samples. Additionally, it was demonstrated that 20 µmol L-1 of 3,4-Dichloro-2(5H)-furanone dosed at 0, 24 and 96 h was able to maintain a lower level of adhered cells (>1 log CFU cm-2; p < 0.05). Since furanones do not pose a selective pressure on bacteria, these results represent an appealing novel strategy for the prevention of L. monocytogenes biofilm grown onto SS.

Tracking bacteriome variation over time in Listeria monocytogenes-positive foci in food industry

The variation in microbial composition over time was assessed in biofilms formed in situ on selected non-food and food contact surfaces of meat and fish industries, previously identified as Listeria monocytogenes-positive foci. First, all samples were analysed for the detection and quantification of L. monocytogenes using ISO 11290-1 and ISO 11290-2 norms, respectively. Although the pathogen was initially detected in all samples, direct quantification was not possible. Psychrotrophic bacteria counts were among resident microbiota in meat industry samples (Mean_max = 6.14 log CFU/cm²) compared to those form fish industry (Mean_max = 5.85 log CFU/cm²). Visual analysis of the biofilms using epifluorescence microscopy revealed a trend to form microcolonies in which damaged/dead cells would act as anchoring structures. 16S rRNA gene metagenetic analysis demonstrated that, although Proteobacteria (71.37%) initially dominated the bacterial communities at one meat industry location, there was a dramatic shift in composition as the biofilms matured, where Actinobacteria (79.72%) became the major phylum present in later samples. This change was largely due to an increase of Nocardiaceae, Micrococcaceae and Microbacteriaceae. Nevertheless, for the other sampling location, the relative abundance of the dominating phylum (Firmicutes) remained consistent over the entire sampling period (Mean = 63.02%). In fish industry samples, Proteobacteria also initially dominated early on (90.69%) but subsequent sampling showed a higher diversity in which Bacteroidetes and Proteobacteria were the most abundant phyla accounting for the 48.04 and 37.98%, respectively by the last sampling period. Regardless of the location, the community profiles of the endpoint samples were similar to those reported previously. This demonstrated that in a given industrial setting there is a trend to establish a determinate biofilm structure due to the environmental factors and the constant incoming microbiota. This information could be used to improve the existing sanitisation protocols or for the design of novel strategies.

Current Knowledge on Listeria monocytogenes Biofilms in Food-Related Environments: Incidence, Resistance to Biocides, Ecology and Biocontrol

Although many efforts have been made to control Listeria monocytogenes in the food industry, growing pervasiveness amongst the population over the last decades has made this bacterium considered to be one of the most hazardous foodborne pathogens. Its outstanding biocide tolerance capacity and ability to promiscuously associate with other bacterial species forming multispecies communities have permitted this microorganism to survive and persist within the industrial environment. This review is designed to give the reader an overall picture of the current state-of-the-art in L. monocytogenes sessile communities in terms of food safety and legislation, ecological aspects and biocontrol strategies.

Dynamics of biofilm formation by Listeria monocytogenes on stainless steel under mono-species and mixed-culture simulated fish processing conditions and chemical disinfection challenges

The progressive ability of a six-strains L. monocytogenes cocktail to form biofilm on stainless steel (SS), under fish-processing simulated conditions, was investigated, together with the biocide tolerance of the developed sessile communities. To do this, the pathogenic bacteria were left to form biofilms on SS coupons incubated at 15°C, for up to 240h, in periodically renewable model fish juice substrate, prepared by aquatic extraction of sea bream flesh, under both mono-species and mixed-culture conditions. In the latter case, L. monocytogenes cells were left to produce biofilms together with either a five-strains cocktail of four Pseudomonas species (fragi, savastanoi, putida and fluorescens), or whole fish indigenous microflora. The biofilm populations of L. monocytogenes, Pseudomonas spp., Enterobacteriaceae, H₂S producing and aerobic plate count (APC) bacteria, both before and after disinfection, were enumerated by selective agar plating, following their removal from surfaces through bead vortexing. Scanning electron microscopy was also applied to monitor biofilm formation dynamics and anti-biofilm biocidal actions. Results revealed the clear dominance of Pseudomonas spp. bacteria in all the mixed-culture sessile communities throughout the whole incubation period, with the in parallel sole presence of L. monocytogenes cells to further increase (ca. 10-fold) their sessile growth. With respect to L. monocytogenes and under mono-species conditions, its maximum biofilm population (ca. 6logCFU/cm²) was reached at 192h of incubation, whereas when solely Pseudomonas spp. cells were also present, its biofilm formation was either slightly hindered or favored, depending on the incubation day. However, when all the fish indigenous microflora was present, biofilm formation by the pathogen was greatly hampered and never exceeded 3logCFU/cm², while under the same conditions, APC biofilm counts had already surpassed 7logCFU/cm² by the end of the first 96h of incubation. All here tested disinfection treatments, composed of two common food industry biocides gradually applied for 15 to 30min, were insufficient against L. monocytogenes mono-species biofilm communities, with the resistance of the latter to significantly increase from the 3rd to 7th day of incubation. However, all these treatments resulted in no detectable L. monocytogenes cells upon their application against the mixed-culture sessile communities also containing the fish indigenous microflora, something probably associated with the low attached population level of these pathogenic cells before disinfection (<10²CFU/cm²) under such mixed-culture conditions. Taken together, all these results expand our knowledge on both the population dynamics and resistance of L. monocytogenes biofilm cells under conditions resembling those encountered within the seafood industry and should be considered upon designing and applying effective anti-biofilm strategies.