When L. monocytogenes persists in food processing environments, it is often in the form of a biofilm (Tompkin 2002; Klaeboe et al, 2010). The biofilm formation gives a degree of protection to environmental stresses for L. monocytogenes. For example, greater numbers of L. monocytogenes cells survived drying on stainless steel surfaces in biofilms compared with unattached cells (Truelstup et al 2011). Although the majority of L. monocytogenes are attached to the biofilm, the cells in the upper layer can move around and were observed to transfer from the biofilms onto smoked or fresh salmon on contact (Truelstup et al 2011).
Routine cleaning and sanitation can be effective in preventing the establishment of biofilms (Aarnisalo et al 2006). However, there are some studies that suggest that effective cleaning and sanitation may not always occur for all surfaces (Salvat et al 1995). When environmental surface samples were taken from a factory after cleaning and sanitation, but before the commencement of processing, Listeria was isolated from some of the samples. When the study was expanded to determine if the original factory was typical, Listeria was isolated from 15 of the 23 factories specifically sampled (65%). In those processing facilities that were Listeria positive, 7.2% of all the surfaces sampled tested positive (Miettinen et al, 2001).
Heat is the preferred method for the removal of L. monocytogenes when it is isolated from processing equipment and environments. However, some sanitising chemicals used at above the usual concentration (after checking the risk to cleaning staff) also have merit. Pan et al, (2006) report increasing resistance for biofilm L. monocytogenes cells to sanitisers. However cells that were removed from the biofilms on peroxide-treated and control coupons were not significantly different in their resistance to sanitising agents. Thus, there may be changes in the composition of polysaccharides and other materials used to construct the biofilm that is the basis of increasing resistance to chemicals by biofilm L. monocytogenes.
One sanitiser formulation that has been reported as effective against L. monocytogenes biofilms is peroxyacetic acid (PAA; an active oxygen-based sanitiser composed of hydrogen peroxide, peracetic acid, and acetic acid in combination; Stopforth et al., 2002). Stopforth created an artificial biofilm onto stainless steel tiles under laboratory conditions. The Stopforth study concluded that PAA, in contrast to a number of other sanitisers that were assessed, was more effective in killing attached cells compared with cells treated in suspension.
Later work by Bagge-Ravn et al (2003) attempted to build on the findings of Stopforth et al. (2002) by determining the effectiveness of PAA in a commercial fish smoking environment. As part of their studies, the Bagge-Ravn team applied a fog of PAA to the slicing area at a salmon smokehouse and compared its effectiveness with that of a foam sanitizer that used sodium hypochlorite as the active agent (the established sanitation process routinely performed at the smokehouse). The effect of each procedure on L. monocytogenes populations was assessed. 223 environmental samples were collected with sponges and swabs after each of the sanitization procedures, and 68 samples were collected post clean during production. Using a selective isolation method, strains of L. monocytogenes were isolated and subsequently genetically characterized by DNA fingerprinting. Following chlorine foam treatment, 14 to 42% of the samples contained <10 cfu L. monocytogenes per site, whereas 29 to 78% of the samples collected after fog sanitisation contained the same proportion of undetected L. monocytogenes. Although a higher proportion of samples had lowered numbers of L. monocytogenes for PAA, the overall prevalence of L. monocytogenes was unchanged. For both treatments, L. monocytogenes was found only in poorly cleaned areas such as drains. The authors make specific note that, in keeping with established dogma, effective cleaning is a pre-requisite for effective sanitation (Bagge-Ravn et al, 2003). The L. monocytogenes types from every single positive drain sample were identical to the type that had persisted in the smokehouse over a seven year period, emphasising the importance of drains as a persistent L. monocytogenes niche. The Bagge-Ravn study is further notable because it demonstrates that the method of sanitiser application can influence the effectiveness of sanitation. The original Stopforth et al (2002) study applied PAA directly to L. monocytogenes-contaminated films and demonstrated effective kill. Although more convenient for commercial premises, when the PAA was applied as a fog by the Bagge-Ravn study, the kill effectiveness was significantly reduced.
A summary of the published literature regarding the effective removal of L. monocytogenes by commonly encountered sanitising agents is provided as Table 1. It is important to note the effect of ineffective cleaning prior to the application of a sanitiser on the number of L. monocytogenes cells killed.
Table 1. The efficacy of sanitisers in removing L. monocytogenes contamination from poorly and properly cleaned surfaces or suspensions (reproduced from Hoelzer et al 2012)
|Sanitiser type||In the absence of protein residues (effective cleaning)||In the presence of protein residues (poor cleaning)|
|No. of studies reviewed||No. of observations||Total no. of replicates||Mean reduction (log cfu)||No. of studies reviewed||No. of observations||Total no. of replicates||Mean reduction (log cfu)|
Aarnisalo,K., Tallavaara,K., Wirtanen,G., Maijala,R. and Raaska,L. (2006) The hygienic working practices of maintenance personnel and equipment hygiene in the Finnish food industry. Food Control 17, 1001-1011
Bagge-Ravin,D., Gardshodn,K., Gram,L. and Vogel,B.F. (2003) Comparison of sodium hypochlorite-based foam and peroxyacetic acid-based fog sanitizing procedures in a salmon smokehouse: Survival of the general microflora and Listeria monocytogenes. Journal of Food Protection 66, 592-598.
Hoelzer,K., Pouillot,R., Gallagher,D., Silverman,M.B., Kause,J. and Dennis,S. (2012) Estimation of Listeria monocytogenes transfer coefficients and efficacy of bacterial removal through cleaning and sanitation. International Journal of Food Microbiology 157, 267-277
Klaeboe,H., Lunestad,B.T., Borlaug,K., Paulauskas,A. and Rosef,O. (2010) Persistence and diversity of Listeria monocytogenes isolates in Norwegian processing plants. Veterinarija Ir Zootechnika 50, 42-47
Miettinen,H., Aarnisalo,K., Salo,S., and Sjöberg,A.M. (2001). Evaluation of surface contamination and the presence of Listeria monocytogenes in fish processing factories. Journal of Food Protection 64, 635-639
Pan,Y., Breidt, F., and Kathariou, S. (2006). Resistance of Listeria monocytogenes biofilms to sanitizing agents in a simulated food processing environment. Applied and Environmental Microbiology 72, 7711-7717.
Salvat,G., Toquin,M.T., Michel,Y., and Colin, P. (1995). Control of Listeria monocytogenes in the delicatessen industries: the lessons of a listeriosis outbreak in France. International Journal of Food Microbiology, 25, 75-81.
Stopforth,J.D., Samelis,J., Sofos,J.N., Kendall,P.A. and Smith,G.C. (2002) Biofilm formation by acid-adapted nonadapted Listeria monocytogenes in fresh its beef decontamination washings and its subsequent inactivation with sanitizers. Journal of Food Protection 65, 1717-1727
Truelstrup,H.L. and Vogel,B.F. (2011) Desiccation of adhering and biofilm Listeria monocytogenes on stainless steel: Survival and transfer to salmon products. International Journal of Food Microbiology. 146, 88-93