Transfer of bacteria between food and food processing equipment is a hot issue. Every time a blade, dicer, slicer, or other piece of equipment contacts food, there is opportunity for transfer. Some might think that its always the food that gets contaminated from the “dirty” equipment, but that’s often not the case. One industry operations veteran told me once, “We get our clean equipment contaminated by our dirty product.”
Lots of studies have looked at transfer of bacteria to meat from deli slicers or vice versa. Almost all studies look at slicing logs. However, I’ve only seen one industrial slicer studied, and they looked at turkey, salami, and beef bologna logs. It was in Lin et al., 2006 J. Food Prot. 69:71-79, which was a Kraft-Oscar Mayer funded study that was conducted at University of Georgia’s Center for Food Safety across the hall from my lab when I was a grad student (it took them 6 years to get it published). This was one of the only studies that actually looked at a low inoculum (10, 100, or 1000 CFU on the slicer blade). They found that turkey was positive more frequently than bologna and salami and attributed it to moisture both helping transfer the inoculum to the meat and subsequent survival and slight growth during shelf life.
I should define translocation. For the purposes of ready-to-eat foods, translocation pertains to describing the transfer of bacteria (Listeria monocytogenes or surrogate representative of the pathogen) from the surface of product or from the blade into the slices or dices. This could be any depth of the slice.
There are some studies that show how an inoculated blade can transfer L. monocytogenes to sliced RTE meat or fish many, many slices later. This had been shown in the following products: ‘gravad’ salmon (Aarnisalo et al., 2007); turkey breast, bologna, and salami (Vorst et al., 2006); salami (Sheen, 2008); beef (Midelet and Carpentier (2002); and most importantly, three types of hams (Sheen and Hwang, 2008).
The latter study mentioned by Sheen and Hwang seemed to be the most relevant, even though they used a retail deli slicer. They showed that higher inoculation levels on blades (9-log10 CFU/blade) led to transfer to uninoculated ham about 140 slices later and that lower inoculation levels on blades (6-log10 CFU/blade) led to transfer up to 30 slices later. Then they inoculated ham slabs at different levels, sliced them, and then sliced with uninoculated ham to see how many slices later the pathogen could be detected. From the high inoculation method, slices were found to have high counts even 140 slices later. With the lower inoculation method, about 110 slicers later contamination was still being picked up.
Methods for measuring translocation:
As you can imagine, some use CFU/g, some use CFU/cm2, some use CFU/slice. Many use enrichment for presence/absence. As far as visualization of bacteria, none of these studies did that. One study used Glo-Germ powder visualized with UV light to see where on a deli slicer most likely areas of contamination would be (Vorst et al., 2006).
A study by Sheen et al., (2010) did visualize by using a fluorescent viability stain/dye for L. monocytogenes. They were investigating the effect of shear force on a deli slicer blade when used to slice agar logs. They visualized live/dead cells on the slicer blade and on the surfaces of agar slices. They used confocal laser scanning microscopy to visualize live/dead cells. They found that shear force seemed to cause some cell dead, although they couldn’t explain why.
The other way to visualize would be similar to what was done by Gory et al. 2001 (FEMS Microbiology Letters. 194:127-133). They transformed Lactobacillus sakei with the green fluorescent protein (GFP) gene and inserted it downstream of a lactate dehydrogenase gene. This made it so that when the level of fluorescence was a function of expression of that lactate gene. They were doing this to monitor fermentation throughout a meat formulation, which is an interesting find in and of itself, which may come in for different projects. Anyway, they used epifluorescent microscopy to visualize cells.
Options (not either or, but a la carte) :
Inoculate with antibiotic resistant (streptomycin and rifampicin) Listeria innocua. Excise slices carefully and quantify or enrich. Do the entire study in-house.
Use GloGerm and UV light to help determine the degree of infiltration into the products. Would likely need a UV light apparatus.
Use a fluorescent viability stain/dye for some surrogate bacterium (L. innocua or other) and work with outside lab or university that can process samples with confocal laser scanning microscopy.
Use GFP labeled surrogate bacterium (L. sakei or other) and work with outside lab or university that can process samples with epifluorescent microscopy.
Etna Consulting can help you with any of these options. Let’s talk.