Assessing Bacterial Ecology in Sausage Casings

The practice of producing natural sausage casings from animal intestines is centuries old, and still used in meat processing today. Casings are prepared by first washing the intestines and then removing much of the inner and outer content. What remains is connective tissue consisting mostly of collagen. This provides a good balance of tenderness and permeability which helps sausage hold its shape. Despite the preparation processes, casings still retain a wide range of both healthy and pathogenic bacteria causing some researchers to investigate strategies to decrease the bacterial load.¹

Interested in assessing the the bacterial ecology of natural casings from different animal species (hog, ovine and cow), Rebecci et al.² analyzed 13 natural casing samples from an Italian sausage producer. To do this, the team sampled portions of casings made from different parts of the intestines. Rebecci and colleagues used culture-based microbiology to determine what bacteria were present in the casings.

For their experiments, the team plated 100μL of casing suspension into several different types of Oxoid agar plates from Thermo Scientific. As an initial screen, they used Oxoid Plate Count Agar to determine the total visibility. The team also investigated the presence of micrococci and staphylococci on Oxoid Baird Parker Agar with added egg yolk tellurite emulsion. To determine the total coliforms, they plated samples on Oxoid Violet Red Bile Agar (VRBA). They used Oxoid VRBA plates with MUG to determine any E. coli present in samples and Enterobacteriaceae on Oxoid Red Bile Glucose Agar.

Additionally, the team investigated the presence of yeasts and molds using Rose Bengal Agar supplemented with chloramphenicol. The team also used various types of plates from Thermo Scientific to detect Listeria monocytogenes, Salmonella spp., Clostridium perfringens and Bacillus cereus (MRS, Oxoid Baird Parker (BP), or TOS-Mup agar, respectively). After growing bacterial colonies, the team randomly selected two of the highest dilutions and purified them by streaking onto the respective medium. They screened colonies belonging to the Staphylococcus genus using catalase and oxidase testing with oxidase detection strips (Thermo Scientific).

Next, the team of researchers analyzed the microbiological data with ANOVA and purified and isolated DNA from the bacterial colonies to look at bacterial diversity. Rebecci et al. found a complex ecological environment with high diversity among casings from different animal species. Comparing animal to animal, Clostridium levels were higher in the cow, whereas Pediococcus, Lactobacillus, Campylobacter, Staphylococcus, Halanaerobacter, Acetobacter and Salinivibrio were higher in the hog casings. Finally, Vagococcus, Pseudoalteromonas, Peptoniphilus, Weissella, Psycrhobacter and Ornithibacterium were significantly higher in the ovine casings.

Looking at each type of casing separately, the team determined Vagococcus was the most abundant genus in the ovine casings, while Clostridium was the most abundant in cow casings. Interestingly, they found differences in the total microbial counts between samples. In ovine casings for example, they found increasing amounts of microbes from the first casing to the last.

The researchers found the in-house processing vs. pre-made casings made a difference in the bacterial load. Ready-to use casings (which were both hog casings) had higher levels of Lactobacillus, Pediococcus, Staphylococcus and Salinivibrio but near zero levels of more harmful bacteria such as Clostridium, Campylobacter and Acetobacter. The team maintains that the complex bacterial interactions should not be underestimated, and should be studied further.

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References

1. Benli, H., et al. (2008). Biomechanical and microbiological changes in natural hog casings treated with ozone. Meat Sci. 79, (pp. 155–162)

2. Rebecchi, A., et al. (2015) “High-throughput assessment of bacterial ecology in hog, cow, and ovine casings used in sausages production.” International Journal of Food Microbiology pii: S0168-1605(15) 00250-0. doi: 10.1016/j.ijfoodmicro.2015.04.047. [Epub ahead of print] spp., Clostridium perfringens and Bacillus cereus, MRS, Oxoid Baird Parker (BP), or TOS-Mup agar.