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Understanding Salmonella in pork meat processing

Photo: DTU National Food Institute
Photo: DTU National Food Institute

Salmonella behaviour in pork processing and factors to influence persistence and adaption have been investigated at the Technical University of Denmark.

Trine Hansen, PhD student at the National Food Institute, looked at methods of characterising Salmonella in pork meat processing and detecting unknown bacteria in different samples.

The PhD thesis was divided into how Salmonella attach to and detach from pork meat, including which genes that are up- and down regulated and development of a diagnostic tool using 16S rDNA sequencing for screening of unknown bacteria in bottled water without prior cultivation.

For the first part, it was found that cells that have been grown immobilized prior to application on a pork meat surface were more easily removed compared with those grown in a planktonic state.

In a subproject Salmonella Typhimurium exposed to heat shock was more resistant to heat and acid inactivation conditions.

For the second part, results revealed that the method was able to detect B. cereus at levels of 105-106 CFU/L, a level low enough for outbreaks situations.

This means it is a good candidate for screening of bacterial contaminants in water samples, said the research.

However, when the method was tried on more difficult samples, such as different types of feed, it was found that further optimization will be needed.

Nucleic acid based methods

Charlotta Löfström, associate professor, division of food microbiology, said different types of polymerase chain reaction (PCR), DNA sequencing and DNA microarray were used.

“PCR was used to quantify the expression of different genes using so called reverse-transcriptase quantitative PCR (RT-qPCR),” she told FoodQualityNews.com.

“This study was aiming at understanding how Salmonella attach to surfaces and to investigate some of the genes that could be up- and down-regulated in this process.

“Microarray was used to characterize different Salmonella strains isolated from a pig abattoir to see if there was a difference in the content of different genes for strains from different sources, for example the carcass and the slaughter equipment. It was also used to study how genes were up- and down regulated during changes in pH and temperature.

“DNA sequencing was included in a method to detect all existing DNA in a water sample, and could be used when you don´t know what bacteria you are looking for.

“This approach is also referred to as metagenomics. In this study, Bacillus cereus was used as a model organism, but the method is applicable for most bacteria.”  

The project has given an understanding of which factors may contribute to the development of more appropriate processing environments, which can limit the occurrence of Salmonella.

Löfström said the process environment should be less favorable for the persistence, attachment and virulence of Salmonella.

“This could be for example factors such as temperature and pH, and properties of the materials chosen for process equipment,” she said.

“One example to illustrate this from the studies included in the thesis is that S. Typhimurium exposed to a heat shock was more resistance to heat and acid inactivation conditions.

“This might make later decontamination steps more difficult and subsequently lead to a higher risk of contamination of food products.”

Bacteria behaviour

It is important to know how pathogenic bacteria behave, e.g. if they are more virulent, grow faster, attach stronger etc under certain conditions to adjust the process parameters to reduce these risks, said Löfström.

“The adaptation and persistence of Salmonella in the pork production chain has been suggested to partly be a result of bacterial attachment and surface colonization,” she said.

“This colonization is a reservoir that can contribute to the risk of cross contamination. It is therefore important to know if there are factors that might reduce the risk of attachment and/or detachment from the meat.

“We found that cells that were grown immobilized prior application on a pork meat surface were found to be more easily removed.

“In the pork processing, Salmonella might appear in an immobilized state on the pork surfaces where low attachment ability might pose a risk for cross contamination. A stronger attachment to a surface makes decontamination steps more difficult.”

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