Research round-up: Comparing Listeria and starving Salmonella

31-Jan-2017 - Last updated on 31-Jan-2017 at 14:09 GMT

Picture: iStock

The long awaited return of the research round-up series sees Listeria, Salmonella and a review article from the Caribbean.

We start with a review of the published literature over the last 20 years on selected bacterial foodborne zoonoses in the Caribbean.

Salmonella spp., Shigella spp. and Campylobacter spp. are the main bacteria, said de Almeida et al.

Projections indicate changes in the dietary habits of local populations, an increase in meat consumption and rising demand for other animal products such as eggs.

In spite of limited surveillance on foodborne diseases (FBD), records related to bacterial foodborne zoonoses in food-producing animals and their associated epidemiologic significance are poorly documented.

Further epidemiological studies are needed to establish possible correlations between human and animal diseases that could lead to establishing preventive measures.

Seeking Listeria

In other news, teams led by Institut Pasteur scientists Sylvain Brisse and Marc Lecuit have used high-throughput sequencing to analyze the whole genome of hundreds of strains of Listeria monocytogenes to develop a universally applicable genome-wide strain genotyping approach.

They looked at almost 1,700 strains from US, the UK, Canada, Denmark and France.

"We established the precise structure of the Listeria monocytogenes population, calculated the evolutionary rate of the strains, demonstrated that they circulate worldwide, and revealed the extent of heterogeneity in the virulence and genomic characteristics of these strains," said the teams.

Previously, when there was a case of listeriosis, the sample was sent to the nearest center of expertise, which would contact other laboratories to determine provenance of the strain.

Brisse said: "We published all these data on a website hosted by the Institut Pasteur, accessible to all international centers of expertise, with a very straightforward classification system which facilitates the comparative analysis of strains."

Meanwhile, researchers from the University of Surrey developed a new categorisation of food scares, according to a study in the British Food Journal.

They said categorisations are useful in developing strategies for reducing the frequency and severity of scares.

However, those in existence were too simplistic as they did not allow for cross categorisation of factors which could compromise the food chain.

The new categorisation enables a food scare to be classified according to its physical manifestation (chemical/physical or biological contamination) and the origins (wilful deception and/or transparency and awareness issues).

Dr Elizabeth Whitworth from RSK ADAS, and formerly the University of Surrey, said: "The salient feature of the new categorisation is that it distinguishes between scares caused by wilful deception, and those that are caused by transparency and awareness issues."

Using Salmonella against cancer

Also, research from the Salk Institute has shown how bacteria block the appetite loss response in their host to make the host healthier and promote the bacteria’s transmission to other hosts.

Mice orally infected with Salmonella Typhimurium typically experience appetite loss and eventually become much sicker as the bacteria become more virulent - spreading from the intestines to other tissues.

Sick mice that consumed extra calories despite their appetite loss survived longer.

This wasn’t due to a more active immune response by well-fed animals (as measured by levels of the bacteria in the host) but because the Salmonella weren’t spreading outside of the intestines and throughout the body when the mice ate more, which enabled them to stay healthy despite infection.

When the host ate more and survived longer during infection, the Salmonella bacteria were able to spread via feces to other animals and increase transmission between hosts.

The researchers found the bacteria were making a trade-off between virulence (the ability of a microbe to cause disease within one host) and transmission, which is its ability to spread and establish infections between multiple hosts.

“What we found was that appetite loss makes the Salmonella more virulent, perhaps because it needs to go beyond the intestines to find nutrients for itself. This increased virulence kills its host too fast, which compromises the bacteria’s ability to spread to new hosts,” said Sheila Rao, a Salk research associate and first author.

Finally, biomedical engineers at Duke University have used a strain of Salmonella that causes food poisoning in the fight against brain cancer.

Salmonella typhimurium, with a few genetic tweaks, can produce self-destruct orders deep within tumors.

Tests in rat models with extreme cases of glioblastoma showed a 20% survival rate over 100 days - roughly equivalent to 10 human years - with the tumors going into remission.

The team selected a detoxified strain of S. typhimurium that was deficient in an enzyme called purine, forcing the bacteria to seek supplies elsewhere.

Ravi Bellamkonda, Vinik dean of Duke's Pratt School of Engineering and corresponding author, said: [Because] their natural toxicity has been deactivated, they don't cause an immunological response. At the doses we used in the experiments, they were naturally cleared once they'd killed the tumors, effectively destroying their own food source."