'Harmless' Listeria species developing pathogenic resistance

In the food processing industry, the deadly bacteria Listeria monocytogenes is monitored closely. Not only can the bacteria make people extremely ill, it is known to be developing resistance to food safety measures around ...
However, two "harmless" species of Listeria are also developing a surprising number of characteristics potentially harmful to humans.

A study in South Africa, from a team of researchers with first author Dr. Thendo Mafuna at the University of Johannesburg, shows some of the changing characteristics of Listeria found in the country.

The study shows that Listeria innocua strains are developing resistance to temperature, pH, dehydration and other stresses; as well as hypervirulence genetically identical to that of Listeria monocytogenes.
Some strains of L. innocua and L. welshimeri in the study show all three genes for resistance to a widely used disinfectant, from the quaternary ammonium compound (QAC or QUAT) group of chemicals.

Two strains of L. innocua they analyzed have developed three or more concerning pathogenic characteristics, including CRISPR CAS-type adaptive immune systems.

The two non-pathogenic strains of Listeria were sampled in raw, dried and processed meats at commercial food processing facilities in the country.

The study confirms other research showing growing resistance in non-pathogenic Listeria species in other parts of the world.

Food-borne bacteria Listeria innocua and L. welshimeri are often regarded as harmless, compared to the deadly Listeria monocytogenes. But both these species are developing an unexpected number of harmful characteristics that can make food safety measures more difficult to maintain, shows a Whole Genome Sequencing study from South Africa. Some strains analysed have virulence genes identical to that of L. monocytogenes. Credit: Photo of data and graphic by Therese van Wyk. Study published in Microbiology Spectrum by Mafuna et al. (10.1128/spectrum.01189-22) (creativecommons.org/licenses/by/4.0/)
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Food-borne bacteria Listeria innocua and L. welshimeri are often regarded as harmless, compared to the deadly Listeria monocytogenes. But both these species are developing an unexpected number of harmful characteristics that can make food safety measures more difficult to maintain, shows a whole-genome study from South Africa. Some strains analysed have virulence genes identical to that of L. monocytogenes. Credit: Therese van Wyk. Study published in Microbiology Spectrum by Mafuna et al. (10.1128/spectrum.01189-22) (creativecommons.org/licenses/by/4.0/)
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Dr Thendo Mafuna from the Department of Biochemistry at the University of Johannesburg (UJ). In this composite photo, Dr Mafuna is in one of the UJ Biochemistry laboratories, and facing genetic data for Listeria innocua.Food-borne bacteria Listeria innocua and L. welshimeri are often regarded as harmless, compared to the deadly Listeria monocytogenes. But both these species are developing an unexpected number of harmful characteristics that can make food safety measures more difficult to maintain. Credit: Photo of data and Dr Mafuna by Therese van Wyk, University of Johannesburg.
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