Beware the 'Harmless' Listeria: Food Processing Industry Alerted to Growing Resistance in L. Innocua

Researchers from the University of Johannesburg have uncovered a worrying trend of non-pathogenic Listeria strains developing dangerous characteristics through the use of whole genome sequencing (WGS). These "harmless" strains, L. innocua and L. welshimeri, are commonly found in food processing facilities, much like the pathogenic L. monocytogenes.

For the study, the researchers collected L. innocua and L. welshimeri strains from various raw and processed meats at commercial food processing plants across South Africa. Although the study was only conducted in one country, the researchers found that their findings align with other studies showing an increase in resistance traits in Listeria species globally.

The University of Johannesburg team discovered that L. innocua strains are becoming increasingly tolerant to temperature, pH, dehydration, and other stressors that are typically used to combat the pathogen. Some strains of L. innocua and L. welshimeri even displayed genes indicating resistance to Benzalkonium chloride (BC), a commonly used disinfectant.

Furthermore, the researchers observed that L. innocua was developing hypervirulence traits, with a complete LIPI-4 gene sequence that is genetically identical to that of L. monocytogenes. This gene sequence can cause illness in humans. Additionally, two strains of L. innocua were found to have developed three or more concerning pathogenic characteristics, including adaptive immune systems to CRISPR-CAS systems.

The samples and isolates analyzed in the study were collected from 2014 to 2019 by the South African Department of Agriculture, Land Reform, and Rural Development. In total, 258 isolates from butcheries, abattoirs, retail outlets, cold stores, and processing plants across South Africa were studied. The isolates came from raw and whole beef, chicken, and pork meats, as well as processed, dried, and cooked meats. Out of the 258 isolates, 38 were found to be non-pathogenic L. innocua, and three were found to be non-pathogenic L. welshimeri.

Although L. innocua rarely causes human cases of listeriosis, it can still make those with compromised immune systems ill. The researchers express concern over the number of harmful characteristics that the L. innocua strains share with L. monocytogenes. They believe that their study can help predict which types of sequences to keep an eye on.

According to Science Daily, Dr. Thendo Mafuna from the Department of Biochemistry at the University of Johannesburg stated that, “Big industrial food processors may want to investigate how efficient BC or quat disinfectants are in their facilities. This can be done by taking swabs before cleaning and again after cleaning, culturing those, to see how well the disinfectant regimes are working”.