Scientists have discovered how strains have become resistant to benzalkonium chloride
Listeria poses a significant risk to human health and there have been serious outbreaks on both sides of the Atlantic in recent years. The main transmission route involves meat and dairy products, so dairies and food-processing plants are regularly cleaned with with disinfectants to kill such bacteria. However, listeria is developing resistance to the compounds that are most frequently used.
Recent work by Stephan Schmitz-Esser and a group researchers at the University of Veterinary Medicine, Vienna (Vetmeduni Vienna) has revealed the mechanism for listeria’s resistance to one such agent, benzalkonium chloride. The findings were published in the online journal Plos One.*
A number of disinfectants are used to kill the causative agent, normally the bacterium Listeria monocytogenes, in dairies and other food-processing facilities, but quaternary ammonium compounds such benzalkonium chloride are most frequently used.
Unfortunately, many strains of listeria seem to be developing resistance to these agents, although the underlying mechanisms have remained obscure. Together with colleagues in Ireland, Stephan Schmitz-Esser and his researchers at the Vetmeduni’s Institute for Milk Hygiene used next-generation sequencing techniques to determine the DNA sequences of two strains of listeria known to be resistant to benzalkonium chloride.
When they examined the sequences they noticed a region of DNA of ca. 5 kb that was strikingly different in composition from the remainder of the genome. The bacteria seem to have acquired this novel element fairly recently and Schmitz-Esser named it Tn6188.
To ensure that the presence of Tn6188 in two strains resistant to benzalkonium chloride was not merely a coincidence, the researchers then screened an additional 90 strains of listeria for the element, finding it in a further 10 of them. Those strains harbouring Tn6188 turned out to be far less sensitive to benzalkonium chloride.
One of the five proteins that could be encoded by Tn6188, called QacH by Schmitz-Esser and colleagues because of its similarity to proteins of this name from other organisms, was activated by the presence of benzalkonium chloride in culture medium. And in a final experiment, the scientists were able to show that deleting the QacH gene made listeria once again sensitive to benzalkonium chloride.
Schmitz-Esser believes the results show that listeria can acquire new genetic material from other bacteria, making it important to ensure thorough disinfecting of food-processing facilities to prevent reservoirs of resistant bacteria building up and transferring their resistance to listeria.
*Tn6188 - A novel transposon in Listeria monocytogenes responsible for tolerance to benzalkonium chloride by A. Müller, K. Rychli, M. Muhterem-Uyar, A. Zaiser, B. Stessl, C. M. Guinane, P.D. Cotter, M. Wagner and S. Schmitz-Esser was published in the online journal Plos One. PLoS One 8(10): e76835. doi:10.1371/journal.pone.0076835