The mechanism of the gram-positive bacteria’s resistance to benzalkonium chloride was uncovered at Stephan Schmitz-Esser at the University of Veterinary Medicine, Vienna (Vetmeduni Vienna).
Researchers said the disinfectants used for cleaning are often quaternary ammonium compounds (QAC) such as benzalkonium chloride (BC).
Between 10 to 46% of L. monocytogenes strains isolated from food and food processing environments can be regarded as BC tolerant.
Researchers found that a noval piece of DNA, a novel transposon in L. monocytogenes, was conferring tolerance to BC.
The pathogen has recently forced a raft of recalls in the US in meat and salads.
With colleagues in Ireland, a team at Stephan Schmitz-Esser of the Vetmeduni’s Institute for Milk Hygiene has provided evidence that a novel piece of DNA in the bacteria is involved.
The scientists used next-generation sequencing techniques to determine the DNA sequences of two strains of listeria known to be resistant to BC.
When they examined the sequences they noticed a region of DNA of ca. 5kb that was different in composition from the rest of the genome.
The bacteria had acquired the novel element recently and Schmitz-Esser termed it Tn6188.
"Tn6188 is related to Tn554 from Staphylococcus (S.) aureus and other Tn554-like transposons such as Tn558, Tn559 and Tn5406 found in various Firmicutes," said the researchers.
"Tn6188 comprises 5117 bp, is integrated chromosomally within the radC gene and consists of three transposase genes (tnpABC) as well as genes encoding a putative transcriptional regulator and QacH, a small multidrug resistance protein family (SMR) transporter putatively associated with export of BC that shows high amino acid identity to Smr/QacC from S. aureus and to EmrE from E.coli."
One of five proteins that could be encoded by Tn6188, termed QacH, was activated by the presence of BC in culture medium. In the final experiment, they showed that deleting the QacH gene made listeria sensitive again to the drug.
L. monocytogenes strains harboring Tn6188 had higher BC minimum inhibitory concentrations (MICs) (28.5 ± 4.7 mg/l) than strains without (14 ± 3.2 mg/l).
Using quantitative reverse transcriptase polymerase chain reaction (PCR) the researchers showed a significant increase in qacH expression in the presence of BC.
To avoid the results being coincidence, they screened 91 strains of listeria for the element, finding it in ten. These strains harbouring Tn6188 were far less sensitive to BC.
“…the higher BC resistance of Tn6188 positive strains might nevertheless be an important advantage in food processing environments if e.g. dosage failures during sanitation occur or in niches which are difficult to sanitize e.g. where biofilms can form,” said the researchers.
“In such niches the effective concentrations of sanitizers might be significantly lower and Tn6188 positive strains might better survive.”
Source: PLoS ONE
Online ahead of print: doi:10.1371/journal.pone.0076835
“Tn6188 - A Novel Transposon in Listeria monocytogenes Responsible for Tolerance to Benzalkonium Chloride”
Authors: Anneliese Müller, Kathrin Rychli, Meryem Muhterem-Uyar, Andreas Zaiser, Beatrix Stessl, Caitriona M. Guinane, Paul D. Cotter, Martin Wagner, Stephan Schmitz-Esser