Novel plasmid-borne gene qacJ mediates resistance to quaternary ammonium compounds in equine Staphylococcus aureus, Staphylococcus simulans, and Staphylococcus intermedius.

The research article presents the discovery of a new gene called qacJ, present in certain types of bacteria found in horses, that makes these bacteria resistant to a group of disinfectants called quaternary ammonium compounds (QACs). The gene’s action could explain why certain types of chronic bacterial infections in horses are resistant to treatments involving these disinfectants.

Identification of the qacJ gene

• The researchers found a new gene, qacJ, in three species of bacteria (Staphylococcus aureus, Staphylococcus simulans, Staphylococcus intermedius) known to cause chronic infections in horses.
• This gene was found on a novel plasmid (a small DNA molecule within a cell that is physically separated from a chromosomal DNA) named pNVH01, belonging to a known family of plasmids (pC194) that propagates using a mechanism known as rolling-circle replication.
• The researchers determined that qacJ encodes a protein (QacJ) which has similarities to other known proteins associated with drug resistance, including Smr/QacC, QacG, and QacH.

Resistance to Disinfectants

• The bacterium containing the qacJ gene showed higher resistance than otherwise identical bacteria containing Smr, QacG, or QacH to benzalkonium chloride, a type of QAC disinfectant.
• This indicates that the presence of qacJ increases the resistance of these bacteria to certain disinfectants.

Spread of qacJ and pNVH01

• The researchers analyzed the spread of the bacteria containing qacJ and pNVH01 in horses using pulsed-field gel electrophoresis, a technique used for the separation and identification of large DNA molecules.
• They found evidence suggesting that a strain of Staphylococcus aureus containing qacJ and pNVH01 had spread around the horses’ bodies, and that pNVH01 had transferred within and between different bacterial species.
• The identical sequence of pNVH01 in different bacterial species indicates that recent plasmid transfer has occurred.

Selection Pressure from Disinfectant Usage

• A skin preparation containing a QAC disinfectant, cetyltrimethylammonium bromide, had been used extensively in three of the horses for several years.
• The researchers hypothesize that this may have led to selection pressure driving the proliferation of bacteria containing the QAC resistance gene.