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Plasmid-borne smr gene causes resistance to quaternary ammonium compounds in bovine Staphylococcus aureus
1Department of Reproduction and Forensic Medicine, National Veterinary Institute, Oslo, Norway. jostein.bjorland@veths.no
Abstract:
Resistance to quaternary ammonium compounds (QAC) in staphylococci is common in hospital environments and has been described in the food industry. Little is known about staphylococcal QAC resistance associated with animal disease, although such disinfectants are widely used in veterinary medicine. In order to investigate the occurrence of QAC resistance in staphylococci isolated from QAC-exposed animals, 32 penicillin- and tetracycline-resistant and 23 penicillin- and tetracycline-susceptible Staphylococcus aureus isolates collected from milk from cows with mastitis during a 4-year period were selected for QAC susceptibility studies and genetic characterization. The isolates originated from four different herds that used a common pasture with a joint milking parlor in the summer. During the pasture season, a teat cream containing the QAC cetyltrimethylammonium bromide had been used daily for more than 10 years for mastitis control. Three of the penicillin- and tetracycline-resistant isolates, which were recovered from three different cows during a 20-month period, were resistant to QAC. Plasmid analysis, PCR, and DNA sequencing revealed a novel plasmid of 2,239 bp containing the smr gene. The plasmid, designated pNVH99, has similarities to small, smr-containing staphylococcal plasmids previously found in human and food isolates. pNVH99 is a new member of the pC194 family of rolling-circle replication plasmids. The three QAC-resistant isolates, as well as 28 of the 29 remaining penicillin- and tetracycline-resistant isolates, were indistinguishable by pulsed-field gel electrophoresis. The study indicates that the occurrence and spread of QAC-resistant S. aureus among dairy cows may be a problem that needs further investigation.
Insights
Quaternary ammonium compound (QAC) resistance in Staphylococcus aureus is emerging in dairy cows due to disinfectant use. A novel plasmid carrying the smr gene was identified in resistant isolates, highlighting a potential veterinary concern.
Area of Science:
- Veterinary Microbiology
- Antimicrobial Resistance
- Molecular Biology
Background:
- Quaternary ammonium compounds (QACs) are widely used disinfectants in veterinary medicine.
- Staphylococcal resistance to QACs is documented in human and food settings, but less is known in animal populations.
- Mastitis control in dairy cows often involves QAC-based teat creams.
Purpose of the Study:
- To investigate the occurrence of QAC resistance in Staphylococcus aureus isolated from dairy cows.
- To characterize the genetic basis of QAC resistance in these isolates.
- To assess the potential spread of QAC-resistant S. aureus in a veterinary context.
Main Methods:
- Susceptibility testing of Staphylococcus aureus isolates to QACs.
- Plasmid analysis, PCR, and DNA sequencing to identify resistance genes and plasmids.
- Pulsed-field gel electrophoresis (PFGE) for strain typing.
Main Results:
- Three QAC-resistant Staphylococcus aureus isolates were identified among penicillin- and tetracycline-resistant strains from mastitis milk.
- A novel 2,239 bp plasmid, designated pNVH99, containing the smr gene was discovered in the resistant isolates.
- pNVH99 shares similarities with previously identified small, smr-containing staphylococcal plasmids.
Conclusions:
- The study indicates that QAC resistance in Staphylococcus aureus can occur in dairy cows, potentially linked to historical disinfectant use.
- The identification of the novel plasmid pNVH99 provides insight into the genetic mechanisms of QAC resistance in veterinary isolates.
- Further investigation is warranted to understand the prevalence and implications of QAC-resistant S. aureus in dairy herds.