Clinical Pharmacist and Master’s student in Clinical Pharmacy with research interests in pharmacovigilance, behavioral interventions in mental health, and AI applications in clinical decision support. Experience includes digital health research with Bloomsbury Health (London) and pharmacovigilance practice in patient support programs. Published work covers drug awareness among healthcare providers, postpartum depression management, and patient safety reporting. 
Clinical Pharmacist and Master’s student in Clinical Pharmacy with research interests in pharmacovigilance, behavioral interventions in mental health, and AI applications in clinical decision support. Experience includes digital health research with Bloomsbury Health (London) and pharmacovigilance practice in patient support programs. Published work covers drug awareness among healthcare providers, postpartum depression management, and patient safety reporting. What was studied?
This original research article investigated the nationwide prevalence and molecular characteristics of livestock-associated methicillin-resistant Staphylococcus aureus (LA-MRSA) in Korea, specifically across the pork production chain. The study focused on determining how widespread LA-MRSA is from pig farms through to slaughterhouses and retail markets, and examined the molecular epidemiology and resistance profiles of isolated strains. Special attention was given to the occurrence of multidrug resistance (MDR), tetracycline (TET) resistance, and zinc resistance, especially among clonal complex 398 (CC398) strains, which are globally recognized as significant LA-MRSA lineages. The study aimed to clarify the role of antimicrobial and heavy metal (zinc) selection pressures in the establishment and persistence of LA-MRSA within Korean swine production, providing valuable insights relevant to both public health and heavy metal certification programs.
Who was studied?
The study encompassed a comprehensive sampling of the pork production chain in Korea. Researchers collected 1,657 samples between July 2017 and August 2018, including swabs from pigs, pig farmers/workers, the farm environment, pig carcasses in slaughterhouses, and pork meat from retail markets. Specifically, samples were obtained from 19 pig farms, 7 slaughterhouses, and 35 retail markets, distributed across eight provinces. Additionally, 152 swabs were collected from 39 workers involved in these different sectors. This diverse and systematic sampling allowed the study to capture MRSA prevalence and characteristics at multiple critical points in the production and supply chain, including both animal and human reservoirs, as well as environmental sources.
Most important findings
| Critical Points | Details |
|---|---|
| Prevalence and Distribution of LA-MRSA | The overall prevalence of MRSA was 2.4% (40/1,657 samples): 3.4% in pig farms, 0.6% in slaughterhouses, and 0.4% in retail markets. Most isolates (37/40) came from pig farms. Among the 40 MRSA strains, 28 were from pigs, 5 from workers, and 4 from the environment, underlining pig farms as the primary reservoir. Transmission down the production chain was limited. |
| Dominant Clonal Lineages and Resistance Profiles | Multilocus sequence typing revealed that ST398 (n=25) and ST541 (n=6) were the dominant clones (collectively 77.5%). All CC398 isolates (ST398 and ST541) carried SCCmec type V and displayed multidrug resistance, including 100% resistance to tetracycline. These isolates also harbored multiple TET-resistance genes (tetM, tetK, tetL) and showed significantly higher minimum inhibitory concentrations (MICs) for TET than non-CC398 isolates. Non-CC398 isolates exhibited lower MDR and TET resistance. |
| Zinc Resistance and Genetic Markers | All MRSA isolates were phenotypically resistant to zinc chloride (MIC >2 mM), but high-level resistance (MIC ≥10 mM) was observed only among czrC-positive ST398 isolates. CC398 isolates had significantly higher zinc chloride MICs than non-CC398 isolates. This suggests that zinc supplementation in animal feed may contribute to the selection and maintenance of CC398 LA-MRSA strains via co-selection pressure, since the czrC gene (conferring zinc resistance) is colocalized with mecA (methicillin resistance) on SCCmec V. |
| Virulence and Enterotoxin Genes | While CC398 LA-MRSA isolates were less frequently positive for classical virulence genes (PVL, TSST-1), some did carry staphylococcal enterotoxin (SE) genes (sea, sec, sed) and exfoliative toxin genes (etb). Non-CC398 isolates had a slightly higher prevalence of these virulence genes. However, the overall frequency was low, and none of the CC398 isolates harbored PVL or TSST-1, limiting immediate foodborne pathogenic risk but underscoring the importance of ongoing surveillance. |
| Heavy Metal and Antimicrobial Co-selection Implications | The study provides strong evidence that co-selection pressures from both extensive antimicrobial (especially tetracycline) use and zinc supplementation in swine feed contribute to the prevalence and persistence of MDR CC398 LA-MRSA strains. The correlation between TET and zinc resistance, as well as the presence of resistance genes, highlights the need for integrated management of both antibiotic and heavy metal exposures in livestock production to control the spread of LA-MRSA. |
Key implications
This research underscores the critical role of both antibiotic and heavy metal (zinc) use in driving resistance in LA-MRSA, particularly CC398, in Korean swine production. For heavy metal certification programs, monitoring and restricting zinc supplementation in animal feed, alongside prudent antimicrobial stewardship, are essential measures to limit the co-selection and persistence of multidrug-resistant LA-MRSA, thereby safeguarding public health and supporting responsible production practices.
Citation
Back SH, Eom HS, Lee HH, Lee GY, Park KT, Yang SJ. Livestock-associated methicillin-resistant Staphylococcus aureus in Korea: antimicrobial resistance and molecular characteristics of LA-MRSA strains isolated from pigs, pig farmers, and farm environment. J Vet Sci. 2020;21(1):e2. doi:10.4142/jvs.2020.21.e2
Heavy metals are high-density elements that accumulate in the body and environment, disrupting biological processes. Lead, cadmium, arsenic, mercury, nickel, tin, aluminum, and chromium are of greatest concern due to persistence, bioaccumulation, and health risks, making them central to the HMTC program’s safety standards.
