Apparent prevalence and selected risk factors of methicillin-resistant Staphylococcus aureus and non-aureus staphylococci and mammaliicocci in bulk tank milk of dairy herds in Indiana, Ohio, and Michigan

• Published in: JDS communications 2023-11, Vol.4 (6), p.489-495
• Main Authors: Goncalves, Juliano L., Mani, Rinosh, Sreevatsan, Srinand, Ruegg, Pamela L.
• Format: Article
• Language: English
• Subjects: Health, Behavior, and Well-being
• ISSN: 2666-9102; 2666-9102
• DOI: 10.3168/jdsc.2023-0386

[Display omitted] •No MR S. aureus was recovered from BTM of 300 dairy farms and the prevalence of MR NASM was 4.3%.•Our results suggest the use of a pre-enrichment method followed by Mueller-Hinton agar with antibiotics to screen for phenotypic resistance to methicillin and mannitol salt agar to screen for phenotypic susceptible staphylococci and mammaliicocci.•Dairy farms that contained ≤200 lactating cows and that had swine located on the farm had a higher prevalence of MR NASM as compared with farms that did not contain swine. The purpose of this study was to determine the apparent prevalence and risk factors of methicillin-resistant Staphylococcus aureus and non-aureus staphylococci and mammaliicocci (NASM) in bulk tank milk (BTM) obtained from 300 dairy farms that belong to a cooperative collecting milk from Indiana, Michigan, and Ohio. Dairy field personnel recorded information about selected farm level risk factors and collected and froze BTM samples (n = 300) that were sent to Michigan State University researchers. Milk samples were thawed at room temperature and pre-enriched by adding 1 to 4 mL of Mueller-Hinton broth supplemented with 6.5% NaCl and incubated at 37°C for 24 h. Subsequently, 10 µL was plated on mannitol salt agar and Mueller-Hinton agar supplemented with 2.5% NaCl containing 2 mg/L oxacillin and 20 mg/L aztreonam. Colonies that grew on the selective media were subcultured on blood agar and identified using MALDI-TOF mass spectrometry. Phenotypic methicillin resistance was tested using cefoxitin disk diffusion. Conventional PCR was used to detect mecA and mecC in phenotypically resistant isolates. Of 550 isolates that were obtained from mannitol salt agar plates and 10 isolates from Mueller-Hinton agar plates, 16 species of NASM accounted for 84% of staphylococci, while S. aureus accounted for the remaining 16%. Among S. aureus, 4 isolates from 4 farms (1.3%) demonstrated phenotypic resistance to methicillin resistance but none carried mecA or mecC genes. Among NASM, 45 isolates from 40 farms (13.3%) demonstrated phenotypic resistance to methicillin. However, only 13 NASM isolates (7 Mammaliicoccus sciuri, 2 Staphylococcus haemolyticus, 1 Mammaliicoccus fleuretti, 1 Staphylococcus epidermidis, 1 Staphylococcus saprophyticus, and 1 Staphylococcus hyicus) from 13 farms were positive for mecA, whereas all were negative for mecC. Thus, the prevalence of mecA-positive NASM in BTM was 4.3%. Based on molecular results, this study demonstr