title: Prevalence and characterization of Staphylococcus aureus, including methicillin-resistant Staphylococcus aureus, isolated from bulk tank milk from Minnesota dairy farms.
abstract: Staphylococcus aureus is a common causative agent of bovine mastitis in dairy herds. The emergence of methicillin-resistant Staphylococcus aureus (MRSA) in hospitals as well as the community is a significant and costly public health concern. S. aureus-related bovine mastitis is a common reason for therapeutic and/or prophylactic use of antibiotics on dairy farms. In this study, herd prevalence of S. aureus, including MRSA, was estimated from bulk tank milk (BTM) from Minnesota farms. A total of 150 pooled BTM samples from 50 farms, collected over 3 seasons (spring, summer, and fall of 2009), were assessed. Herd prevalence of methicillin-susceptible S. aureus (MSSA) was 84%, while MRSA herd prevalence was 4%. A total of 93 MSSA isolates and 2 MRSA isolates were recovered from 150 BTM samples. Antibiotic susceptibility testing of S. aureus isolates showed pansusceptibility in 54 isolates, resistance to a single antibiotic class in 21 isolates, resistance to two antibiotic classes in 13 isolates, and resistance to ≥3 antibiotics classes and thus multidrug resistance in 5 isolates. The two MRSA isolates displayed resistance to β-lactams, cephalosporins, and lincosamides and were multiresistant. Staphylococcal protein A gene (spa) typing identified spa types t529 and t034 most frequently among methicillin-susceptible isolates, while t121 was observed in MRSA isolates. Seven isolates, including the two MRSA isolates, produced staphylococcal enterotoxins B, C, D, and E on overnight culture. MRSA isolates were further genotyped using multilocus sequence typing (MLST) and pulsed-field gel electrophoresis (PFGE). Of the 2 MRSA isolates, one had a composite genotype profile of MLST ST 5-PFGE USA100-unknown spa type, which has been reported among hospital-associated MRSA isolates, while the second isolate carried the MLST ST 8-PFGE USA300-spa type t121 genotype, commonly identified among community-associated MRSA isolates. These results suggest that MRSA genotypes associated with hospitals and community can be isolated from milk at very low rates.
authors.affiliation: Veterinary Population Medicine, University of Minnesota, St. Paul, Minnesota, USA.
authors.forename: K P
authors.initials: KP
authors.lastname: Haran
authors.forename: S M
authors.initials: SM
authors.lastname: Godden
authors.forename: D
authors.initials: D
authors.lastname: Boxrud
authors.forename: S
authors.initials: S
authors.lastname: Jawahir
authors.forename: J B
authors.initials: JB
authors.lastname: Bender
authors.forename: S
authors.initials: S
authors.lastname: Sreevatsan
mesh_terms: D000818:Animals; D000829:Animals, Domestic; D000900:Anti-Bacterial Agents / Q000494:pharmacology; D002417:Cattle; D016000:Cluster Analysis; D016521:Electrophoresis, Gel, Pulsed-Field; D004768:Enterotoxins / Q000235:genetics; D005838:Genotype; D008826:Microbial Sensitivity Tests; D008892:Milk / Q000382:microbiology*; D008910:Minnesota; D058885:Multilocus Sequence Typing; D015995:Prevalence; D013205:Staphylococcal Protein A / Q000235:genetics; D013211:Staphylococcus aureus / Q000187:drug effects / Q000302:isolation & purification*
chemical_list: D000900:Anti-Bacterial Agents; D004768:Enterotoxins; D013205:Staphylococcal Protein A