A sensitive and cost-effective method for isolation of Staphylococcus aureus from agricultural soils suitable for antimicrobial resistance surveillance programs

Abstract

The inclusion of antimicrobials in selective media introduces culture bias that may confound surveillance studies aimed at exploring the origins of Antimicrobial Resistance (AMR). We describe a cost-effective procedure to isolate Staphylococcus aureus, a pathogen susceptible to acquisition of AMR, from agricultural soil without the use of selective antibiotics. Environmental stress was modelled using heat and ultraviolet light and S. aureus recoveries assessed using seeded sterile soils with several combinations of enrichment and isolation media. Suitable combinations were then applied to native soils. Using Modified Vogel-Johnson enrichment Broth (MVJB) and Baird-Parker Agar (BPA), as few as 3 cfu ml-1 of stressed S. aureus ATCC 25923 cells seeded into 25 g of sterile soil were retrieved. This combination detected S. aureus in 100% of 40 dairy farm soil samples and enabled differentiation from non-target organisms, with all isolates selected confirmed by species-specific PCR. We believe this cost-effective method could assist long-term surveillance studies aimed at exploring the origin and evolution of antimicrobial resistance in S. aureus.