Sorbitol macconkey agar

STEC O157 was isolated by conventional selective culture (conventional) and immunomagnetic separation (IMS) methods. For the conventional method, approximately 1 g of each sample was homogenized in 9 mL modified EC broth (mEC; Becton, Dickinson and Company, USA) supplemented with novobiocin (20 mg/L, Oxoid, UK) and incubated overnight at 37℃. Following incubation, one loop of mEC broth culture was streaked onto sorbitol MacConkey agar (Becton, Dickinson and Company, USA) supplemented with potassium tellurite (T-SMAC; 2.5 mg/L, Sigma-Aldrich, Canada) and incubated at 37℃ overnight. A maximum of six typical colonies were subcultured onto MacConkey agar (MAC; Becton, Dickinson and Company, USA) and CHROMagar O157 (CHROM; CHROMagar Microbiology, France) and then incubated overnight at 37℃. Typical colonies, pink in MAC and mauve in CHROM, were selected for testing using the E. coli O157 latex test kit (Oxoid, UK).
For the IMS method, Dynabeads MAX anti-E. coli O157 (Dynal; Invitrogen, USA) was used according to the manufacturer's instructions. The suspension of immunomagnetic beads was spread onto T-SMAC and incubated at 37℃ overnight. Up to four typical colonies were selected and identified by applying the same criteria as described in the conventional method. If the four colonies yielded no STEC O157 strain, up to four additional colonies were tested.

Ground Hanwoo beef samples (n=32) were divided into 4 groups to evaluate the antimicrobial activity of ozone at 4℃ for 4 d. Two groups were inoculated with 100 μL of E. coli O157:H7 (10⁷ CFU/mL), which was applied to the meat surface and spread with sterile spatulas. Subsequently, the inoculated and non-inoculated groups were each placed in either an air or ozone chamber.
To determine the CFU of the meat samples, duplicate samples were taken every 24 h. Each sample (1 g) was diluted with 9 mL of 0.1% sterile peptone water and homogenized for 2 min in a Stomacher (Lab blender 400 Seward Laboratory, UK). Decimal dilutions were then performed, and diluted samples were plated on Sorbitol MacConkey agar (Becton Dickinson) for total E. coli and Plate Count Agar (PCA; Difco) for total aerobic and anaerobic bacteria. The plates were incubated aerobically at 37℃ for 24 h to enumerate the E. coli and total aerobic bacteria. For anaerobic bacteria enumeration, the PCA plates were placed on an anaerobe container (BD GasPak EZ, USA). The container was incubated at 37℃ for 24 h.