Antimicrobial resistance was assessed by a disk diffusion assay according to CLSI guidelines15 using the breakpoints of Staphylococcus species resistance because no resistance criteria exist for Listeria susceptibility testing in the CLSI guidelines.23 (link), 24 (link), 25 (link)
Cells were grown at 35 °C for 24 h in tryptic soy broth (TSB; HiMedia, São Paulo, SP, Brazil), suspended in a saline solution and diluted to 0.5 points on the McFarland scale (ca. 108 cfu). The suspension of cells was inoculated in Mueller-Hinton (MH) agar (HiMedia, São Paulo, SP, Brazil) using a swab. Bacterial growth was recorded after 24 h of incubation at 35 °C. One Staphylococcus aureus (ATCC 33591) positive control was used for each set of analyzed samples. The isolates were tested against a panel of eight antimicrobial agents: penicillin G (PEN) (10 IU), erythromycin (ERY) (15 μg), tetracycline (TET) (30 μg), oxacillin (OXA) (1 μg), cefoxitin (CEF) (30 μg), vancomycin (VCM) (30 μg), streptomycin (STR) (10 μg) and ciprofloxacin (CIP) (5 μg). The antibiotic discs were purchased from Laborclin (Pinhais, PR, Brazil). The zones of inhibition were measured (mm) at 24 h.
Cells were grown at 35 °C for 24 h in tryptic soy broth (TSB; HiMedia, São Paulo, SP, Brazil), suspended in a saline solution and diluted to 0.5 points on the McFarland scale (ca. 108 cfu). The suspension of cells was inoculated in Mueller-Hinton (MH) agar (HiMedia, São Paulo, SP, Brazil) using a swab. Bacterial growth was recorded after 24 h of incubation at 35 °C. One Staphylococcus aureus (ATCC 33591) positive control was used for each set of analyzed samples. The isolates were tested against a panel of eight antimicrobial agents: penicillin G (PEN) (10 IU), erythromycin (ERY) (15 μg), tetracycline (TET) (30 μg), oxacillin (OXA) (1 μg), cefoxitin (CEF) (30 μg), vancomycin (VCM) (30 μg), streptomycin (STR) (10 μg) and ciprofloxacin (CIP) (5 μg). The antibiotic discs were purchased from Laborclin (Pinhais, PR, Brazil). The zones of inhibition were measured (mm) at 24 h.