Mic test strip

Bacteriostatic susceptibility was performed using Liofilchem MIC test strip (Liofilchem S.R.I., Italy) for chloramphenicol, erythromycin, sulfamethoxazole, tetracycline and trimethoprim according to the manufacturers’ instructions. The strips were placed on a Luria Berthani (LB) Agar inoculated with 0.5 ml LB pure colonies’ suspension of Lp^NC8 and Lp^NIZO2877, separately. Three independent replicates were performed for each antibiotic. Subsequently, the agar plates were incubated at 37 °C and observed for Minimum Inhibitory Concentration (MIC) reading after 24 h. MIC value was determined at the intersection of the strip and the growth inhibition ellipse, as reported by Kowalska-Krochmal et al.^{71 (link)}.

S. Choleraesuis isolates were tested for susceptibility to antimicrobial agents using the agar disk diffusion method (Kirby Bauer test) following the guidelines and criteria of the Clinical and Laboratory Standards Institute (CLSI) [26 ]. The antibiotics tested (Oxoid, England) were ampicillin (10 µg), amoxicillin–calvulanic acid (20/10 µg), aztreonam (30 µg), cefepime (30 µg), ceftazidime (30 µg), cefoxitin (30 µg), cefotaxime (30 µg), cefpodoxime (10 µg), cephalothin (30 µg), cefuroxime (30 µg), ceftriaxone (30 µg), ceftiofur (30 µg), ciprofloxacin (5 µg), chloramphenicol (30 µg), nalidixic acid (30 µg), norfloxacin (10 µg), streptomycin (10 µg), tetracycline (30 µg), and trimethoprim/sulfamethoxazole (25 µg). Furthermore, cefotaxime-resistant S. Choleraesuis isolates were screened for ESBL producers by the combination disk method (CLSI, 2008) [12 (link)]. The MICs for ciprofloxacin (32–0.002 µg/mL), cefotaxime (256-0.016 µg/mL), ceftazidime (256-0.016 µg/mL), ceftriaxone (256-0.016 µg/mL) and cefpodoxime (256-0.016 µg/mL) were determined using the Liofilchem MIC test strip (Liofilchem, Roseto degli Abruzzi, Italy), according to the manufacturer’s instructions. Escherichia coli ATCC 25922 was used as the quality control strain for antimicrobial susceptibility testing.