Azithromycin

Antimicrobial susceptibility testing was performed for Enterobacteriaceae in accordance with the Clinical and Laboratory Standards Institute (CLSI) performance standards [61 ] using the disk-diffusion method on Mueller Hinton plates (Oxoid, Hampshire, UK) and the antibiotics ampicillin (AM), amoxicillin with clavulanic acid (AMC), azithromycin (AZM), cefazolin (CZ), cefepime (FEP), cefotaxime (CTX), chloramphenicol (C), ciprofloxacin (CIP), fosfomycin (FOS), gentamicin (G), kanamycin (K), nalidixic acid (NA), nitrofurantoin (F/M), streptomycin (S), sulfamethoxazole trimethoprim (SXT) and tetracycline (TE) (Becton Dickinson, Allschwil, Switzerland). Results were interpreted according to CLSI standards [61 ]. For azithromycin, an inhibition zone of ≤12 mm was interpreted as resistant.

AST was performed using the disk diffusion method according to the guidelines of the Clinical and Laboratory Standards Institute (Clinical and Laboratory Standards Institute, 2020 ). Antimicrobial substances included ampicillin, amoxicillin/clavulanic acid, cefazolin, cefotaxime, cefepime, nalidixic acid, ciprofloxacin, sulfamethoxazole‐trimethoprim, fosfomycin, azithromycin, nitrofurantoin, streptomycin, kanamycin, gentamicin, chloramphenicol, and tetracycline (Becton, Dickinson). Results were interpreted according to CLSI breakpoints for human clinical isolates (Clinical and Laboratory Standards Institute, 2020 ). In the absence of clinical breakpoints for azithromycin resistance in Citrobacter spp. and E. coli, a zone diameter of ≤12 mm was interpreted as resistant, based on data reported by Meerwein et al. (2020 ).

Antimicrobial susceptibility testing (AST) was performed using the disc-diffusion method according to the guidelines of the Clinical and Laboratory Standards Institute (CLSI) [31 ], and the antibiotics ampicillin (AM), amoxicillin with clavulanic acid (AMC), azithromycin (AZM), cefazolin (CZ), cefepime (FEP), cefotaxime (CTX), chloramphenicol (C), ciprofloxacin (CIP), fosfomycin (FOS), gentamicin (G), kanamycin (K), nalidixic acid (NA), nitrofurantoin (F/M), streptomycin (S), sulfamethoxazole/trimethoprim (SXT) and tetracycline (TE) (Becton Dickinson, Allschwil, Switzerland). Results were interpreted according to CLSI performance standards [31 ]. For azithromycin, an inhibition zone of less than or equal to 12 mm was interpreted as resistant. For isolates harbouring mcr-1, the minimum inhibitory concentration (MIC) of colistin was determined by broth microdilution according to the European Committee on Antimicrobial Susceptibility Testing (EUCAST), 2019 (eucast.org). The results were interpreted according to the breakpoints suggested by EUCAST for Enterobacterales (susceptible, MIC ≤ 2 mg l⁻¹; resistant, MIC > 2 mg l⁻¹).
Isolates displaying resistance to three or more classes of antimicrobials were defined as multidrug-resistant (MDR), as proposed by Magiorakos et al. [32 (link)], counting β-lactams as one class.

Antimicrobial susceptibility testing was performed using the disk diffusion method with commercially available disks. The disks used included ampicillin (AMP₁₀), ceftriaxone (CRO₃₀), chloramphenicol (C₃₀), erythromycin (E₁₅), gentamicin (GM₁₀), nalidixic acid (NA₃₀), ciprofloxacin (CIP₅), ofloxacin (OFX₅), norfloxacin (NOR₁₀), meropenem (MEM₃₀), streptomycin (STR₁₀), azithromycin (AZM₁₅), tetracycline (TET₃₀), trimethoprim/sulfamethoxazole (SXT_1.25/23.75), neomycin (N₃₀) (Becton Dickinson, Sparks Glencoe, MD, USA) in accordance with the criteria recommended by Clinical and Laboratory Standards Institute [15 ]. According to CLSI guidelines, breakpoints for Enterobacteriaceae were used to determine antimicrobial susceptibility. Escherichia coli ATCC 25922 was used as a quality control strain.

Antibiotic susceptibility pattern of isolates was performed using the disc diffusion method on Mueller Hinton Agar (Condalab, Spain) according to the Clinical and Laboratory Standards Institute (CLSI) guideline for the following antibiotics: penicillin (10 μg), cefepime (30 μg), cefoxitin (30 μg), sulfamethoxazole (23/75 μg), tazobactam (10 μg) + piperacillin (100 μg), cefotaxime (30 μg), ceftriaxone (30 μg), meropenem (10 µg), clindamycin (2 μg), clarithromycin (15 μg), levofloxacin (5 μg), azithromycin (15 μg) (BD-BBL Company, USA). The Minimum Inhibitory Concentration (MIC) of colistin and vancomycin was determined using the broth microdilution method. Bacterial isolates resistant to three or more different antimicrobial classes were identified as MDR isolates. S. aureus (ATCC 25923) and E. coli (ATCC 25922) were used as control strains.