Erythromycin

The Kirby–Bauer disc-diffusion method on Mueller–Hinton agar was used to test the susceptibility of the isolates against amikacin, gentamicin, tobramycin, kanamycin, tetracycline, erythromycin, clindamycin, linezolid, teicoplanin, ciprofloxacin, rifampicin, quinupristin-dalfopristin and trimethoprim-sulfamethoxazole (Mast Co., Bootle, UK) based on the CLSI guideline. Susceptibility to vancomycin, mupirocin, tigecycline and fusidic acid was assessed by the broth microdilution method to determine the MIC titre. The European Committee for Antimicrobial Susceptibility Testing (EUCAST) breakpoints was used to determine MIC titres of fusidic acid and tigecycline (EUCAST 2018). The results of other antibiotics were interpreted by using the CLSI 2018 breakpoints. Low-level and high-level mupirocin resistance (LLMUPR, HLMUPR), inducible macrolide-lincosamide-streptogramin group B (iMLS_B) and constitutive (cMLS_B) macrolide-lincosamide-streptogramin group B were identified based on the CLSI guideline. Susceptibility testing was quality controlled using S. aureus ATCC 25923, ATCC 43300 and ATCC 29213 strains. Powders of antibiotics were all obtained from Sigma Chemical Co. (St Louis, MO, USA).

The antimicrobial susceptibility test was done with the disk diffusion method (Bauer et al, 1966) using Mueller–Hinton agar (Difco). Initially, an emulsion of sample in saline solution was prepared by adjustment to the 0.5 McFarland turbidity standards. The susceptibility of the E. coli strains was tested in relation to several antibiotics, including: chloramphenicol (CAF) (30 μg), ciprofloxacin (5 μg), gentamycin (10 μg), trimethoprim (1.25 μg-sulfamethoxazole 23.75 μg), erythromycin (15 μg), streptomycin (10 μg) and tetracycline (30 μg) (Mast Group Ltd., Merseyside, UK). Using sterile tweezers, commercially available antibiotic disks were placed individually on the surface of Mueller-Hinton agar. After 24 h of incubation at 35°C, the strains were scored as “susceptible”, “intermediate”, or “resistant” to each antibiotic based on the measurement of the inhibition zone, as recommended by clinical laboratory standard institute (CLSI).26

The disk diffusion test was employed to determine the susceptibility of the isolates to vancomycin (30 µg), teicoplanin (30 µg), gentamicin (120 µg), linezolid (30 µg), ciprofloxacin (5 µg), erythromycin (15 µg), ampicillin (10 µg), tetracycline (30 µg) and rifampicin (5 µg) (Mast Group Ltd., Merseyside, UK). The microdilution broth method was used to determine the minimal inhibitory concentration (MIC) of vancomycin for the isolates that showed resistance phenotype after the disk diffusion method. The results were interpreted according to the guidelines of the Clinical and Laboratory Standards Institute (CLSI) [10 ]. Isolates that showed intermediate levels of susceptibility were classified as non-susceptible. Multidrug-resistance (MDR) was defined as resistance to three or more different classes of antibiotics [11 (link)].