Chloramphenicol

Eight K. pneumoniae isolates were received from the Medical Microbiology laboratories of three hospitals in Armenia between January 2019 and August 2019. All isolates were recovered from various clinical specimens (urine, sputum, throat, blood, and stool) of hospitalized patients. The isolates were identified using a matrix-assisted laser desorption ionization–time of flight mass spectrometry (MALDI-TOF-MS) as described previously (59 (link)).
All isolates were tested using a disk diffusion method for susceptibility to a panel of 11 antibiotics, including ampicillin (10 mg), piperacillin-tazobactam (30/6 mg), amoxicillin-clavulanic acid (20 and 10 mg, respectively), ceftazidime (10 mg), cefepime (30 mg), norfloxacin (10 mg), levofloxacin (5 mg), amikacin (30 mg), imipenem (10 mg), meropenem (10 mg), and chloramphenicol (30 mg) (Mast Group, Merseyside, United Kingdom) according to the European Committee on Antimicrobial Susceptibility Testing protocol (EUCAST v.6.0, 2017) (60 ). The antibiotics chosen were those most frequently used in clinical settings in Armenia. Isolates resistant to three or more antibiotic classes were considered multidrug resistant.

The methicillin resistance of 40 S. aureus isolates were tested by using the Oxacillin agar screen method on the Muller Hinton agar modified by adding NaCl (4 %) + Oxacillin (6 µg/ml) (Dhanalakshmi et al. 2012 ). The strains that could grow on this medium were then tested to be revealed whether they carry the mecA gene (Kot et al. 2020 (link); Ubukata et al. 1989 (link)), as described in the next section.
The disc diffusion method was conducted to determine the antibiotic resistance pattern of the identified methicillin-resistant S. aureus strains to Vancomycin (30 µg), Ciprofloxacin (5 µg), Erythromycin (15 µg), Tetracycline (30 µg), Gentamycin (10 µg), Ceftriaxone (30 µg), Amikacin (30 µg), Mupirocin (5 µg), Oxacillin (30 µg), Chloramphenicol (30 µg) and Trimethoprim/Sulfamethoxazole (1.25/23.75 µg) purchased from MAST Group, Merseyside, UK.

The antimicrobial susceptibility test was performed using the disk diffusion method, in Muller-Hinton agar (MHA, Merck, Germany) plate. The antibiotics disc tested based on the minimal growth inhibitory zone diameter were nalidixic acid (NA = 30 μg), ciprofloxacin (Cip = 5 μg), ofloxacin (OF = 5 μg), chloramphenicol (CL = 30 μg), cefotaxime (30 μg), ceftazidime (30 μg) and aztreonam (AZT = 30 μg) (Mast Group, Bootle, UK) [13 (link)]. The results were interpreted as resistant, intermediate or sensitive, in accordance with the guidelines of the CLSI (2015) and the manufacturer protocols [4 (link)]. The strains used for quality control were E. coli ATCC 25922 and Enterococcus fecalis ATCC 29212.