Cefalotin

All GNB isolates which were non-naturally colistin-resistant and grown on the LBJMR medium were subjected to an AST according to the current (DD) test method (Kirby-Bauer procedure). The minimal inhibition concentration (MIC) was confirmed by CLSI and EUCAST guidelines [28 ]. AST was performed with a definite turbidity bacterial suspension in NaCl (0.5 McFarland; 1.5 × 10⁸ cells/mL). Antibiogram test included the following sixteen antibiotics: amoxicillin (AMX), amoxicillin-clavulanic acid (AMC), cefepime (FEP), piperacillin/tazobactam (TPZ), cefalotin (KF), ceftriaxone (CRO), ertapenem (ETP), imipenem (IMP), fosfomycin (FF), nitrofurantoin (F), trimethoprim-sulfamethoxazole (SXT), amikacin (AK), ciprofloxacin (CIP), doxycycline (DO), colistin (CT), and gentamicin (GT) (Bio-Rad, Marne-la-Coquette, France). Hierarchical clustering of the antibiotic resistance phenotype was performed using Multi-Experiment Viewer (MeV 4.9.0).
Strains with a narrow diameter zone of inhibition (ZOI) less than 15 mm were picked out to confirm the minimal inhibition concentration value using other complementary tests, namely the E-tests method (BioMérieux) and UMIC test (Biocentric Bandol, France) [29 (link)]. Furthermore, strains were considered to be multidrug-resistant (MDR) if bacteria were resistant to more than three different classes of antibiotics.

Antibiotic susceptibility testing was performed by the disc diffusion method on Mueller Hinton agar according the standard recommendations of the French Society of Microbiology (CA-SFM, 2010) [8 ]. The following antibiotic discs were tested: ampicillin, amoxicillin+ acid clavulanic, ticarcillin, cefalotin, cefoxitin, cefuroxime, cefotaxim, imipenem, nalidixic acid, aztreonam, norfloxacin, ciprofloxacin, gentamycin, tetracycline, trimethoprim-sulfamethoxazole, streptomycin (Bio-Rad, France). For carbapenems, ertapenem (10 μg/ml) antibiotic discs were used (Bio-Rad, France). The zones of inhibition were measured to assess resistance or susceptibility.

Antibiotic susceptibility was determined by the disk diffusion method, on Mueller-Hinton (MH) agar, in accordance with the guidelines of the Antibiogram Committee of the French Society for Microbiology (CA-SFM 2006)³⁹ . The following antimicrobial drugs (Bio-Rad, Marnes-la-Coquette, France) were tested: ampicillin (AMP), cefalotin (CEF), cefotaxime (CTX), streptomycin (STR), chloramphenicol (CHL), azithromycin (AZM), sulfonamides (SUL), trimethoprim-sulfamethoxazole (SXT), vibriostatic agent O/129 (O129), tetracycline (TET), nalidixic acid (NAL), ciprofloxacin (CIP), nitrofurantoin (FUR), polymyxin B (PB) and colistin (CS). Escherichia coli CIP 76.24 (ATCC 25922) was used as a control.