3.4. Antimicrobial susceptibility
Overall, the E. ictaluri isolates were susceptible to antimicrobials belonging to the following classes/subclasses: β-lactam/β-lactamase inhibitor combination, cephems, tetracyclines, and fluoroquinolones, while they were resistant to antimicrobials belonging to the penicillin, macrolide, sulfonamide, amphenicol, and glycopeptide subclasses (Tables 4 and 5). More than 80% of the isolates were susceptible to cefotaxime, ceftriaxone, cefuroxime, doxycycline, oxytetracycline, ofloxacin, and norfloxacin; 73.1% of the isolates were also susceptible to the combination of amoxicillin and clavulanate (Tables 4 and 5). However, 80.8%–100% of isolates were resistant to amoxicillin, oxacillin, erythromycin, sulfamethoxazole/trimethoprim, florfenicol, and vancomycin. The resistance frequencies of E. ictaluri isolates to nalidixic acid and neomycin were 27.0% and 19.2%, respectively (Table 5). The MAR values of E. ictaluriisolates ranged from 0.25–0.5, corresponding to 4–8 antibiotics or 12 resistant phenotypes (Table S1). The highest frequency of the isolates (34.6%) was observed to resist seven tested antimicrobial agents, followed by five and six agents (23% both). The frequencies of the isolates resistant to 2 and 8 drugs were 11.5 and 7.7%, respectively (Table S1).