The use of highly effective combination of antimicrobials (CA) is a promising strategy for increasing treatment efficacy, controlling the bacterial drug resistance and reducing the use of antimicrobials in aquaculture. In the present work, we investigated the in vitro activity and in vivo therapeutic effectiveness of thiamphenicol (TAP) combined with florfenicol (FFC) against Aeromonas hydrophila isolated from diseased Nile tilapia Oreochromis niloticus. Individual minimum inhibitory and bactericidal concentrations (MIC and MBC) of each antimicrobial were determined for 26 strains of Aeromonas isolated from pacu Piaractus mesopotamicus and Nile tilapia during disease outbreaks in Brazil. MICs were used to perform the checkerboard microdilution assay to evaluate the in vitro effect of the combination (fractional inhibitory concentration index – FICI). Recommended doses of each antimicrobial (G1: TF – 10 mg kg–1 bw, and G2: FFC – 10 mg kg–1 bw) and two doses of TAP combined with FFC (G3: TAP + FFC – 5 + 2.5 mg kg–1 bw, and G4: TAP + FFC – 2.5 + 1.25 mg kg–1 bw) correspondent to FICI deduction, were administered by intraperitoneal injection to Nile tilapia (70 ± 12 g) challenged by gavage with A. hydrophila, to evaluate the therapeutic effectiveness of CA.
In fish aeromoniosis model, combinations of TAP and FFC (G3 and G4) were effective as the standards doses of the single antimicrobial (G1 and G2), and all medicated groups were statistically different (p <0.05) from the unmedicated group (G5) (Figure 1).
Figure 1. Cumulative survival (%) of Nile tilapia challenged with A. hydrophila and medicated with thiamphenicol (TAP) or florfenicol (FFC) alone and combined. G1: TAP (10 mg kg-1); G2: FFC (10 mg kg-1); G3: [TAP + FFC] (5 + 2.5 mg kg-1); G4: [TAP + FFC] (2.5 + 1.25 mg kg-1); G5: unmedicated; and CG: Control group (Phosphate Buffered Saline).
These findings suggests the potentiality of CA involving TAP and FFC as an effective approach to control aeromoniosis with lower doses of antimicrobials.