Purpose: To investigate the hypothesis that synergistic antimicrobial effect between FF and TAP exists for certain bacteria species.
Methods: A total of 57 field isolates including Escherichia coli, Riemerella anatipestifer, Salmonella enterica, and Staphylococcus aureus from avian, pig, cattle, dog and cat origins were collected. In addition, two ATCC strains, E. coli (ATCC 25922) and S. aureus (ATCC29213) were also used. The checkerboard microdilution assay and agar dilution assay were employed to determine the minimal inhibitory concentration (MIC) and synergism while time-kill curves were also applied to further validate the synergistic effect on susceptible strains.
Results: The synergistic effect was observed on 36% of E. coli isolates (n = 22) and 62% of S. aureus isolates (n = 13). The MIC of FF could be reduced by 87.5% (1/8 MIC) when combined with 1/2 MIC of TAP to exhibit antimicrobial activity comparable to the respective drugs at its original strength (1×MIC). The fractional inhibitory concentration indexes were ≤ 0.5 in 3 S. aureus strains and the synergism was further demonstrated by the time-kill assay. Some of the FF resistant strains also showed susceptibility to the synergistic combination. In contrast, the synergism was not observed in the 18 Salmonella and 4 R. anatipestifer strains tested.
Significance: This antibiotics synergism may offer potential alternatives for effective therapy against infections of these two pathogens. Food safety could be improved by reduced drug use and associated residues/toxicity at lower cost.