In vitro and in vivo synergistic effects of cryptdin 2 and ampicillin against Salmonella

In view of the emergence of multidrug-resistant Salmonella strains, there is a need for therapeutic alternatives. To reduce the dose of antibiotic required in order to decrease the associated side effects, the present study was aimed at evaluating the synergism between cryptdin 2 (a Paneth cell antimicrobial peptide) and ampicillin (Amp) against Salmonella enterica serovar Typhimurium. The synergy was evaluated in terms of the fractional bactericidal concentration (FBC) index, time-kill assay results (in vitro), macrophage functions, i.e., intracellular killing, lipid peroxidation, superoxide dismutase activity, and generation of nitrite (ex vivo), and decreases in CFU of salmonellae in livers, spleens, and small intestines of infected mice treated with cryptdin 2 and/or Amp (in vivo). In vitro synergism between the two agents was observed on the basis of the FBC index and time-kill assays. When the agents were used in combination, ex vivo studies revealed an enhanced effect on macrophage functions, particularly exhibiting a synergetic effect in terms of SOD levels. In vivo synergy was indicated by larger log unit decreases in all target organs of mice treated with the combination than those for the drugs used alone. These results point toward the possible use of cryptdin 2 as an adjunct to ampicillin and may help in developing alternate strategies to combat Salmonella infections.