Antimicrobial Activity of Bioactive Peptides on Resistant Enterobacteriaceae and the Viability of Giardia duodenalis Cysts Isolated from Healthy Dogs

<i>Enterobacter</i> spp. and <i>Giardia duodenalis</i> are two resistant and globally distributed pathogens widely reported in humans and animals worldwide. To contrast the antimicrobial resistance phenomenon, this study aimed to evaluate the in vitro activity of antimicrobial peptides (AMPs), such as temporins, on the viability of <i>Enterobacter</i> spp. and <i>G. duodenalis</i> cysts isolated from naturally infected dogs. A total of 50 clinically healthy dogs were sampled, with 25 in each of the two sampling sessions. Rectal and oral swabs for <i>Enterobacter</i> spp. and fecal samples for <i>G. duodenalis</i> isolation were carried out. The AMPs were synthesized by a solid-phase peptide synthesis approach. <i>Enterobacter ludwigii</i> and <i>E. hormaechei</i> strains were isolated from two different dogs, while 16 dogs were positive for <i>G. duodenalis</i>. Subsequently, cysts and bacterial strains were treated with the following peptides: TL-34, TL-48, TL-42, TL-51, RB-71, and RB-58 at different concentrations (i.e., 250 mg/mL, 125 mg/mL, 62. mg/mL, and 31.25 for <i>Enterobacter</i> spp. strains and 0.15 mg/mL, 0.3 mg/mL, 0.6 mg/mL, and 1.2 mg/mL for <i>G. duodenalis</i> cysts). The results showed antimicrobial activity of TL-34, TL-48, and RB-71 against the two <i>Enterobacter</i> strains, while TL-51/TL-42 and RB-58 were not effective. In contrast, for <i>G. duodenalis,</i> TL-34, TL-48, TL-42, and RB-58 were effective at the two highest concentrations, inhibiting the viability of 100% of <i>Giardia</i> cysts, while the other two peptides (TL-51 and RB-71) showed less effectiveness. The AMPs used in this study have in vitro antimicrobial activity on cysts and bacterial strains and appear to have potential for the treatment of these infections in dogs.