Mariana Andrade

Background/Objectives: Antimicrobial-resistant Staphylococcus aureus is a growing threat to human health for which alternative therapeutic options are needed. In this study, we aimed to evaluate the efficacy of amlodipine (AML) and imipramine (IMI) to treat S. aureus infection in the Galleria mellonella larval model by targeting efflux and biofilms, which are relevant contributors to antimicrobial resistance and virulence in S. aureus. Methods: In-house reared G. mellonella were used in virulence assays to determine the infective dose of two S. aureus strains differing in the expression of norA (gene encoding the native NorA efflux pump). Toxicology assays were conducted to determine the drugs’ LD₅₀ for G. mellonella. Drug efficacy assays were performed to evaluate the potential of amlodipine, imipramine and the control drugs ciprofloxacin (CIP) and enalapril (ENA) to clear S. aureus infection in G. mellonella. Results: Survival analysis defined the infective dose as 1 × 10⁷ CFU/larva for both strains. High LD₅₀ values were determined (CIP: >1000 mg/kg; AML: >640 mg/kg; IMI: 1141 mg/kg; ENA: >1280 mg/kg), revealing a high tolerance of G. mellonella to these drugs. AML at 15 mg/kg and IMI at 100 mg/kg increased the larvae survival by 20% (p = 0.04) and 11% (p = 0.11), respectively, also positively affecting health score indexes. In agreement with the literature, ciprofloxacin at >100 mg/kg promoted larvae survival by >73%. Conclusions: Amlodipine and imipramine show mild potential as new therapeutic options for managing S. aureus infections but are promising as new lead molecules. This study also reinforces G. mellonella as a sustainable, reliable model for drug evaluation.

Coagulase-positive staphylococci (CoPS) account for most bacteria-related pyoderma in companion animals. Emergence of methicillin-resistant strains of Staphylococcus pseudintermedius (MRSP), Staphylococcus aureus (MRSA) or Staphylococcus coagulans (MRSC), often with multidrug-resistant (MDR) phenotypes, is a public health concern. The study collection comprised 237 staphylococci (S. pseudintermedius (n = 155), S. aureus (n = 55) and S. coagulans (n = 27)) collected from companion animals, previously characterized regarding resistance patterns and clonal lineages. Biofilm production was detected for 51.0% (79/155), 94.6% (52/55) and 88.9% (24/27) of the S. pseudintermedius, S. aureus and S. coagulans, respectively, and was a frequent trait of the predominant S. pseudintermedius and S. aureus clonal lineages. The production of biofilm varied with NaCl supplementation of the growth media. All S. pseudintermedius and S. aureus strains carried icaADB. Kaplan–Meier survival analysis of Galleria mellonella infected with different CoPS revealed a higher virulence potential of S. aureus when compared with other CoPS. Our study highlights a high frequency of biofilm production by prevalent antimicrobial-resistant clonal lineages of CoPS associated with animal pyoderma, potentially related with a higher virulence potential and persistent or recurrent infections.