Antibacterial Efficacy of Recombinant Endolysin S.AgEndo1332 in Combination with Antibiotics against Mastitis-Associated S. aureus in a Mouse Mastitis Model.

Bovine mastitis is one of the diseases that affect cattle and has a considerable detrimental impact on both the economics and animal welfare internationally. S. aureus is among the most relevant causative agents of this disease. Due to developing bacterial resistance and public health regulations, the number of antibiotics to treat mastitis is restricted. Endolysins, cell-wall-degrading enzymes of bacteriophage origin, are emerging as a promising alternative to traditional antibiotics. Moreover, combination therapy is frequently utilized to combat drug-resistant bacterial pathogens. In this study, we evaluated the therapeutic efficacy of the recombinant endolysin S.AgEndo1332 in combination with the antibiotic gentamicin. The synergistic activity of this combination was assessed in vitro using checkerboard and time-kill assays, and in vivo using a mouse mastitis model induced by mastitogenic S. aureus. S.AgEndo1332, a homolog of GBS phage B30 lysin gene, was custom synthesized, expressed by prokaryotic expression system, and purified under native purification protocol. S.AgEndo1332 showed strong antimicrobial activity against Streptococcus agalactiae (MIC 25 µg/mL) and S. aureus (MIC 50 µg/mL) strains in Tris-HCl buffer system at pH 7.5. The lytic activity of S.AgEndo1332 wasretained at -80 °C for 12 weeks. The combination of S.AgEndo1332 (12.5 µg/mL) with gentamicin (0.0594 µg/mL) was showing synergistic activity by checkerboard assay with FIC index 0.375. The dosage for the combination treatment (10X concentration, S.AgEndo1332: 125 µg/mL and gentamicin: 0.594 µg/mL) used for the in vivo trial was determined based on cell viability by the MTT assay (> 93% cell viability), time kill kinetics study (30 min) and lytic activity in milk (within 30 min). In S. aureus induced-mouse mastitis model, combination therapy significantly reduced the intramammary bacterial load and result was further supported by consistent findings of expression of pro and anti-inflammatory cytokine genes (TNF-α, IL-6, IFN-γ, and IL-10), and histopathological analysis of the mammary gland tissue section. Overall, our findings highlight the potential of combining endolysin with antibiotics as a promising alternative treatment for S. aureus-induced bovine mastitis.