Commercially available antiseptics show high in vitro efficacy against pathogens most commonly associated with canine and feline infectious keratitis

Purpose: To determine the minimal bactericidal concentration (MBC) of polyhexanide (PHMB), povidone-iodine (PVP-I), N-acetylcysteine (NAC), and hypochlorous acid (HOCl) for bacterial species commonly found in canine and feline infectious keratitis.

Methods: MBCs for clinical isolates of Staphylococcus (S.) pseudintermedius (n = 11), including 3 methicillin-resistant strains, Pseudomonas (P.) aeruginosa (n = 8), and Streptococcus (Str.) canis (n = 11), including the corresponding control strains, were examined. All testing substances were serially diluted in phosphate-buffered saline (PBS) and cation-adjusted Mueller–Hinton Broth (CAMHB) and inoculated with the bacterial suspension for 10 min. Afterwards, a neutralisation with Dey–Engley neutralising broth was performed, followed by plating onto Columbia sheep–blood agar. After incubation, plates were visually examined for bacterial growth. Tests were carried out in triplicate.

Results: MBCs in PBS for polyhexanide ranged 0.8–1.6 mg/L for S. pseudintermedius and 1.6–3.2 mg/L for P. aeruginosa and Str. canis. For povidone-iodine, MBCs in PBS were observed at concentrations ranging 8–32 mg/L for S. pseudintermedius and P. aeruginosa and 8–16 mg/L for Str. canis. MBCs in PBS for NAC were recorded at a range of 6,400–12,800 mg/L for S. pseudintermedius, whereas those for P. aeruginosa and Str. canis ranged 3,200–6,400 mg/L. Results for HOCl in PBS ranged 0.4–1.6 mg/L for S. pseudintermedius and 0.4–0.8 mg/L for P. aeruginosa and Str. canis. MBCs in CAMHB for polyhexanide were found in the range between 3.2 and >12.8 mg/L, those for povidone-iodine between 6,400 and >12,800 mg/L, and for NAC between 6,400 and >12,800 mg/L, across the tested species. When dissolved in CAMHB, no antimicrobial effect could be observed for HOCl in concentrations up to 137.5 mg/L.

Conclusion: All tested substances had an in vitro bactericidal effect against all three bacterial species with MBCs below known tolerated ocular concentrations when dissolved in PBS. Povidone-iodine and hypochlorous acid showed a marked reduction in their in vitro efficacy in the presence of protein. Nevertheless, our results provide a promising outlook on alternatives or adjuvants to antibiotics in ophthalmology that align with the One Health approach.