In Vitro Antibacterial Efficacy of Cetirizine and N-Acetylcysteine Alone and in Combination with Cefalexin on Canine Methicillin-Sensitive and -Resistant Staphylococcus pseudintermedius

Staphylococcus (S.) pseudintermedius, as a commensal of the skin and mucosa, leads to a variety of diseases in dogs, most commonly skin and ear infections. The development of methicillin-resistant S. pseudintermedius (MRSP) is an emerging risk for animals and humans. The aim of this study was to test cetirizine and N-acetylcysteine as synergistic substances with cephalexin for treating S. pseudintermedius infections. Each of the five methicillin-sensitive S. pseudintermedius (MSSP) isolates and five MRSP isolates, and one control strain were tested. The minimal inhibitory concentration (MIC) of the substances was tested by broth microdilution assay. In a checkerboard assay, the MIC of cefalexin alone was compared to the MIC of the substances combined. The determined dose reduction index (DRI) shows the influence each substance had on the efficacy of cefalexin. Furthermore, the minimal bactericide concentration (MBC) of N-acetylcysteine (NAC) was identified, and a time kill assay was performed to determine its time-related efficacy on selected isolates. Cetirizine showed no inhibition on bacterial growth or influence on antibiotic efficacy. NAC inhibited bacterial growth at 2 mg/mL. A significant synergistic influence was shown against the MRSP (p < 0.001) and MSSP isolates (p < 0.01). The MBC of the MSSP isolates and control strain was 12.8 and 25.6 mg/mL for the MRSP isolates. The time kill assay showed that NAC is bactericidal within 120 s at the prior determined MBC concentrations. NAC showed an antibacterial effect alone and a synergistic influence on cefalexin’s antibacterial properties. Thus, NAC shows promising efficacy in treating infections with S. pseudintermedius; according to the preliminary study conducted here, this effect may be independent of the resistance profile.