TDK conference 2011

Antibiotic resistance is a growing global problem in human and veterinary medicine alike. Long term treatment in recurrent dermatological cases increases the incidence of resistance development in bacteria. Combination of different classes of antimicrobials helps fighting against the emergence of resistant strains. Pyoderma and otitis externa are common diseases in dogs and cats. These diseases have a multifactorial aetiology, while the most frequently isolated bacteria are staphylococci, streptococci and Pseudomonas aeruginosa.

The aim of this study was to evaluate the in vitro activity of gentamicin, marbofloxacin and the combination of these antibacterial agents against bacteria from canine and feline external ear and skin infections isolated in Hungary. In this study 129 strains of Staphylococcus intermedius (110 canine and 19 feline isolates) , 6 strains of S.aureus (2 canine and 4 feline strains), 43 strains of Streptococcus canis (38 canine and 5 feline isolates) and 56 strains of Pseudomonas aeruginosa (52 canine and 4 feline isolates) were investigated.

To determinate the MIC (minimum inhibitory concentration) values broth microdilution method was used according to CLSI (Committee of Laboratory Standards Institute) recommendations. The MIC50 of gentamicin ranged from 0.125 to 16 µg/ml. MIC90 ranged from 1 to 16 µg/ml. In case of marbofloxacin MIC50 was between 0.125 and 2 µg/ml, MIC90 was found between 0.25-8 µg/ml. Staphylococci were the most susceptible bacteria both for gentamicin and marbofloxacin. The most effective ratio of marbofloxacin and gentamicin was 1:1. MIC50 for the combination was 0.125-2 µg/ml, while MIC90 ranged from 0.25 to 2 µg/ml. In case of staphylococci the combination showed 83.7% additive effect, 11.1% synergism and 5.2% antagonistic effect. According to CLSI breakpoints the 1:1 combination of marbofloxacin and gentamicin showed 100% effectiveness on all the Staphylococcus strains tested, no resistant strain was found. The combination represented 79.1% additive influence, 16.3% synergism and 4.6% antagonistic effect against Streptococcus canis strains. Four strains (9.3%) proved to be resistant to the combination. Against Pseudomonas aeruginosa isolates gentamicin and marbofloxacin presented 73.2% synergism, 23.2% additive and 3.6% antagonistic effect. Only 1.8% of the strains proved to be resistant.

Based on our results it can be stated that the 1:1 ratio of marbofloxacin and gentamicin have excellent activity against the most common bacteria from dermal and ear infections of dogs and cats, especially Pseudomonas aeruginosa. The combination increased the ratio of susceptible bacteria and reduced the ratio of moderately susceptible and resistant strains to the individual substances.