Comparison between clinical evaluations and laboratory findings and the impact of biofilm on antimicrobial susceptibility in vitro in canine otitis externa

Background: In canine otitis externa (OE), biofilm-producing bacteria are frequently present but biofilm may be underdiagnosed clinically. Hypothesis/Objectives: The study aimed to investigate an association between clinical and cytological findings with bacteriological data from dogs with OE, to establish, through Environmental Scanning Electron Microscope (ESEM) examination, whether the presence of biofilm in vivo can be predicted and to evaluate the impact of biofilm on antimicrobial susceptibility tests. Materials and Methods: Fifty- six dogs showing clinical signs of OE were enrolled. One cotton swab each was collected for ESEM, bacterial culture and susceptibility testing and for cytology. Staphylococcus pseudintermedius (n = 42, 48.8%) and Pseudomonas aeruginosa (n = 26, 30.2%) were tested for their ability to form biofilm. Minimum Inhibitory Concentrations (MIC), Minimal Biofilm Inhibitory Concentrations (MBIC) and Minimal Biofilm Eradication Concentrations (MBEC) towards enrofloxacin, gentamicin, polymyxin B and rifampicin were determined. Results: Pseudomonas aeruginosa was positively associated with the biofilm clinical evaluation (p < 0.01) and neutrophils (p < 0.05), nuclear streaks (p < 0.01) and rods bacteria (p < 0.01) on cytology. S. pseudintermedius was associated with a low presence of neutrophils. There was a statistical correlation between clinical and cytological biofilm presence (p <= 0.01), but none with the biofilm production assay nor ESEM biofilm detection. No differences were found comparing the results of MIC and MBIC. MBEC results showed higher values than MIC and MBIC for all antimicrobials tested ( p <= 0.001). Conclusions and Clinical Relevance: Biofilm presence in OE was often underdiagnosed. Even if there is no specific clinical or cytological pattern related to biofilm, its presence should always be suspected.