Antimicrobial
therapy using a combination of
polymyxin B and
miconazole is effective against the main bacterial pathogens associated with
otitis externa in dogs, and a synergistic effect of both drugs has been shown previously. The objective of the present investigation was to visualize ultrastructural changes after exposure of Escherichia coli, Pseudomonas aeruginosa, Staphylococcus pseudintermedius and Malassezia pachydermatis to
polymyxin B and
miconazole by transmission electron microscopic (TEM). For this, cultures of E. coli, P. aeruginosa, S. pseudintermedius and M. pachydermatis were exposed to
polymyxin B and
miconazole, alone or in combination for 24 h. Ultrastructural changes were observed most frequently in the cell envelope of the four microorganisms. Exposure to
polymyxin B seemed to cause more damage than
miconazole within the range of concentrations applied. Treatment resulted in changes of the cell size: in E. coli, cell size increased significantly
after treatment with either compound alone; in P. aeruginosa, cell size decreased significantly
after treatment with
polymyxin B and with
miconazole; exposure of S. pseudintermedius to
miconazole caused a decrease in cell size; in M. pachydermatis, cell size increased significantly
after treatment with
polymyxin B.; in E.coli, S. pseudintermedius and M. pachydermatis, cell size changed highly significant, in P. aeruginosa significantly after exposure to the combination of both compounds. In conclusion, by using a different approach than previous investigations, this study confirmed a clear combinatory effect of
polymyxin B and
miconazole against the tested microorganisms involved in canine
otitis externa. It is the first time that visualization technologies were applied to compare the effect of single drugs to their combinatory effects on cellular and subcellular entities of selected bacterial and yeast species.