OBJECTIVES

This study aimed to compare the antimicrobial activity in vitro of two ear cleansers, Epi-Otic with spherulites (Virbac, France) and Otoclean (Janssen, France), against 5 randomly selected canine clinical isolates of Staphylococcus intermedius, Pseudomonas aeruginosa and Malassezia pachydermatis. The relative rates of microbicidal activity of the two ear cleansers were determined against the three major pathogens isolated from cases of otitis externa in dogs.

MATERIALS

Recent canine isolates of the three organisms were grown overnight at 37°C in brain heart infusion agar or, for Malassezia, on modified Dixon's agar (MDA) for 3 days at 34°C. Growth was then suspended in Mueller Hinton broth and 2 µl of each suspension added to 2 ml of either undiluted test product, a 1/5 dilution in sterile distilled water of the test product (Epi-otic or Otoclean) or PBS (controls). The suspensions were held at room temperature (21°C). Duplicate aliquots were removed after 1, 2, 4, 8, 16 and 30 minutes, spread plated on 5% blood agar or MDA and incubated for 48h at 37°C (bacteria) or 72 h at 34°C (Malassezia). Colony forming units (cfu) on the media were counted and then converted to cfu/ml of the original suspension.

RESULTS

The staphylococcal and Malassezia isolates failed to survive 1 minute of exposure with both test products (Epi-Otic and Otoclean), either undiluted or diluted 1/5. Growth was not inhibited in control suspensions, which remained above 1.25 x 10e3 cfu/ml for staphylococci, and 2.3 x 10e3 cfu/ml for Malassezia, throughout the 30-min. period. Epi-Otic killed all 5 of the Pseudomonas isolates within 1 minute, both undiluted and at 1/5 dilution. However, two isolates were able to survive for 2 minutes, and one for 8 minutes, with the 1/5 diluted Otoclean. Control Pseudomonas suspensions remained above 2.9 x 10e4 cfu/ml throughout the 30-min. period.

CONCLUSION

This study demonstrates that both ear cleansers have excellent potency in vitro against S. intermedius and M. pachydermatis. In addition, Epi-Otic has rapid and complete activity against Ps. aeruginosa whilst Otoclean has delayed activity against some isolates when diluted 5-fold. This could be important since exudates and glandular secretions, which may be present in significant amounts in cases of otitis externa, are likely to dilute the ear cleanser in vivo. The potent activity of Epi-Otic against Pseudomonas is also of special interest in view of the increasing resistance shown by this genus against antimicrobials that may be used following cleansing agents in the therapy of otitis externa.