When Chronic Otitis Doesn't Respond: Are You Missing Biofilms and Hidden Pathogens?

Every veterinarian has encountered the frustrating ear case that has received multiple ear drops, several antibiotic courses, and yet keeps coming back.

Increasingly, evidence suggests that hidden bacterial populations and biofilms may be part of the reason.

Chronic Otitis Is Rarely a Single-Bacteria Disease

Traditional culture often identifies one or two dominant bacteria.

However, molecular testing has shown that chronic otitis frequently involves complex bacterial communities, including organisms that routine culture may miss.

Among these were¹:

• Corynebacterium spp.
• Porphyromonas cangingivalis
• Finegoldia magna
• Bacteroides pyogenes
• Fusobacterium species

Many of these are anaerobic bacteria that are not routinely cultured in veterinary practice.

Corynebacterium: More Than Just a Contaminant?

Many clinicians consider Corynebacterium a harmless secondary organism.

Recent findings suggest otherwise.

In several chronic otitis cases, molecular testing showed Corynebacterium as the dominant bacterial population, even when routine culture failed to detect it².

Even more concerning, multidrug-resistant strains have been reported, including reduced susceptibility to antibiotics commonly used in otic preparations.

So when treatment repeatedly fails despite apparently appropriate therapy, reconsider whether Corynebacterium deserves greater clinical attention.

Is That Thick White Exudate Really Pus?

One surprising observation from recent research was that some dogs with classic whitish, slimy ear discharge had no neutrophils on cytology¹.

This raises an interesting possibility.

Rather than being true purulent exudate, some of these cases may represent mature bacterial biofilms.

Although routine practice cannot easily confirm biofilms, clinicians should suspect them when¹:

• Thick mucoid discharge persists
• Multiple antibiotic courses have failed
• Cytology and culture don't correlate
• Pseudomonas is present

In these patients, thorough ear cleaning becomes just as important as selecting the correct antimicrobial.

Should We Be Using Gram Staining More Often?

Most practices rely on Diff-Quick staining for ear cytology, which remains rapid and clinically useful.

However, Gram staining may provide additional information, particularly for identifying Gram-positive bacilli such as Corynebacterium, allowing clinicians to better appreciate the bacterial population before culture results arrive^1,2.

While not necessary for every case, it may prove valuable in chronic or treatment-resistant otitis

Breed Matters

One important clinical observation was the overrepresentation of Cocker Spaniels in chronic suppurative otitis cases¹

Many had experienced recurrent disease for months—or even years.

For breeds with known predisposition, repeated symptomatic treatment should prompt a more comprehensive investigation rather than another empirical antibiotic course.

Clinical Pearl

Successful management of chronic otitis requires more than selecting the "right" antibiotic.

It requires understanding:

• Mixed bacterial populations
• Biofilm formation
• Previous treatment history
• Limitations of diagnostic tests
• Breed predisposition
• Underlying primary disease

The more chronic the ear, the more valuable a multimodal diagnostic approach becomes.

References

1. Leonard C, Thiry D, Taminiau B, Daube G, Fontaine J. External ear canal evaluation in dogs with chronic suppurative otitis externa: Comparison of direct cytology, bacterial culture and 16S amplicon profiling. Veterinary sciences. 2022 Jul 18;9(7):366. https://www.mdpi.com/2306-7381/9/7/366 
2. Parasana DK, Makwana PM, Shobha K, Poshiya PJ, Kalyani IH. Isolation and Identification of Corynebacterium Species from cases of skin and ear infection of dogs. https://www.researchgate.net/profile/Dixit-Parasana/publication/376029655