Chronic Canine Otitis: Is Culture Report Telling the Whole Story?

When a chronic otitis case doesn't respond despite selecting antibiotics based on culture and sensitivity, it's tempting to blame antimicrobial resistance. But what if the culture itself isn't showing the complete picture?

Recent research suggests that in chronic suppurative otitis, relying solely on bacterial culture may cause clinicians to overlook important pathogens.

Culture Is Still Essential—But It Has Limitations

Bacterial culture remains the gold standard for selecting antibiotics in recurrent and chronic otitis. It identifies viable bacteria and provides susceptibility testing, making it indispensable for antimicrobial stewardship¹.

However, culture doesn't always detect every organism present within the ear canal.

Advanced molecular testing (16S amplicon profiling) has demonstrated that several bacterial species may be present even when they fail to grow in culture.

For clinicians, this explains why some "culture-guided" treatments still fail.

Cytology, Culture and Molecular Testing: Each Answers a Different Question

Think of these three diagnostic tools as complementary rather than competing.

Ear Cytology²

• Immediate chairside information
• Identifies cocci, bacilli and yeast
• Guides empirical treatment
• Excellent for monitoring treatment response

Bacterial Culture²

• Identifies viable bacteria
• Provides antimicrobial susceptibility
• Best suited for chronic or recurrent infections

16S Amplicon Profiling²

• Detects bacterial DNA, including difficult-to-culture organisms
• Reveals mixed bacterial populations
• Mainly a research tool at present

No single test is perfect.

In fact, researchers found that agreement between culture and molecular profiling varied considerably depending on how bacterial abundance was interpreted.

Don't Ignore Cytology When Culture Looks "Normal"

One common clinical scenario is²:

• Numerous bacilli visible on cytology
• Culture grows only a few organisms—or none at all

This discrepancy has been reported repeatedly in chronic otitis cases.

Possible explanations include:

• Previous antibiotic therapy suppressing bacterial growth
• Fastidious organisms
• Biofilm-associated bacteria
• Mixed bacterial populations

The takeaway?

Never disregard what you see on cytology simply because the culture report appears less dramatic.

Pseudomonas Still Deserves Special Attention

Among chronic suppurative otitis cases, Pseudomonas aeruginosa continues to be one of the most clinically significant pathogens³.

Research found that every case identified by molecular testing was also detected by bacterial culture, suggesting that culture remains highly reliable for identifying Pseudomonas.

When Pseudomonas is isolated, clinicians should anticipate²:

• Biofilm formation
• Multidrug resistance
• Longer treatment duration
• Need for aggressive ear cleaning
• Frequent reassessment

Previous Medication Can Change Your Results

Many referral cases have already received multiple topical antibiotics or corticosteroids before samples are collected.

Although these treatments fail clinically, residual antibiotics may still suppress bacterial growth during culture.

This means the laboratory report should always be interpreted alongside²:

• Clinical examination
• Otoscopic findings
• Cytology
• Previous treatment history

Clinical Pearl

If chronic otitis isn't responding as expected, avoid asking only "Which antibiotic should I change to?"

Instead ask:

• Is there a biofilm?
• Have I addressed the underlying disease?
• Do cytology and culture agree?
• Has previous treatment affected my culture result?

Sometimes the problem isn't antibiotic selection—it's incomplete diagnosis.

Reference

1. Lehner G, Sauter Louis C, Mueller RS. Reproducibility of ear cytology in dogs with otitis externa. Veterinary Record. 2010 Jul;167(1):23-6. https://www.researchgate.net/profile/Georg-Lehner/publication/45096936
2. 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 
3. Secker B, Shaw S, Atterbury RJ. Pseudomonas spp. in canine otitis externa. Microorganisms. 2023 Oct 28;11(11):2650. https://www.mdpi.com/2076-2607/11/11/2650