Ear swab

Ear swabs are collected for the diagnosis of otitis externa only. They are not useful in the diagnosis of otitis media as the tympanic membrane (ear drum) separates the middle ear from the ear canal.

Otitis externa is a bacterial infection of the external auditory canal usually caused by Pseudomonas aeruginosa, Staphylococcus aureus, Streptococcus pneumoniae, Group A streptococcus or fungus / yeast. Polymicrobial infections are not unusual.

Setup

Direct Examination

Gram stain

Culture

Media Incubation
Blood Agar (BA) CO2, 35°C x 48 hours
MacConkey Agar (MAC) O2, 35°C x 48 hours
Chocolate Agar (CHOC) CO2, 35°C x 48 hours

If fungus is requested, add:

Sabouraud’s Heart Infusion Agar (SABHI) O2, 30°C x 4 weeks
SABHI with Chloramphenicol (SABHI-C) O2, 30°C x 4 weeks

Interpretation Examine the culture plates after 24 and 48 hours incubation. Any growth of S. aureus, P. aeruginosa, S. pneumoniae, Group A streptococcus or yeast is significant.

For specimens from neonates only, identify and report Group B streptococcus.

For other organisms, a significant result is determined by the presence of a moderate to heavy growth of an organism which correlates with the predominant organism on the Gram stain. The Gram stain should also show >1+ pus cells. Full identification is required for all significant organisms except yeasts.

Reporting

Gram stain

Report with quantitation the presence of pus cells and organisms.

Culture

Negative Report
“Normal skin flora” or “No growth”.

Positive Report
Quantitate all significant isolates with appropriate susceptibilities. If normal flora is also present, report with quantitation.

References H.D. Isenberg. 2004. Specimen Collection, Transport and Acceptability p. 2.1.1 – 2.1.28. In Clinical Microbiology Procedures handbook, 2nd Edition, Vol 1 ASM Press, Washington, D.C

H.D. Isenberg, 2004. Wound Cultures – Wound and Soft Tissue Cultures, p. 3.13.1.1 – 3.13.1.16. In Clinical Microbiology Procedures Handbook, 2nd Edition, Vol 1 ASM Press, Washington, D.C.

H.D. Isenberg. 2004. Otitis Cultures p. 3.11.5.1 – 3.11.5.6. In Clinical Microbiology Procedures Handbook, 2nd Edition, Vol 1 ASM Press, Washington, D.C.