UK standards for microbiology investigations of ear infection (SMI B1) are inadequate for the recovery of fungal pathogens and laboratory diagnosis of otomycosis: a real-life prospective evaluation

Mycoses. 2022 Jan 26. doi: 10.1111/myc.13423. Online ahead of print.


BACKGROUND: We evaluated the recovery of fungal pathogens from clinical external ear samples from patients with otitis externa (OE) using the UK national Standard Microbiology Investigations of ear infection (SMI B1).

METHOD: The UK SMI B1 protocol including a single Sabouraud dextrose agar with chloramphenicol (SABC) incubated at 37°C for 48 hours was compared to a standard fungal-specific culture method using two SABC agar plates incubated at 28°C and 37°C for 2 weeks with an extra Candida chromogenic agar incubated at 37°C for 5 days. This was undertaken on ear samples from patients with OE from January 2020 to December 2020.

RESULTS: Altogether, 304 individual patient ear swabs were prospectively examined. The positivity rate of UK standard was 14% (42/304) versus 26% (79/304) for the fungal-specific protocol (p<0.05). The standard protocol identified seven compared to 17 species using the fungal-specific protocol. A total of 93 fungal isolates were recovered; nine different yeasts and eight filamentous fungal species. Candida parapsilosis (38/304; 13%), C. albicans (10/304; 3%) and C. orthopsilosis (6/304; 2%) were common yeast species. Aspergillus niger complex (16/304; 5%) was the most common mold, followed by A. fumigatus complex (3/304; 1%). Many less common and emerging yeasts and molds were only isolated from samples cultured using a fungal-specific protocol.

CONCLUSION: Our results suggest that the UK SMI B1 media and procedures are inadequate to detect all fungal agents causing otomycosis. Fungal-specific culture protocols increase the recovery rate and diversity of fungal pathogens isolated from ear samples.

PMID:35080057 | DOI:10.1111/myc.13423

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