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.

Abstract

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). The UK SMI B1 protocol including a single Sabouraud dextrose agar with chloramphenicol (SABC) incubated at 37°C for 48hours was compared with a standard fungal-specific culture method using two SABC agar plates incubated at 28 and 37°C for 2weeks with an extra Candida chromogenic agar incubated at 37°C for 5days. This real-life evaluation was undertaken on ear samples from patients with OE from January 2020 to December 2020. 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<.05). The standard protocol identified seven compared with 17 species using the fungal-specific protocol. A total of 93 fungal isolates were recovered; nine different yeasts and eight filamentous fungal species. Candidaparapsilosis (38/304; 13%), C.albicans (10/304; 3%) and C.orthopsilosis (6/304; 2%) were common yeast species. Aspergillusniger complex (16/304; 5%) was the most common mould, followed by A.fumigatus complex (3/304; 1%). Many less common and emerging yeasts and moulds were only isolated from samples cultured using a fungal-specific protocol. 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 external ear samples.