This study describes Thermoactinomyces candidus, a new species of thermophilic actinomycete isolated from home environments including humidifiers, air conditioners, and house dust. Researchers identified 25 strains of this organism during an investigation of thermophilic actinomycetes in homes of individuals with allergic pneumonitis. T. candidus differs from the previously known species T. vulgaris and T. sacchari through distinct biochemical characteristics. The new species hydrolyzes esculin and splits arbutin but does not attack tyrosine, hypoxanthine, or starch—properties that distinguish it from T. vulgaris. Compared to T. sacchari, T. candidus grows faster, produces abundant aerial mycelia, causes hemolysis on blood agar, and decomposes esculin and arbutin. The organisms exhibit type III cell wall composition containing mesodiaminopimelic acid. T. candidus grows optimally at 45-60°C with thermoresistant spores surviving 100°C for one hour. The type strain T-106 (ATCC 27868) was isolated from an air conditioner. Antiserological studies confirmed these strains are distinct from T. vulgaris, supporting their classification as a new species.

Key findings

• T. candidus is a newly identified thermophilic actinomycete species isolated from home environments, differing from T. vulgaris by hydrolyzing esculin and arbutin but not tyrosine, hypoxanthine, or starch
• The species exhibits type III cell wall composition with mesodiaminopimelic acid and produces thermoresistant spores capable of surviving boiling water for one hour
• T. candidus grows optimally at 45-60°C, produces abundant white aerial mycelia, and causes hemolysis on blood agar, distinguishing it from other Thermoactinomyces species
• Immunological cross-reactivity studies confirmed T. candidus antigens fail to cross-react with T. vulgaris antiserum, supporting its status as a distinct species
• The type strain T-106 was deposited in the American Type Culture Collection as ATCC 27868 for reference purposes