The skin microflora and the formation of human axillary odour

Synopsis We have examined the relationship between human axillary skin microflora and underarm odour (UAO), in particular, the ability of cutaneous bacteria to transform steroids. A study was made of bacterial population density and odour intensity of the axillae of 34 normal male subjects. There was a statistically significant association between population density of aerobic coryneform bacteria and UAO intensity. No associations could be found between population densities of staphylococci, micrococci or propionibacteria and UAO intensity. An in vitro model for formation of UAO was developed, and used to test individual bacterial isolates. Only aerobic coryneforms could produce axillary odour in vitro, most notably C. xerosis. Many aerobic coryneforms could transform testosterone, the principal metabolites being 5alpha- and 5beta-DHT, androstenedione, and 5alpha- and 5beta-androstanedione. UAO positive coryneforms were more metabolically active than UAO negative bacteria. Micrococci also transformed testosterone to androstenedione, whilst staphylococci and propionibacteria could not metabolize it. A hypothesis for the role of aerobic coryneforms in the formation of human axillary odour is discussed.