Opacification of Middlebrook agar as an aid in identification of Nocardia farcinica.

One hundred twenty seven human isolates of Nocardia asteroides complex were identified to the species level by drug susceptibility testing, acetamide utilization, thermotolerance studies, studies of arylsulfatase activity, and studies of acid production from rhamnose. N. asteroides complex organisms which were not identifiable as N. farcinica or N. nova were designated N. asteroides sensu stricto. All of the N. farcinica isolates and none of the other isolates produced a milky-white opacity surrounding colonies on Middlebrook 7H10 agar. This finding may facilitate the identification of N. farcinica.