Introduction

Nocardioform and coryneform bacteria are Grampositive, soil-dwelling microorganisms with a high G + C content genome and include a large number of species of medical, agricultural, or industrial interest. Many mycobacterial diseases such as tuberculosis (caused by Mycobacterium tuberculosis) and leprosy (M. leprae) have been under investigation for a long time, whereas, more recently, opportunistic infections with mycobacteria (M. avium) have been described in patients treated with immunosuppressive drugs and in individuals with acquired immunodeficiency syndrome. Mycobacterium bovis bacille Calmette-Guérin (BCG), an avirulent strain of M. bovis, has been used as a vaccine for the prevention of tuberculosis and has nonspecific immuno-stimulating properties. Mycobacterial components are frequently added as adjuvants to stimulate the immune response to foreign antigens.

Nocardioform bacteria of the genus Rhodococcus are mostly saprophytic soil organisms but include human and animal pathogens (e.g. Rhodococcus bronchialis isolated from sputum of patients with pulmonary diseases; R. equi, which causes a purulent bronchopneumonia in foals, cattle, swine and occasionally humans) (Goodfellow & Minnikin, 1981; Goodfellow, 1986). Rhodococcus fascians is a pathogen on a large range of plants, causing fasciation, a disease characterized by the loss of apical dominance and the development of adventitious shoots. In severe infections, these stunted shoots have the appearance of a ‘leafy gall’ (Tilford, 1936; Lacey, 1939).