The genus Rhodococcus organisms are Gram-positive and strict aerobic actinomycetes. The cell wall of Rhodococcus organisms is similar to those of mycobacterium by containing mycolic acid and tuberculostearic acid, which makes the bacteria acid fast. The genus contains 20 species out of which at least seven species are known to cause disease in humans.
Rhodococcus equi, formerly known as Corynebacterium equi, isthe most important human pathogen. R. equi is a pleomor-phic Gram-positive coccobacillus showing some degrees of branching. It is weakly acid fast. Rhodococcus grows well on routinely used media aerobically. On blood agar, on pro-longed incubation at room temperature, the bacteria produce colonies resembling those of Klebsiella, with production of pink pigments. The bacteria do not ferment carbohydrates and show a variable biochemical reaction. R. equi is an intra-cellular pathogen, which multiplies in macrophages. This causes granulomatous reaction, resulting in the formation of abscess. The bacteria cause invasive pulmonary disease, such as pneumonic and lung abscess. Subsequently, the infection may spread to distant sites of the body causing meningitis, pericarditis, etc.
In immunocompetent hosts, rhodococci cause opportunis-tic infections, such as traumatic endophthalmitis, peritonitis in patients undergoing dialysis, and post-traumatic skin infection. The diagnosis of the condition is made by demon-stration of weakly acid-fast and Gram-positive pleomorphic bacilli in clinical specimens. Definitive diagnosis by culture is often difficult. Rhodococci are difficult bacteria to treat, par-ticularly in immunocompromised patients. They are resistant to penicillins and cephalosporins. Treatment with vancomy-cin or combination of erythromycin and rifampicin is effective.