Nocardia

Nocardia is a genus of Gram-positive, catalase-positive, rod-shaped bacteria; some species are pathogenic (nocardiosis).^[1] Nocardia are found worldwide in soil that is rich with organic matter. Most Nocardia infections are acquired by inhalation of the bacteria or through traumatic introduction.

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Culture and staining

Nocardia colonies have a variable appearance, but most species appear to have aerial hyphae when viewed with a dissecting microscope, particularly when they have been grown on nutritionally-limiting media. Nocardia grow slowly on non-selective culture media, and are strict aerobes with the ability to grow in a wide temperature range. Some species are partially acid fast (meaning that a less concentrated solution of sulfuric or hydrochloric acid should be used during the staining procedure) due to the presence of intermediate-length mycolic acids in their cell wall. Majority of strains possess the cord factor(trehalose 6-6' dimycolate) an important virulence factor. They are catalase positive and can grow easily on the most commonly used media with colonies becoming evident in 3-5 days. However sometimes prolonged incubation periods (2-3 weeks) are needed. The genus includes at least 30 different species with ten of them isolated from humans.

Virulence

The various species of Nocardia are pathogenic bacteria with low virulence; therefore clinically significant disease most frequently occurs as an opportunistic infection in those with a weak immune system, such as small children, the elderly, and the immunocompromised. Nocardial virulence factors are the enzymes catalase and superoxide dismutase (which inactive reactive oxygen species that would otherwise prove toxic to the bacteria), as well as a "cord factor" (which interferes with phagocytosis by macrophages by preventing the fusion of the phagosome with the lysosome).

Clinical disease and microbiological diagnosis

Nocardia asteroides is the species of Nocardia most frequently infecting humans, and most cases occur as an opportunistic infection in immunocompromised patents. Other species of medical interest are N. brasiliensis and N. caviae.

The most common form of human nocardial disease is a slowly progressive pneumonia, whose common symptoms include cough, dyspnoea (shortness of breath), and fever. It is not uncommon for this infection to spread to the pleura or to chest wall. Pre-existing pulmonary disease, especially pulmonary alveolar proteinosis, increases the risk of contracting a Nocardia pneumonia. Every organ can be affected if a systemic spread takes place.

In about 25-33% of people Nocardia infection will take the form of encephalitis and/or cerebral abscess formation.

Nocardia may also cause a variety of cutaneous infections such as actinomycetoma (especially Nocardia brasiliensis), lymphocutaneous disease, cellulitis and subcutaneous abscesses.

Nocardia isolation from biological specimens can be performed using agar medium enriched with yeast extract and activated charcoal (BCYE) , the same utilized for Legionella spp. Selective media for Mycobacteria or fungi can also be inoculated. The most suitable specimens are the sputum or, when clinically necessary, bronchoalveolar lavage or biopsy. Further biochemical tests for species identification are not routinely performed. Serological or cutaneous tests are not available.

Treatment

Antibiotic therapy with a sulfonamide is the treatment of choice. The most common sulfonamide used is trimethoprim-sulfamethoxazole. People who take trimethoprim-sulfamethoxazole for other reasons, such as prevention of pneumocystis jiroveci infection in AIDS, have fewer nocardia infections. High-dose imipenem and amikacin have also been used in refractory cases. Antibiotic therapy may have to be continued for six months to a year. Proper wound care is also critical.

Genetics

Despite that Nocardia has interesting and important features such as production of antibiotics and aromatic compound-degrading or converting enzymes, the genetic study of this organism has been hampered by the lack of genetic tools. However, practical Nocardia–E. coli shuttle vectors have been developed recently.^[2]

References

Further reading

• MicrobeWiki's N. farcinica Page, Current as of June 2007