This infection is an exogenous infection caused by the inhalation of bacilli.
Norcardia species can be found in the environment as saprophytes in the soil.
They have a broad temperature growth range.
They are branched, Gram positive, strictly aerobic bacteria closely related to the Mycobacteria.
Nocardia causes opportunistic diseases to human.
Infection occurs via inhaling of spores.
Medically important main species are N. brasiliensis and N. asteroids. N. brasiliensis causes mycetoma.
Occasionally they caused lympho cutaneous and pulmonary infections.
N. asteroids causes pulmonary diseases specillay in immunocompromised patients.
They occasionally causes brain abscesses.
Other Nocardia species like N. caviae, N. abscessus, N. farcinica, N. brasiliensis, N. brevicatena, N. otitidiscaviarum, N. nova and N. transvalensis causes wider range of diseases in immunosuppressed patients.
Principally N. asteroids cause opportunist pulmonary disease to the immunocompromised individuals.
The patients receiving post-transplant immunosuppressive therapy or chemotherapy for cancer and patents with acquired immune deficiency syndrome (AIDS) are risk groups.
Sputum, pus and infected tissues for culture and microscopy are the specimens useful for the diagnosis of the infection.
It depends on the site of infection.
Microscopic examination
Nocardia species are non-spore forming bacteria which are non-motile and having branching filaments which get easily fragmented.
Microscopic examination of sputum may be useful for the presumptive diagnosis of pulmonary nocardiasis.
Normally sputum contains number of lymphocytes and macrophages.
When stained with Gram stain they can be seen as Gram-positive bacilli.
They shows pleomorphism. They stain unevenly by Gram stain.
When stained with Modified Ziehl-Neelsen stain, the branches appear as acid fast and bacilli appears weakly acid fast.
In Mycetoma conditions soft, small whitish yellow granules are discharged in the pus.
They can be detected by using a magnifying lens.
In tissue biopsies stained by Gram or modified Ziehl– Neelsen methods, Nocardiae cannot be seen easily.
However, they can be seen easily in the preparations stained by Gram–Weigert or Gomori methenamine silver methods.
Culture
These organisms are aerobes. They are slow growers.
Growth is visible after incubation between 2 days to 1 month. They will grow at 35–37ºC, room temperature and 45ºC.
When we incubated at 45ºC, it will help to isolate Nocardiae from other contaminant commensals.
Nocardiae can be cultured on blood agar, Sabouraud agar containing chloramphenicol as a selective agent, enriched media like Löwenstein– Jensen medium and brain–heart infusion agar.
When cultured on Sabouraud agar for 3-14 days at 35–37ºC, orange pink colour, waxy, folded colonies can be seen. These colonies may dry, chalky and they have firmly adhere to the medium.
Most strains are beta-lactamase producers.
Nocardia are catalase positive and urease positive.
But identification is done mainly in reference laboratories.
Gene sequencing methods are used for the identification (16S rRNA gene sequences).Â
Treatment
Trimethoprim-sulfamethoxazole is widely used for the treatment.
Dose should be administered prolong basis for about 3–6 months.
It may cause adverse reactions due to prolong drug usage and some Nocardia species resistant to Trimethoprim-sulfamethoxazole.
Therefore, alternatively high-dose of imipenem with amikacin for 4–6 weeks can be given.
This may be given in severe disease.
Antibiotics like Minocycline, third generation cephalosporins, amoxicillin–clavulanate combinations and linezolid, an oxazolidinone, are also effective.
Always it is necessary do formal antibiotic susceptibility testing to ensure the optimal antibiotic therapy.
Mycetoma caused by Nocardiae can be treated more easily than Mycetoma caused by fungi.
Nocardia are relatively resistant to penicillin.
Prevention
There is no vaccine or prophylactic drug available for Nocardiosis.
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