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Nocardia
A number of Nocardia species have been identified as the source of both local and disseminated disease in children and adults. These organisms are primarily opportunistic pathogens infecting immunocompromised persons. Infection caused by these bacteria is termed nocardiosis , which consists of acute, subacute, or chronic suppurative infections with a tendency for remissions and exacerbations.
Etiology
Nocardia spp. are obligate aerobes and will grow on a variety of culture media, including simple blood agar, brain-heart infusion agar, and Lowenstein-Jensen media. Colonies can appear as early as 48 hr, but typically growth of Nocardia is slower than in other bacteria and may take 1-2 wk. Growth appears as waxy, folded, or heaped colonies at the edges, and yield is best achieved in conditions that include a temperature of 37°C (98.6°F) with 10% carbon dioxide. However, many isolates of Nocardia are thermophilic and will grow at temperatures up to 50°C (122°F). Microscopically, Nocardia spp. are weakly gram-positive rod-shaped filamentous bacteria. For some isolates, there may be alternating areas of gram-positive and gram-negative staining, giving a beaded appearance often described with Nocardia. These organisms are also weakly acid fast, and the modified Kinyoun acid-fast staining technique can be helpful to identify organisms from clinical specimens such as a tissue biopsy or bronchoalveolar lavage (BAL).
Approximately 100 distinct Nocardia spp. have been identified, almost 20 of which have been associated with human infection. The distribution of Nocardia spp. causing disease varies across observational studies, partly because of variation in taxonomic classification over time. Currently, the predominant species to cause disease are Nocardia farcinica , N. cyriacigeorgica , N. abscessus, and N. nova . Species identification can be critical for optimal clinical outcomes because of variability in virulence strategies and antibiotic resistance profiles (see Treatment later). Traditional approaches to speciation require biochemical processing that can be laborious and inefficient. Techniques such as 16S rDNA polymerase chain reaction (PCR) or matrix-assisted laser desorption/ionization (MALDI) time of flight (TOF) mass spectrometry can more efficiently speciate Nocardia. Of these, MALDI-TOF technology is likely to become more available in clinical microbiology laboratories in the near future.
Epidemiology
Once thought to be a rare human disease, nocardiosis is being recognized more frequently and has been diagnosed in persons of all ages. Pediatric patients with compromised cellular immunity are at particular risk, including children receiving immune suppression after solid-organ or stem cell transplantation, chemotherapy for malignancy, prolonged corticosteroid therapy, children with poorly controlled HIV infection, or those with a primary immunodeficiency, especially chronic granulomatous disease (see Chapter 156 ). Notably, nocardiosis has been described in patients without an identified immune defect, although in these clinical scenarios, other predisposing factors such as bronchiectasis are often present.
Multiple contemporary retrospective studies have been performed in Australia, France, and Spain to better define the epidemiology of nocardiosis in children and adults. The incidence of nocardiosis has been estimated to be 6 cases per 100,000 hospital admissions. This rate is much higher in susceptible hosts, such as in solid-organ transplant recipients, in whom the rate is as high as 20 per 1000 transplants.
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