Nocardia

Includes several species that can infect and cause nocardiosis. Be aware that Nocardia nomenclature and classification has changed dramatically over the years. As a result, some isolates that were once commonly associated with nocardiosis no longer are. For example, several isolates formerly identified as Nocaria asteroides have been reclassified. Microbiology: Weakly acid-fast, with a delicate "beaded" appearance. Aerobic and catalase-positive, with slow growth. Unique aerial hyphae, with filaments that grow upward from the colony. Nocardia are not considered part of the normal human microflora. Found in soil. Gain entry to human hosts via inhalation and aspiration. Individuals with defective cellular immunity are particularly susceptible to infection. Pathogenic Nocarida have multiple ways to avoid phagocytic destruction, which is key to innate immunity. The enzymes catalase and superoxide dismustase protect them from the harmful effects of phagocytic reactive oxygen species. Cord factor: When phagocytosed by macrophages, Nocarida cord factor (aka, trehalose dimycolate) prevents phagosome-lysosome fusion, which means the bacteria avoid bactericidal molecules. Nocardia can survive and replicate within macrophages, which travel throughout the body. Overall, Nocardiosis is rare, and manifests as non-specific symptoms. However, it should be ruled out early to avoid delayed diagnosis and treatment. Fortunately, the bacteria's weakly acid-fast nature and aerial hyphae make it easy to identify. Lung infection is most common; illness onset is associated with nonspecific symptoms. Lung abscesses and necrosis can develop, and dissemination to other organs can occur. CNS infection is the most serious form of nocardiosis, and results in abscesses with non-specific symptoms (such as fever and headache); meningitis is possible but infrequent. Cutaneous infections manifest as granulomas, ulcers, or cellulitis, and may involve nearby lymphatics. Infection can be primary or secondary. Treatment: Nocardia response to antibiotics varies, so testing for antimicrobial susceptibility is crucial. In general, pulmonary infections are treated with trimethoprim-sulfamethoxazole (TMP-SMX) and amikacin; CNS infections are treated with trimethoprim-sulfamethoxazole and imipenem or cephalosporin. Prolonged antibiotic treatment is recommended to avoid relapse.