Genus of weakly staining Gram-positive, catalase-positive, rod-shaped bacteria.
It forms partially acid-fast beaded branching filaments (acting as fungi, but being truly bacteria).
Majority of strains possess the cord factor (trehalose 6-6′ dimycolate) an important virulence factor.
The genus includes at least 30 different species with ten of them isolated from humans. Some species are non-pathogenic while others are responsible for nocardiosis.
Nocardia are oral microflora found in healthy gingiva as well as periodontal pockets.
Most Nocardia infections are acquired by inhalation of the bacteria or through traumatic introduction.
Nocardia are pathogenic bacteria with low virulence: clinically significant disease most frequently occurs as an opportunistic infection in those with a weak immune system (small children, elderly, HIV, etc).
Nocardia asteroides: species most frequently infecting humans. Other species of medical interest are N. brasiliensis and N. caviae. Because it is acid-fast to some degree, it stains only weakly gram positive.
The most common form of human nocardial disease is a slowly progressive pneumonia, whose common symptoms include cough, dyspnea (shortness of breath), and fever. Every organ can be affected if a systemic spread takes place.
Therapy:
Important considerations: The choice of an agent should be based on local antimicrobial sensitivities, site of infection, cost, and comorbid conditions. Generally, the most common agents/regimens are listed first. Listed dosages may need to be adjusted for renal dysfunction. Specific therapy depends on the species of Nocardia.
Bactrim (TMP-SMX) – Mild-moderate infection: 5 to 10 mg/kg/day (based on trimethoprim component) IV/oral divided in 2-4 doses. Severe infection: 15 to 20 mg/kg/day (based on trimethoprim component) IV, given in equally divided doses every 6 to 8 hours.
Bactrim (TMP-SMX) – Mild-moderate infection: 5 to 10 mg/kg/day (based on trimethoprim component) IV/oral divided in 2-4 doses. Severe infection: 15 to 20 mg/kg/day (based on trimethoprim component) IV, given in equally divided doses every 6 to 8 hours PLUS Imipenem500mg IV every 6 hours.
Linezolid 600 mg orally or IV q 12h + Meropenem 0.5 – 1 gram IV q8h (life-threatening infection -unlabeled use: 2 grams IV q8h)
Imipenem500mg IV every 6 hours + Amikacin 5 – 7.5 mg/kg/dose q8-12h (patient-specific pharmacokinetic dosing required)
Minocycline 100-200 mg po bid (unlabeled use). Usual dosage: 200 mg x 1, followed by 100mg q12h.
