Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (1315 page)

   
Positive results with
Nocardia
:
Delicate, branching filaments that retain the carbol-fuchsin stain.

   Limitations

   
Nocardia
may stain poorly in direct staining of patient specimens. Other species of aerobic actinomycetes, such as
Rhodococcus equi
and occasionally coryneform bacteria, may be modified acid-fast stain positive.

Suggested Readings

Al-Moamary M, Black W, Bessuille E, et al. The significance of the persistent presence of acid-fast bacilli in sputum smears in pulmonary tuberculosis.
Chest
1999;116:726–731.

Winn WC Jr, Allen SD, Janda WM, et al.
Koneman’s Color Atlas and Textbook of Diagnostic Microbiology
, 6th ed. Baltimore, MD: Lippincott Williams & Wilkins; 2006.

AEROBIC CULTURE

   Definition and Use

   Aerobic cultures are indicated for the detection of common aerobic bacterial pathogens in patient specimens taken from sites with signs and symptoms of bacterial infection (e.g., swelling, redness, heat, pus, or exudate). Site-specific bacterial cultures (e.g., sputum culture, genital culture) are recommended, if available. Specimens may be inoculated on several types of aerobic culture plates and broth media and may include selective and enriched media. Typical media for aerobic cultures include

   Supportive media to isolate nonfastidious organisms, like sheep blood agar (SBA).

   Enriched media to isolate organisms with special nutritional requirements, like chocolate agar.

   Selective media to suppress the growth of specific types of bacteria. Selective media are often formulated so that colonies of different types of organisms that are able to grow on the media have different appearances. MacConkey is an example: Selective: nonfastidious gram-negative bacilli are able to grow. Differential: lactose fermenters are distinguished from lactose nonfermenters.

   Solid versus Broth Media

   Culture media may be prepared in a solid or broth phase.

   Solid media (culture plates) are inoculated with a small amount of specimen. Mixed cultures are recognized by differences in colony morphology. The amount of each type of organism (and relative proportions in mixed cultures) can be estimated (e.g., rare, light, moderate, or heavy).

   Pyogenic infections are usually associated with growth of a single (or predominant) pathogen in moderate or heavy amounts.