Direct microscopic examination of Gram-stained preparations of clinical specimens is of utmost importance in the diagnosis of infections caused by the aerobic actinomycetes. Often, the demonstration of gram-positive, branching or partially branching beaded filaments provides the first due to the presence of an aerobic actinomycete (Figure 21-1). Unfortunately, the actinomycetes do not always exhibit such characteristic morphology; many times these organisms are not seen at all or appear as gram-positive cocci, rods, or short filaments. Nevertheless, if gram-positive, branching or partially branching organisms are observed, a modified add-fast stain should be performed (i.e., 1% sulfuric add rather than 3% hydrochloric add as the decolorizing agent). Of note, the modified add-fast stain is positive in only about half of these smears showing gram-positive beaded, branching filaments subsequently confirmed as Nocardia.16 Histopathologic examination of tissue spedmens using various histologic stains, such as Gomori's methenamine-silver (GMS) stain, can also detect the presence of actinomycetes.
Of importance, any biopsy or drainage material from actinomycetomas should be examined for the presence of granules. If observed, the granules are washed in saline, emulsified in 10% potassium hydroxide or crushed between two slides. Gram-stained, and examined microscopically for the presence of filaments.3
Figure 21-1 A, Gram stain of sputum obtained from patient with pulmonary nocardiosis caused by Nocardia asteroides. B, The same sputum stained using a modified acid-fast stain. The arrows indicate the organism.
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