Figure 6-7 Gram stains of direct smears can reveal infectious etiologies other than bacteria, such as the yeast Candida tropicalis.
are categorized as being non-acid-fast, a trait typical of most other clinically relevant bacteria. However, some degree of acid-fastness is a characteristic of a few nonmycobacterial bacteria, such as Nocardia spp., and cocdidian parasites, such as Cryptosporidium spp.
Procedure. The classic acid-fast staining method, Ziehl-Neelsen, is outlined in Figure 6-9. The procedure requires heat to allow the primary stain (carbolfuchsin) to enter the wax-containing cell wall. A modification of this procedure, the Kinyoun acid-fast method, is more commonly used today. Because of a higher concentration of phenol in the primary stain solution, heat is not required for the intracellular penetration of carbolfuchsin. This modification, which is presented in more detail in Chapter 45, is referred to as the "cold" method. Another modification of the acid-fast stain that is used for identifying certain nonmycobacterial species is described and discussed in Pan m, Section 14. When the acid-fast-stained smear is read with lOOOx magnification, acid-fast-positive organisms stain red. Depending on the type of counterstain used (e.g., methylene blue or malachite green), other microorganisms, host cells, and debris stain a blue to blue-green color (Figures 6-9 and 6-10).
As with the Gram stain, the acid-fast stain is used to detect acid-fast bacteria (e.g., mycobacteria) directly in clinical specimens and provide preliminary identification information for suspidous bacteria grown in culture. Because mycobacterial infections are much less common than infections caused by other non-add-fast bacteria, the add-fast stain is only performed on specimens from patients highly suspected of having a mycobacterial infection. That is, Gram staining is a routine part of most bacteriology procedures while add-fast staining is reserved for spedfic situations. Similarly, the add-fast stain is only applied to bacteria grown in culture when it is suspected based on other characteristics that mycobacteria are present (for more information regarding identification of mycobacteria see Chapter 45).
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