Comparison of organism recovery and staining properties for 3 methods of Gram stain preparation

One possible way to improve accuracy of blood culture Gram stain analyses is increasing the concentration of organisms on the slide prepared from the blood culture broth.

From each positive blood culture bottle, 1 direct smear, a 1-drop concentrated preparation, and 1 cytospin/cytofuge preparation were Gram stained and evaluated. There were 2 evaluators who ranked the 3 preparations from most to fewest organisms seen. Each preparation was also scored as acceptable or unacceptable for both organism and background stain quality.

The 1-drop slide exhibited the highest concentration of organisms compared with both the cytospin and direct smear but presented frequent difficulties with interpretation of Gram staining acceptability with both the background and the organisms. Although cytospin preparations are known to concentrate liquid specimens, the current study found no enrichment of microorganisms over the direct smear preparation. The cytospin was, however, advantageous for enhancing organism morphology by creating a monolayer of elements and reducing background artifacts.

The 1-drop preparation is simple, inexpensive, and effective at increasing organism concentration for analysis, making it a good option when processing culture bottles with low organism loads, but more investigation into methods of improving stain quality is necessary.