When interpreting a bacterial stain, it is important to know what type of sample you are working with. In a pure culture, all bacteria are identical. A mixed culture contains different species of bacteria. Clinical samples such as blood or CSF may include both bacterial and human cells. Since human cells do not have peptidoglycan, they will appear readdish pink in the Gram stain.

The microscopic view of CSF. Big, red, irregularly shaped structures are loosely aligned to each other on the white background. In between them, tiny pink spheres are visible, some as single spheres, some as clusters of two or three. One red structure is enclosed in black circle and marked with the letter A. One cluster of two tiny pink spheres is also enclosed in vlack circle and marked with the letter B.

Figure 1: CSF sample with Gram-negative bacteria. The big circular reddish pink structures (A) are human cells, while the small pink structures are Gram-negative bacteria (B).