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.
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).