In this simplified diagram of gram staining, bacterial cells are first treated with a purple dye called crystal violet. The dye gets into the cells’ peptidoglycan layer (gray), tinting it purple.

Gram staining is a type of differential staining used to distinguish between Gram positive and Gram negative bacterial groups, based on inherent differences in their cell wall constituents. Credit ...

Gram-positive bacteria are bacteria with thick cell walls. In a Gram stain test, these organisms yield a positive result. Here’s why knowing whether the result is positive or negative is important.

Gram staining is one way scientists can identify bacteria. Gram-positive bacteria, such as Staphylococcus, appear blue or purple under a microscope when scientists apply a stain. Conversely, Gram ...

Gram negative bacteria. Gram negative bacteria appear a pale reddish color when observed under a light microscope following Gram staining. This is because the structure of their cell wall is unable to ...

Credit: Katryna Kon/Shutterstock.com. Gram staining was developed in the 1800s as a quick and simple procedure to distinguish between Gram positive and Gram negative bacteria.

Gram negative bacteria. Gram negative bacteria appear a pale reddish color when observed under a light microscope following Gram staining. This is because the structure of their cell wall is unable to ...

More information: Çağatay Işıl et al, Virtual Gram staining of label-free bacteria using dark-field microscopy and deep learning, Science Advances (2025). DOI: 10.1126/sciadv.ads2757.