Introduction The Gram stain was developed by Christian Gram in 1800, a Danish bacteriologist. (Smith and Hussey, 2005) It was the first differential staining technique and the most commonly used in microbiology. Furthermore, the bacteria are transparent and cannot be seen under a microscope. For this reason, the Gram stain is an important tool for distinguishing between two main types of bacteria Gram-positive and Gram-negative. Gram stain differentiates Gram positive and Gram negative cells based on the structure of their cell wall. (Menard, et al.,20150) Most bacteria are Gram positive or Gram negative, but there are some Gram variable bacteria and very small bacteria without a cell wall that do not have a Gram reaction. For the purposes of this lab we will focus on the two main types of bacteria Gram+ and Gram-. Bacteria are prokaryotes with a cell wall; they are classified as Bacillus (rod-shaped), Coccus (spherical), and Spirillum (spirals) (Menard, et al., 2015). Therefore, in this lab we will look at how. After using Gram staining techniques, the gram positive bacteria turned purple and the gram negative bacteria pink, showing the difference in the characteristics of each bacterium. Therefore, observing the morphological characteristics of the bacteria, under a bright field microscope, Citrobacter freundii is a gram positive bacterium and Staphylococcus aureus is a gram negative bacterium. We also learned what can go wrong when using the Gram stain that could produce errors in testing. This is a testament to the different results that occur when too much decolorizer is added or when the slide is heat fixed incorrectly. Learning Gram stain techniques helped us understand how to identify bacteria for proper treatment and future bacterial studies