Gram-positive bacilli


noun, singular: gram-positive bacillus

A group of rod-shaped bacterial cells that appears violet through Gram's method


Gram's method has become an essential laboratory diagnostic tool for the rapid identification of a bacterial species. Through this method, the observer can classify if the bacterial species is a gram-positive or not. Bacterial cells that stain violet or bluish in color under the microscope are described as gram-positive. This indicates that they resist the red dye counterstain. Those that appear pinkish under the microscope are described as gram-negative, indicating that they have taken the counterstain.

Gram-positive bacteria are not able to take the color of the counterstain because of their thick peptidoglycan layer. This layer of peptidoglycan chains forms a thick and rigid cell wall. This cell wall becomes dehydrated when treated with a decolorizing agent in Gram's method. As a result, the primary dye cannot readily diffuse and is retained in the cell. Thus, they appear violet or bluish under the microscope. Despite the thick cell walls of gram-positive bacteria, they are generally more susceptible to antibiotic treatments than gram-negative bacteria.1

Based on gram staining results and cell morphology as observed under the microscope, gram-positive bacteria may be gram-positive cocci or gram-positive bacilli (rods). Gram-positive bacilli appear rods in contrast to cocci that are spherical in shape. Gram-positive bacilli include the Bacillus spp., Clostridium spp., Listeria spp. and Corynebacterium spp.

See also:

1 Madigan M; Martinko J (editors). (2005). Brock Biology of Microorganisms (11th ed.). Prentice Hall.

Please contribute to this project, if you have more information about this term feel free to edit this page

This page was last modified on 30 November 2014, at 00:24. This page has been accessed 421 times. 
What links here | Related changes | Permanent link