A group of rod-shaped bacteria that are positive to Gram’s method and form endospores
Gram-positive endospore-forming rods belong to a group of gram-positive bacteria. These bacteria are those whose cells appear violet rods under the microscope through Gram’s method in contrast to those bacteria that appear as spherical (gram-positive cocci) and to those that appear as pink cells (gram-negative bacteria). Gram-positive bacteria appear violet or blue under the microscope because the crystal violet dye fixed with the iodine mordant is retained in their outer covering. Gram-positive bacteria do not appear pink indicating that they did not take the safranin counterstain as gram-negative bacteria would. The main reason for this is the thick cell wall that gram-positive bacteria have. Their cell wall has a thick layer of peptidoglycan chains that cross-linked by the enzyme DD-transpeptidase. The rigid cell wall becomes dehydrated when treated with a decolorizing agent during gram staining. This prevents the efflux of initial stain. Thus, gram-positive bacteria remain violet even when a counterstain is introduced. Gram-positive bacteria may be cocci or bacilli depending on the cell shape. Gram-positive bacilli would appear violet rods after gram staining. Gram-positive bacilli may be endospore-forming or not. Endospores are structures indicating a dormant stage in certain bacteria. Gram-positive endospore-forming bacilli include Bacillus spp. and Clostridium spp.
1 Madigan M; Martinko J (editors). (2005). Brock Biology of Microorganisms (11th ed.). Prentice Hall.