Scientists classify bacteria by gram stain. This staining method was developed in 1884 by a Danish bacteriologist named Christian Gram. This method is still used in labs today. Bacteria that are called gram negative have this name because they lose the first stain that is used in the staining process, while gram positive bacteria do not.
Gram Positive Cocci
S. aureus and S. pneumoniae are gram positive because they keep the stain in the gram staining process; they have a round (cocci) shape. S. aureus can cause infections on your skin and in your heart and bones. S. pneumoniae can cause pneumonia. According to Warren Levinson, M.D., PhD, Professor of Microbiology at the University of California, all gram positive bacteria have a thick cell wall which is made out of peptidoglycan. Peptidoglycans are peptides (amino acids) that are attached to sugars (glycans). Gram positive bacteria also have a substance called teichoic acid, which can cause you to go into septic shock. Some can even make a capsule that surrounds them and gives protection against your immune system.
Gram Positive Rods
C. botulinum, C. tetani and C. anthracis are all examples of gram positive rods. C. botulinum causes botulism, C. tetani causes tetanus, and C. anthracis causes anthrax. Also called gram positive bacilli, they keep the first stain, but have a rod shape. They all have a thick cell wall made out of peptidoglycan and all have teichoic acid. Some of the gram positive rods can also make a capsule.
Gram Negative Cocci
N. meningitidis, N. gonorrhoeae and M. catarrhalis are all examples of gram negative cocci. N. meningitidis causes meningitis, N. gonorrhoeae causes gonorrhea and M. catarrhalis causes pneumonia. They are called gram negative because they do not hold onto the first stain. They are called cocci because they have a round shape. All gram negative bacteria have an outer membrane that surrounds them. John Ingraham writes in “Introduction to Microbiology,” that the outer membrane holds the toxin that can make you go into shock. The cell wall of all gram negatives is thin, but is made of peptidoglycan. All gram negatives have a space, called a periplasmic space, where some have enzymes that can destroy penicillin. Some gram negatives can make a capsule.
Gram Negative Rods
E. coli, Salmonella, Shigella and C. jejuni are all gram negative rods. Most urinary tract infections are due to E. coli. Salmonella, Shigella and C. jejuni are gram negative rods that cause diarrhea. They are rod-shaped, do not keep their first stain and are also referred to as gram negative bacilli. They all have an outer membrane, a thin cell wall and a periplasmic space. Like the gram negative cocci, the outer membrane holds a toxin, the periplasmic space can contain enzymes and some can make a capsule.
Bacteria That Do Not Stain Well
There are some bacteria that are considered gram positive or negative based on their structure, but do not stain well with a gram stain. Among this group is T. pallidum. This bacterium causes syphilis and must be viewed using a special (dark-field) microscope. M. tuberculosis causes tuberculosis and has to be viewed using an acid-fast stain, because it has so many lipids in its cell wall that the usual staining dyes do not penetrate it. Dr. Levinson explains in “Medical Microbiology & Immunology,” that R. rickettsii causes rocky mountain spotted fever, and must be seen using a Giemsa stain because this bacterium is so small. L. pneumoniae needs extra staining time; this bacterium causes Legionnaires’ Disease, a pneumonia.
About this Author
Based in North Carolina, Ruth Coleman has written articles and manuals for 25 years. Her writing has appeared in community newspapers, places of employment and comprises work done in medical college, of which she is a recent graduate. Ruth Coleman holds a Bachelor of Science in biology from Salem College, and is the recipient of numerous academic awards.