Gram-negative bacteria are those that do not retain crystal violet dye in the Gram staining protocol. Gram positive bacteria will retain the dark blue dye after an alcohol wash, but Gram negative do not. In testing for a gram stain, a counterstain is added after the crystal violet, which colors all gram-negative bacteria a red or pink color. The test itself is useful in classifying two distinctly different types of bacteria based on structural differences.
Many species of Gram-negative bacteria are pathogenic, meaning they can cause disease in a host organism. This pathogenic capability is usually associated with certain components of Gram-negative cell walls, particularly the lipopolysaccharide (endotoxin) layer.
Characteristics of Gram-Negative Bacteria
- Only contain a few layers of peptidoglycan -- the building block for strong, rigid cell walls
- Contain an outer membrane, external to the peptidoglycan, called the lipopolysaccharide
- The space between the layers of peptidoglycan and the secondary cell membrane is called periplasmatic space
- The S-layer is directly attached to the outer membrane, rather than the peptidoglycan
- Any flagella, if present, have 4 supporting rings instead of two
- No teichoic acids are present
Example Species
The proteobacteria are a major group of Gram-negative bacteria, including for instance Escherichia coli, Salmonella, and other Enterobacteriaceae, Pseudomonas, Moraxella, Helicobacter, Stenotrophomonas, Bdellovibrio, acetic acid bacteria, Legionella and a great many others. Other notable groups of Gram-negative bacteria include the cyanobacteria, spirochaetes, green sulfur and green non-sulfur bacteria.
Medically relevant Gram-negative cocci include 3 organisms, which cause a sexually transmitted disease (Neisseria gonorrhea), a meningitis (Neisseria meningitidis), and respiratory symptoms (Moraxella catarrhalis).
Medically relevant Gram-negative bacilli include a multitude of species. Some of them cause primarily respiratory problems (Hemophilus influenzae, Klebsiella pneumoniae, Legionella pneumophila, Pseudomonas aeruginosa), primarily urinary problems (Escherichia coli, Proteus mirabilis, Enterobacter cloacae, Serratia marcescens), and primarily gastrointestinal problems (Helicobacter pylori, Salmonella enteritidis, Salmonella typhi).
The Outer Membrane
One of the several unique characteristics of gram-negative bacteria is the outer membrane. The outer membrane is responsible for protecting the bacteria from several antibiotics, dyes, and detergents that would normally damage the inner membrane or cell wall (peptidoglycan). Because of the outer membrane, these bacteria are resistant to lysozyme and penicillin. Fortunately, alternative treatments like lysozyme with EDTA and ampicillin have been developed to combat the protective outer membrane.
References
- Baron, Samuel (1996). Medical Microbiology, 4th ed., The University of Texas Medical Branch at Galveston. ISBN 0-9631172-1-1.
- Madigan, Michael; Martinko, John (editors) (2005). Brock Biology of Microorganisms, 11th ed., Prentice Hall. ISBN 0131443291.
- This article contains material from the Science Primer published by the NCBI, which, as a US government publication, is in the public domain at http://www.ncbi.nlm.nih.gov/About/disclaimer.html.
The content of this page is retrieved from http://en.wikipedia.org/wiki/Gram-negative_bacteria under GFDL
