Everything about Gram Positive totally explained
Gram-positive bacteria are those that are stained dark blue or violet by
Gram staining. This is in contrast to
Gram-negative bacteria, which can't retain the crystal violet stain, instead taking up the
counterstain (
safranin) and appearing red or pink. Gram-positive organisms are able to retain the crystal violet stain because of the high amount of
peptidoglycan in the
cell wall. Gram-positive cell walls typically lack the outer membrane found in Gram-negative bacteria.
Characteristics
The following characteristics are generally present in a Gram-positive bacterium:
- Cytoplasmic lipid membrane
- Thick peptidoglycan layer
- Capsule polysaccharides (only in some species)
- Flagellum (only in some species)
- If present, it contains two rings for support as opposed to four in Gram-negative bacteria because Gram-positive bacteria have only one membrane layer.
Classification
In the original bacterial phyla, the Gram-positive organisms made up the
phylum Firmicutes, a name now used for the largest group. It includes many well-known genera such as
Bacillus,
Listeria,
Staphylococcus,
Streptococcus,
Enterococcus, and
Clostridium. It has also been expanded to include the Mollicutes, bacteria like
Mycoplasma that lack cell walls and can't be Gram stained, but are derived from such forms.
Actinobacteria are the other major group of Gram-positive bacteria, which have a high
guanosine and
cytosine content in their genomes (high G+C group). This contrasts with the Firmicutes, which have a low G+C content.
Both Gram-positive and Gram-negative bacteria may have a membrane called an
S-layer. In Gram-negative bacteria, the S-layer is directly attached to the
outer membrane. In Gram-positive bacteria, the S-layer is attached to the peptidoglycan layer. Unique to Gram-positive bacteria is the presence of
teichoic acids in the cell wall. Some particular teichoic acids, lipoteichoic acids, have a lipid component and can assist in anchoring peptidoglycan, as the lipid component is embedded in the membrane.
Exceptions
The
Deinococcus-Thermus bacteria have Gram-positive stains, although they're structurally similar to Gram-negative bacteria.
Pathogenesis
Most pathogenic bacteria in humans are gram-negative organisms. Classically, six gram-positive organisms are typically pathogenic in humans. Two of these,
Streptococcus and
Staphylococcus, are
cocci (round bacteria). The remaining organisms are bacilli (rod-shaped bacteria) and can be subdivided based on their ability to form spores. The non-spore formers are
Corynebacterium and
Listeria, while
Bacillus and
Clostridium produce spores. The spore-forming bacteria can again be divided based on their
respiration:
Bacillus is a facultative anaerobe, while
Clostridium is an obligate anaerobe.
Further Information
Get more info on 'Gram Positive'.
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