Gram staining (or Gram's method) is a method of differentiating bacterial species into two large groups (Gram-positive and Gram-negative).
It is based on the chemical and physical properties of their cell walls. Primarily, it detects peptidoglycan, which is present in a thick layer in Gram positive bacteria.[1] A Gram positive results in a purple/blue color while a Gram negative results in a pink/red color.
The Gram stain is almost always the first step in the identification of a bacterial organism, and is the default stain performed by laboratories over a sample when no specific culture is referred.
While Gram staining is a valuable diagnostic tool in both clinical and research settings, not all bacteria can be definitively classified by this technique, thus forming Gram-variable and Gram-indeterminate groups as well.
The word Gram is always spelled with a capital, referring to Hans Christian Gram, the inventor of Gram staining.
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The method is named after its inventor, the Danish scientist Hans Christian Gram (1853–1938), who developed the technique while working with Carl Friedländer in the morgue of the city hospital in Berlin. Gram devised his technique not for the purpose of distinguishing one group of bacteria from another but to enable bacteria to be seen more readily in stained sections of lung tissue.[2] He published his method in 1884, and included in his short report the observation that the Typhus bacillus did not retain the stain.[3]
Gram staining is a bacteriological laboratory technique[4] used to differentiate bacterial species into two large groups (Gram-positive and Gram-negative) based on the physical properties of their cell walls.[5] Gram staining is not used to classify archaea, formally archaeabacteria, since these microorganisms yield widely varying responses that do not follow their phylogenetic groups.[6]
The Gram stain is not an infallible tool for diagnosis, identification, or phylogeny, and it is of extremely limited use in environmental microbiology. It has been largely superseded by molecular techniques even in the medical microbiology lab. Some organisms are Gram-variable (that means, they may stain either negative or positive); some organisms are not susceptible to either stain used by the Gram technique. In a modern environmental or molecular microbiology lab, most identification is done using genetic sequences and other molecular techniques, which are far more specific and information-rich than differential staining.
Gram stains are performed on body fluid or biopsy when infection is suspected. Gram stains yield results much more quickly than culture, and is especially important when infection would make an important difference in the patient's treatment and prognosis; examples are cerebrospinal fluid for meningitis and synovial fluid for septic arthritis.[4][7] .
Gram-positive bacteria have a thick mesh-like cell wall made of peptidoglycan (50-90% of cell wall), which are stained purple by crystal violet, whereas Gram-negative bacteria have a thinner layer (10% of cell wall), which are stained pink by the counter-stain. There are four basic steps of the Gram stain:
Crystal violet (CV) dissociates in aqueous solutions into CV+ and chloride (Cl−) ions. These ions penetrate through the cell wall and cell membrane of both Gram-positive and Gram-negative cells. The CV+ ion interacts with negatively charged components of bacterial cells and stains the cells purple.
Iodine (I− or I−
3) interacts with CV+ and forms large complexes of crystal violet and iodine (CV–I) within the inner and outer layers of the cell. Iodine is often referred to as a mordant, but is a trapping agent that prevents the removal of the CV–I complex and, therefore, color the cell.[10]
When a decolorizer such as alcohol or acetone is added, it interacts with the lipids of the cell membrane. A Gram-negative cell will lose its outer lipopolysaccharide membrane, and the inner peptidoglycan layer is left exposed. The CV–I complexes are washed from the Gram-negative cell along with the outer membrane. In contrast, a Gram-positive cell becomes dehydrated from an ethanol treatment. The large CV–I complexes become trapped within the Gram-positive cell due to the multilayered nature of its peptidoglycan. The decolorization step is critical and must be timed correctly; the crystal violet stain will be removed from both Gram-positive and negative cells if the decolorizing agent is left on too long (a matter of seconds).
After decolorization, the Gram-positive cell remains purple and the Gram-negative cell loses its purple color. Counterstain, which is usually positively charged safranin or basic fuchsin, is applied last to give decolorized Gram-negative bacteria a pink or red color.[11][12]
Some bacteria, after staining with the Gram stain, yield a Gram-variable pattern: a mix of pink and purple cells are seen. The genera Actinomyces, Arthobacter, Corynebacterium, Mycobacterium, and Propionibacterium have cell walls particularly sensitive to breakage during cell division, resulting in Gram-negative staining of these Gram-positive cells. In cultures of Bacillus, Butyrivibrio, and Clostridium, a decrease in peptidoglycan thickness during growth coincides with an increase in the number of cells that stain Gram-negative.[13] In addition, in all bacteria stained using the Gram stain, the age of the culture may influence the results of the stain.
Gram-positive bacteria have generally a single membrane (monoderm) surrounded by a thick peptidoglycan. This rule is followed by two phyla — the Firmicutes (except for the classes Mollicutes and Negativicutes) and the Actinobacteria.[5][14] In contrast, members of the Chloroflexi (green non-sulfur bacteria) are monoderms but possess a thin or absent (class Dehalococcoidetes) peptidoglycan and can stain negative, positive or indeterminate.[5][14] Members of the Deinococcus-Thermus group, stain positive but are diderms with a thick peptidoglycan.[5][14]
Historically, the Gram-positive forms 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 so cannot be stained by Gram, but are derived from such forms.
Gram-negative bacteria generally possess a thin layer of peptidoglycan between two membranes (diderms). Most bacterial phyla are Gram-negative, including the cyanobacteria, spirochaetes and green sulfur, and most Proteobacteria (exceptions being some members of the Rickettsiales and the insect-endosymbionts of the Enterobacteriales).[5][14]
Gram-indeterminate bacteria do not respond to Gram staining and, therefore, cannot be determined as either Gram-positive or Gram-negative. Examples of them, but not limited to, are Gram-variable and acid fast bacteria.
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