Gram stain: Difference between revisions
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The '''Gram stain''' is a laboratory technique in [[microbiology]], which causes bacteria with certain biologically and medically important characteristics to be colored violet or red when viewed under a [[microscope]]. '''Gram-positive''' bacteria are those that are stained dark blue or violet by the process. Organisms that are Gram-positive have cell walls containing multiple layers of [[peptidoglycan]] bound together by [[amino acid]] bridges. | The '''Gram stain''' (also known as '''Gram's stain''' or '''Gram's method''') is a laboratory technique in [[microbiology]], which causes bacteria with certain biologically and medically important characteristics to be colored violet or red when viewed under a [[microscope]]. '''Gram-positive''' bacteria are those that are stained dark blue or violet by the process. Organisms that are Gram-positive have cell walls containing multiple layers of [[peptidoglycan]] bound together by [[amino acid]] bridges. | ||
'''Gram-negative''' organisms do not take up the peptidoglycan-selective coloring, and are stained red by the default second dye. | '''Gram-negative''' organisms do not take up the peptidoglycan-selective coloring, and are stained red by the default second dye. | ||
Not all bacteria reliably take either stain. ''[[Mycobacteria]]'', for example, tend to need the [[Ziehl-Neelsen]] stain, which uses heat and acid to drive a stain into dye-resistant cell walls. | Not all bacteria reliably take either stain. ''[[Mycobacteria]]'', for example, tend to need the [[Ziehl-Neelsen]] stain, which uses heat and acid to drive a stain into dye-resistant cell walls. | ||
==Technique== | ==Technique== | ||
#Prepare a heat-fixed smear of the material containing the bacteria of interest | #Prepare a heat-fixed smear of the material containing the bacteria of interest | ||
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#Decolorize with an alcohol or acetone-alcohol | #Decolorize with an alcohol or acetone-alcohol | ||
#Counterstain with [[safranin]], a red dye | #Counterstain with [[safranin]], a red dye | ||
==Clinical correlations== | ==Clinical correlations== | ||
Broad-spectrum antibiotics affect both Gram-positive and [[Gram-negative bacteria]]. No antibiotic attacks all Gram-positive, all Gram-negative, or any other total population of bacteria. Gram staining is a guide to antibiotic selection, but [[bacterial culture]] and [[antibiotic sensitivity testing]] as well as clinical experience — especially local to a given area — remain the gold standard of selecting antibiotics for treatment. | Broad-spectrum antibiotics affect both Gram-positive and [[Gram-negative bacteria]]. No antibiotic attacks all Gram-positive, all Gram-negative, or any other total population of bacteria. Gram staining is a guide to antibiotic selection, but [[bacterial culture]] and [[antibiotic sensitivity testing]] as well as clinical experience — especially local to a given area — remain the gold standard of selecting antibiotics for treatment. | ||
===Gram-positive=== | ===Gram-positive=== | ||
The first clinically useful antibiotic, [[penicillin]], of the [[beta-lactam]] class, tended to attack only [[Gram-positive bacteria]]. | The first clinically useful antibiotic, [[penicillin]], of the [[beta-lactam]] class, tended to attack only [[Gram-positive bacteria]]. | ||
===Gram-negative=== | ===Gram-negative=== | ||
Other classes, such as [[streptomycin]], a member of the [[aminoglycoside]] class, tended to attack Gram-negative bacteria. | Other classes, such as [[streptomycin]], a member of the [[aminoglycoside]] class, tended to attack Gram-negative bacteria. | ||
===Broad-spectrum=== | ===Broad-spectrum=== | ||
[[Broad-spectrum]] antibiotics, the earliest representatives of which were [[tetracycline]]s, attack both Gram-positive and Gram-negative bacteria. | [[Broad-spectrum]] antibiotics, the earliest representatives of which were [[tetracycline]]s, attack both Gram-positive and Gram-negative bacteria. | ||
Revision as of 18:26, 17 February 2010
The Gram stain (also known as Gram's stain or Gram's method) is a laboratory technique in microbiology, which causes bacteria with certain biologically and medically important characteristics to be colored violet or red when viewed under a microscope. Gram-positive bacteria are those that are stained dark blue or violet by the process. Organisms that are Gram-positive have cell walls containing multiple layers of peptidoglycan bound together by amino acid bridges.
Gram-negative organisms do not take up the peptidoglycan-selective coloring, and are stained red by the default second dye.
Not all bacteria reliably take either stain. Mycobacteria, for example, tend to need the Ziehl-Neelsen stain, which uses heat and acid to drive a stain into dye-resistant cell walls.
Technique
- Prepare a heat-fixed smear of the material containing the bacteria of interest
- Stain with crystal violet
- Treat with iodine solution, usually Lugol's solution. Its mordant solution fixes the crystal violet in Gram-positive cell walls.
- Decolorize with an alcohol or acetone-alcohol
- Counterstain with safranin, a red dye
Clinical correlations
Broad-spectrum antibiotics affect both Gram-positive and Gram-negative bacteria. No antibiotic attacks all Gram-positive, all Gram-negative, or any other total population of bacteria. Gram staining is a guide to antibiotic selection, but bacterial culture and antibiotic sensitivity testing as well as clinical experience — especially local to a given area — remain the gold standard of selecting antibiotics for treatment.
Gram-positive
The first clinically useful antibiotic, penicillin, of the beta-lactam class, tended to attack only Gram-positive bacteria.
Gram-negative
Other classes, such as streptomycin, a member of the aminoglycoside class, tended to attack Gram-negative bacteria.
Broad-spectrum
Broad-spectrum antibiotics, the earliest representatives of which were tetracyclines, attack both Gram-positive and Gram-negative bacteria.