Gram-positive bacteria
Say it: GRAM POZ-ih-tiv bak-TEER-ee-uh
Gram-positive bacteria have a thick peptidoglycan cell wall and no outer membrane, so they keep the purple crystal violet dye in a Gram stain.
Also: Gram-positive bacterium, gram-positive, Gram+ bacteria
Definition
What are Gram-positive bacteria?
Gram-positive bacteria are bacteria that stay purple after Gram staining. The color comes from their cell wall: a thick layer of peptidoglycan, built from many stacked sheets, sits outside the plasma membrane with no outer membrane beyond it. During staining, the complex of crystal violet and iodine becomes trapped in this dense mesh and is not washed out by alcohol. Teichoic acids threaded through the wall help hold it together and help some pathogens stick to host tissues.
The group includes many familiar genera. Staphylococcus grows in grape-like clusters and Streptococcus in chains, and both include common human pathogens. Bacillus and Clostridium can form tough endospores, and species in these genera cause anthrax, tetanus and botulism. Lactobacillus ferments sugars into lactic acid and is used in foods and probiotics, while soil-dwelling Streptomyces produces many of the antibiotics used in medicine. Mycobacterium, the cause of tuberculosis, has a related wall coated in waxy mycolic acids, so it is identified with an acid-fast stain instead.
Because their thick wall is exposed, Gram-positive bacteria are often more sensitive than Gram-negative ones to drugs that block wall building, such as penicillin and vancomycin.
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Terms that go with it
- Gram stainThe Gram stain is a lab staining method that sorts bacteria into Gram-positive (purple) and Gram-negative (pink) groups based on their cell walls.
- Gram-negative bacteriaGram-negative bacteria have a thin peptidoglycan wall wrapped in an outer membrane, so they lose the purple dye in a Gram stain and look pink.
- PeptidoglycanPeptidoglycan is the mesh-like polymer of sugars and short peptides that forms the bacterial cell wall and gives bacteria their shape and strength.
- EndosporeAn endospore is a tough, dormant structure formed inside some bacteria, such as Bacillus and Clostridium, that survives heat, drying and chemicals.
- BacteriumA bacterium is a single-celled prokaryote with no nucleus, found in almost every habitat on Earth, including in and on the human body.
- AntibioticAn antibiotic is a drug that kills bacteria or stops them from growing, used to treat or prevent bacterial infections; it has no effect on viruses.
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Written for biofideX in plain words, after checking these references:
- OpenStax Microbiology, 3.3 Unique Characteristics of Prokaryotic Cells
- OpenStax Microbiology, 4.4 Gram-Positive Bacteria
- MedlinePlus Lab Tests: Gram Stain
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