Gram-positive bacteria- cell wall, examples, diseases, antibiotics
Bacteria are microscopic organisms that can cause various infections and diseases in humans and animals. However, not all bacteria are the same. There are many different types of bacteria that have different characteristics, shapes, and structures. One way to classify bacteria is based on their reaction to a laboratory technique called Gram staining.
Gram staining is a method that uses four chemicals: crystal violet, iodine, alcohol, and safranin, to stain bacteria and differentiate them into two major groups: Gram-positive and Gram-negative. The name of this technique comes from the Danish bacteriologist Hans Christian Gram, who first developed it in 1884.
Gram staining works by exploiting the differences in the cell wall composition of bacteria. The cell wall is a protective layer that surrounds the bacterial cell membrane and gives shape and rigidity to the cell. The cell wall of bacteria is mainly composed of a substance called peptidoglycan, which is a polymer of sugars and amino acids. Peptidoglycan forms a mesh-like structure that can trap or exclude certain molecules.
Gram-positive bacteria have a thick layer of peptidoglycan (about 20-80 nm) in their cell wall, which makes up about 90% of the cell wall mass. They also have molecules called teichoic acids and lipoteichoic acids that are attached to the peptidoglycan layer and extend to the cell surface. These molecules are negatively charged and play a role in bacterial adherence and immunity.
Gram-negative bacteria have a thin layer of peptidoglycan (about 2-7 nm) in their cell wall, which makes up only about 10% of the cell wall mass. They also have an outer membrane that covers the peptidoglycan layer and contains lipids, proteins, and lipopolysaccharides (LPS). LPS are complex molecules that consist of a lipid part (lipid A) and a sugar part (O antigen). LPS are also negatively charged and act as endotoxins that can trigger inflammation and shock.
Gram-positive bacteria can be classified into different shapes based on their morphology under the microscope. The most common shapes are cocci and bacilli, but some Gram-positive bacteria can also form special arrangements such as tetrads and sarcinae.
- Cocci are spherical or oval-shaped bacteria that measure 0.5-1.0 um in diameter. They can occur as single cells, pairs, chains, or clusters.
- Bacilli are rod-shaped bacteria that have round, tapered, square, or swollen ends. They measure 1-10 um in length and 0.3-1.0 um in width. They can occur as single cells, pairs, chains, or palisades (side-by-side arrangement).
- Some Gram-positive bacteria can form tetrads or sarcinae, which are special arrangements of cocci in square or cubic patterns.
The cell wall of Gram-positive bacteria is a thick and rigid structure that surrounds the cytoplasmic membrane and protects the cell from osmotic lysis. It also serves as a site for attachment of various molecules and as a target for antibiotics. The main components of the Gram-positive bacteria cell wall are:
- Peptidoglycan: This is the most abundant and essential component of the cell wall. It is a polymer of alternating sugar units (N-acetylglucosamine and N-acetylmuramic acid) linked by β-1,4 glycosidic bonds. The sugar units are attached to short peptide chains that cross-link with each other to form a mesh-like network. Peptidoglycan provides mechanical strength and shape to the cell wall and also acts as a barrier to large molecules.
- Teichoic acids: These are polymers of glycerol or ribitol phosphate that are covalently attached to the peptidoglycan or the cytoplasmic membrane. They extend beyond the peptidoglycan layer and give a negative charge to the cell surface. Teichoic acids are involved in regulating cell growth, maintaining cell wall integrity, binding metal ions, and mediating adhesion to host cells.
- Lipoteichoic acids: These are a type of teichoic acids that are anchored to the cytoplasmic membrane by a lipid tail. They span the entire cell wall and may interact with the outer surface proteins. Lipoteichoic acids have similar functions as teichoic acids but also play a role in modulating immune responses and triggering inflammation.
- Surface proteins: These are proteins that are either covalently or non-covalently attached to the peptidoglycan layer or the teichoic acids. They have diverse roles in bacterial physiology and pathogenesis, such as enzyme activity, nutrient uptake, toxin secretion, antigenicity, biofilm formation, and invasion of host cells.
Gram-positive bacteria are a diverse group of microorganisms that can cause various infections in humans and animals. Some of the most common and important examples of Gram-positive bacteria and the diseases they cause are:
- Staphylococcus aureus: This bacterium can cause localized and deep skin infections, respiratory infections, cardiovascular infections, and musculoskeletal infections. It can also produce toxins that cause food poisoning and toxic shock syndrome.
- Streptococcus pyogenes: This bacterium can cause acute pharyngitis, impetigo, erysipelas, puerperal sepsis, and invasive group A streptococcal disease. It can also cause streptococcal toxic shock syndrome, acute rheumatic fever, and acute glomerulonephritis as post-infectious complications.
- Streptococcus pneumoniae: This bacterium can cause community-acquired acute bacterial pneumonia, adult bacterial meningitis, otitis media in children, bacteremia/sepsis, and meningitis.
- Bacillus anthracis: This bacterium can cause anthrax disease in humans who are exposed to contaminated materials. Anthrax disease can manifest as pulmonary, cutaneous, or gastrointestinal forms.
- Corynebacterium diphtheriae: This bacterium can cause diphtheria disease in humans who are not immunized. Diphtheria is a localized infection of the upper respiratory tract that produces a pseudomembrane that can obstruct the airway.
- Clostridium botulinum: This bacterium can produce botulinum toxin that causes botulism poisoning in humans who ingest contaminated food or have infected wounds.
- Clostridium tetani: This bacterium can produce tetanus toxin that causes tetanus disease in humans who have deep puncture wounds or burns that are contaminated with spores.
- Clostridium difficile: This bacterium can cause pseudomembranous colitis in humans who have diarrhea associated with antibiotic use or hospitalization.
- Listeria monocytogenes: This bacterium can cause listeriosis in humans who ingest contaminated food or cross the placenta from infected mothers to fetuses.
Antibiotics are drugs that can kill or inhibit the growth of bacteria. Different types of antibiotics have different mechanisms of action, depending on their chemical structure and target site in the bacterial cell. Some antibiotics are effective against both Gram-positive and Gram-negative bacteria, while others are specific for one group.
Gram-positive bacteria are susceptible to a wide range of antibiotics, such as:
- Beta-lactams: These antibiotics inhibit the synthesis of peptidoglycan, a major component of the bacterial cell wall.
- Sulfonamides: These antibiotics interfere with the synthesis of folic acid, a vitamin that is essential for bacterial growth and replication.
- Aminoglycosides: These antibiotics cause misreading of the mRNA code and production of faulty proteins.
- Tetracyclines: These antibiotics prevent the attachment of transfer RNA (tRNA) to the ribosomes for protein synthesis.
- Chloramphenicol: This antibiotic inhibits the formation of peptide bonds during protein synthesis.
- Macrolides: These antibiotics block the exit tunnel of the growing peptide chain during protein synthesis.
- Glycopeptides: These antibiotics prevent the cross-linking of peptidoglycan chains and weaken the cell wall structure.
- Oxazolidinones: These antibiotics interfere with the initiation of protein synthesis.
- Ansamycins: These antibiotics prevent the transcription of DNA into mRNA.
- Quinolones: These antibiotics inhibit the bacterial DNA gyrase and topoisomerase IV, enzymes involved in DNA replication and transcription.
It is important to identify the type and strain of Gram-positive bacteria causing the infection and choose the appropriate antibiotic based on its susceptibility testing to ensure optimal treatment outcomes.
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