Most of these structures are also observed in archaea
Bacteria can function as independent single-celled organisms or in groups like colonies or biofilms
Bacteria have an average size of 1 μm
Pleomorphism refers to variations in cell wall structure due to genetic or nutritional differences
Bacterial shapes and arrangements:
Cocci: spherical or ball-shaped
Rods: cylindrical
Vibrio: gently curved rods
Spirillum: slightly curled or spiral-shaped
Spirochete: spiral cell with periplasmic flagella
Branching filaments: some bacteria produce multiple branches off a basic rod structure
Arrangement of cocci:
Single
Diplococci: pairs
Tetrads: groups of four
Staphylococci or micrococci: irregular clusters
Streptococci: chains
Sarcina: cubical packets of eight, sixteen, or more cells
Arrangement of bacilli:
Single
Diplobacilli: pair of cells with ends attached
Streptobacilli: chain of several cells
Palisades: cells of a chain remain partially attached by a small hinge region at the ends
External structures of bacteria include:
Flagella and axial filaments for motility
Fimbriae and pili for attachment points or channels
Fine points of flagellar function:
Chemotaxis: movement in response to chemical signals
Types of flagellar arrangements: polar, monotrichous, lophotrichous, amphitrichous, peritrichous
Run: rotation counterclockwise for linear direction
Tumble: reversal of flagellum direction causing the cell to change course
Periplasmic flagella are found in spirochetes and are internal flagella enclosed between the cell wall and cytoplasmic membrane
Fimbriae are small fibers sprouting off bacterial cells for tight adhesion and colonization
Pili are used in conjugation between bacterial cells and can transfer genetic material
Glycocalyx:
Coating of repeating polysaccharide or glycoprotein units
Slime layer protects against loss of water and nutrients
Capsule is denser and thicker, producing a sticky character to colonies on agar
Specialized functions of the glycocalyx:
Capsules formed by pathogenic bacteria have greater disease-causing abilities and protect against phagocytes
Biofilms protect bacteria from dislodgement and aid in persistent colonization of medical devices
The cell envelope of bacteria includes the S layer and glycocalyx
Biofilm formation involves the development of plaque-like structures that protect bacteria from environmental factors
The Cell Envelope lies outside the cytoplasm and is composed of two or three basic layers that act as a single protective unit:
Cellwall
Cytoplasmicmembrane
Outer membrane (in some bacteria)
The Cell Wall helps determine the shape of a bacterium and provides strong structural support to prevent bursting or collapsing due to changes in osmotic pressure
Certain drugs target the cell wall, disrupting its integrity and causing cell lysis (disintegration or rupture) of the cell
The cell wall gains its relative rigidity from peptidoglycan
Peptidoglycan is a compound composed of a repeating framework of long glycan (sugar) chains cross-linked by short peptide (protein) fragments, providing a strong but flexible support framework
Gram-Positive Cell Wall is a thick, homogenous sheet of peptidoglycan, 20 to 80 nm in thickness, and contains teichoic acid and lipoteichoic acid that function in cell wall maintenance and enlargement
Gram-Negative Cell Wall is a single, thin sheet of peptidoglycan, 1 to 3 nm in thickness, making gram-negative cells more susceptible to lysis
Nontypical Cell Walls: Acid-Fast Bacteria like Mycobacterium and Norcardia contain peptidoglycan and stain gram-positive, but the bulk of the cell wall is composed of unique lipids like Mycolic acid, which contributes to the pathogenicity of the bacteria
Nontypical Cell Walls: Archaea exhibit unusual and chemically distinct cell walls, some entirely composed of polysaccharides or pure protein, lacking true peptidoglycan structure or a cell wall entirely
Mycoplasmas naturally lack a cell wall, with sterols in the cell membrane stabilizing the cell against lysis, such as Mycoplasma pneumoniae causing "walking pneumonia"
The Gram-Negative Outer Membrane is similar in composition to most membranes but contains specialized polysaccharides and proteins like Lipopolysaccharide, serving as signaling molecules and receptors, including Endotoxin
Cytoplasmic Membrane is a lipid bilayer with proteins embedded, serving as a site for energy reactions, nutrient processing, synthesis, and regulating transport of nutrients and wastes, being selectively permeable
Differences in Cell Envelope Structure show that the outer membrane of gram-negative bacteria contributes an extra barrier, making them more resistant to certain antimicrobial chemicals compared to gram-positive bacteria
The Cytoplasm is 70 to 80% water and a complex mixture of sugars, amino acids, and salts
Bacterial Chromosomes and Plasmids exist in the bacterial chromosome, with DNA aggregated in the nucleoid, and plasmids being nonessential pieces of DNA conferring protective traits like drug resistance
Ribosomes are the site of protein synthesis, composed of rRNA and protein, with a large and small subunit together forming the bacterial ribosome
The Cytoskeleton in some bacteria produces long polymers of protein similar to eukaryotic cells, contributing to cell shape and being a potential target for antibiotic development
Endospores are dormant bodies produced by Bacillus, Clostridium, and Sporosarcina, resisting extremes of heat, drying, freezing, radiation, and chemicals that would kill vegetative cells
The Medical Significance of Bacterial Endospores includes pathogens like Bacillus anthracis, Clostridium tetani, Clostridium perfringens, Clostridium botulinum, and Clostridioides difficile causing serious diseases
Archaea are a third cell type more closely related to domain Eukarya than bacteria, exhibiting unique characteristics like extremophiles living in extreme conditions