Bacterial Classification: Shape and Cell Walls

• Bacteria are relatively simple, single-celled (unicellular), microscopic living organisms ranging from 0.5-2.0 micron in diameter, visible under a light microscope with 100x oil immersion lenses
• Bacterial cells are called prokaryotes because their genetic material is not enclosed in a special nuclear membrane
• Bacteria can be found everywhere
• Bacteria can be beneficial, as in fermentation and decomposition, or pathogenic, causing disease by entering a living body and spreading through water, air, soil, or physical contact

Classification of Bacteria Based on Shape:

• Bacillus: Rod-shaped; e.g., Escherichia coli (E. coli)
• Spirilla/Spirochete: Spiral-shaped; e.g., Helicobacter pylori
• Coccus: Sphere-shaped; e.g., Streptococcus pneumoniae
• Vibrio: Comma-shaped; e.g., Vibrio cholerae

Classification of Bacteria Based on Cell Wall Composition:

• Peptidoglycan Cell Wall: Gram-positive bacteria
• Lipopolysaccharide Cell Wall: Gram-negative bacteria

Classification of Bacteria Based on Mode of Nutrition:

• Autotrophic Bacteria: Cyanobacteria
• Heterotrophic Bacteria: Disease-causing bacteria

Classification of Bacteria Based on Mode of Respiration:

• Aerobic Bacteria: Bacillus, Pseudomonas
• Anaerobic Bacteria: Clostridium
• Facultative Anaerobes: Escherichia coli
• Microaerophiles: Helicobacter pylori

Bacterial Nomenclature

• Established in 1735 by Carolus Linnaeus
• Scientific names are latinized from Latin, traditionally used by scholars
• Each organism has two names, the genus and the specific epithet
• Genus is always capitalized
• Specific epithet (species name) follows and is not capitalized
• Both genus and specific epithet are underlined or italicized
• After the first mention, the genus name can be abbreviated to its initial
• Names describe the organism, honor a researcher, or identify the habitat of a species
• Staphylococcus aureus is commonly found on human skin, where "Staphylo-" describes clustered arrangement, "-coccus" indicates spheres, and "aureus" is Latin for golden (the color of many colonies)
• Escherichia coli is named after Theodor Escherich; the epithet "coli" points to its presence in the colon or large intestine

Structure and Functions of Bacterial Cell Envelope:

• The cell envelope provides a structural and physiological barrier between the protoplasm and the external environment
• Protects against osmotic lysis, and provides shape and rigidity
• The cell envelope consists of two components: a cell wall and plasma membrane

Cell Wall:

• Prokaryotic cells have a rigid, chemically complex cell wall located between the cell membrane and capsule/slime layer
• Peptidoglycan is the main component and provides shape and strength
• Peptidoglycan is a disaccharide made of N-acetyl glucosamine and N-acetyl muramic acid, joined by short peptide chains

Gram-Positive Cell Wall:

• Thick (15–80 nm) and homogenous
• Contains a large amount of peptidoglycan (40–80% of the dry weight of the cell wall) in several layers
• Consists primarily of teichoic and teichuronic acids, accounting for up to 50% of the dry weight of the wall and 10% of the dry weight of the total cell

Gram-Negative Cell Wall:

• More complex
• Less peptidoglycan compared to Gram-positive
• 1–2 peptidoglycan layers (2–8 nm) are present just outside the cell membrane
• Contains three main components: Lipoprotein layer, Outer membrane, Lipopolysaccharides

Periplasmic Space

• It is a distinct space between the cell membrane and outer membrane in Gram-negative bacteria
• Contains a loose layer of peptidoglycan matrix
• Contains proteins for nutrient acquisition, hydrolytic enzymes, beta lactamases, binding proteins, and peptidoglycan synthesis enzymes
• Contains membrane-derived oligosaccharides for osmoregulation
• Less distinct in Gram-positive cell walls

Capsule and Slime Layer:

• Gel-like layer outside the envelope in many bacteria, both Gram-positive and Gram-negative
• Capsule: well-defined condensed layer around the bacterial envelope, visible by light microscopy
• Microcapsule: narrower layer, detectable by serological or electron microscopy
• Slime/Glycocalyx: amorphous viscid colloidal material secreted extracellularly

Capsule:

• Mostly polysaccharides (exopolysaccharides)
• Bacillus anthracis contains polyamino acids like D-glutamic acid

Slime (S) Layer:

• Structured paracrystalline protein layer, usually a single protein with carbohydrates
• Resistant to proteolytic enzymes and protein-denaturing agents
• Protects against wall-degrading enzymes and bacteriophages
• Maintains cell shape and can be involved in cell adhesion

Surface Appendages:

• Flagella and fimbriae/pili are surface appendages of bacteria

Flagella:

• Thread-like appendages embedded in the cell envelope
• Responsible for motility
• Arrangements vary

Types of Flagella Arrangements:

• Monotrichous: Single polar flagellum (e.g., Vibrio cholerae)
• Lophotrichous: Multiple polar flagella (e.g., Spirilla)
• Peritrichous: Flagella distributed over the entire cell (e.g., Salmonella Typhi, E. coli)
• Amphitrichous: Single flagellum at both ends

Functions of Flagella:

• Primarily responsible for motility by chemotaxis
• Role in bacterial survival and pathogenesis

Pili (Fimbriae):

• Hair-like filaments extending from the cell surface, mostly in Gram-negative bacteria
• Composed of pilin subunits
• Adhesins located at the tips are responsible for attachment
• Pili are shorter and straighter than flagella
• Consist of helices of protein named pilins
• Sex pili are specialized for attachment in bacterial conjugation

Sporulation:

• Differentiation process forming endospores
• Resting phase for survival in adverse conditions (e.g., starvation) for Bacillus and Clostridium species
• Process begins in nutrition-deprived conditions
• Each cell forms a single internal spore, forms single vegetative cell

Properties of Spores:

• Resistant to boiling, disinfectants, and heating
• Destroyed by autoclaving at 121°C for 15 minutes
• Germination: conversion of spore to vegetative cell in suitable conditions through activation, initiation, and outgrowth

Normal Human Microbiota:

• Microorganisms on skin and mucous membranes of healthy individuals
• Provides a first line of defense against microbial pathogens
• Assists in digestion, toxin degradation, and contributes to immune system maturation

Bacterial Diseases in Humans:

• Pulmonary Tuberculosis: Mycobacterium tuberculosis
• Diphtheria: Corynebacterium diphtheriae
• Cholera: Vibrio cholerae
• Leprosy: Mycobacterium leprae
• Pertussis: Bordetella pertussis
• Tetanus: Clostridium tetani
• Plague: Yersinia pestis
• Gonorrhoea: Neisseria gonorrhoeae
• Salmonellosis: Salmonella enteritis

Explore the classification of bacteria based on shape, including bacillus, spirilla, coccus, and vibrio forms. Learn about cell wall composition, differentiating between peptidoglycan (Gram-positive) and lipopolysaccharide (Gram-negative) structures. Understand how these classifications aid in bacterial identification and study.