Bacterial Morphology & Anatomy

Shape and Size of Bacteria

• Bacteria's shape is determined by its rigid cell wall.
• Microscopic appearance is a key identification criterion.
• Bacterial arrangements are important.
• Bacteria range in size from 0.2 to 5 micrometers.

Structural Components of Bacteria

• Nucleoid (DNA)
• Ribosomes
• Cell membrane
• Cell wall

Three Architectural Regions

• Appendages attach to the cell surface via flagella and pili (fimbriae).
• The cell envelope consists of a capsule, cell wall, and plasma membrane.
• The cytoplasmic region contains the cell chromosome (DNA), ribosomes, and inclusions.

The Cell Envelope

• The cell envelope includes layers enclosing the protoplasm.
• The cell cytoplasm is surrounded by the plasma membrane, cell wall, and capsule.

Capsules

• Most prokaryotes have a polysaccharide layer outside the cell wall polymer.
• A true capsule is a discrete polysaccharide layer outside the cell wall.
• A slime layer or biofilm is a less discrete structure embedding cells.
• Glycocalyx is a thin layer of tangled polysaccharide fibers on cells growing in nature.

Functions of Capsule

• Capsules mediate cell adherence to surfaces.
• Capsules protect bacteria from engulfment by phagocytes (protozoa or white blood cells).
• Capsules protect cells from drying effects.
• Overproduced capsular materials like dextrans store carbohydrates for metabolism.

Cell Wall

• The cell wall is present in bacteria except mycoplasmas.
• The cell wall protects and enhances survival, provides shape, and prevents bursting.

Gram-Positive Bacterium

• Has a thick peptidoglycan layer (60-90%).
• Contains teichoic and lipoteichoic acids.

Teichoic Acid

• Teichoic acids are fibers in the gram-positive cell wall's outer layer.
• They are polymers of glycerol or ribitol phosphate.
• Some glycerol teichoic acid polymers penetrate the peptidoglycan layer and link to lipids in the cytoplasmic membrane and are called lipoteichoic acid.
• Others anchor to the muramic acid of the peptidoglycan.
• Teichoic acids can induce septic shock like endotoxin (LPS) in gram-negative bacteria.
• Teichoic acids mediate staphylococci attachment to mucosal cells and are absent in gram-negative bacteria.

Gram-Negative Bacterium

• Has a thin peptidoglycan layer and an outer membrane containing lipopolysaccharide, phospholipids, and proteins.
• The periplasmic space between membranes contains transport, degradative, and cell wall synthesis proteins.
• The outer membrane joins to the cytoplasmic membrane at adhesion points and attaches to the peptidoglycan with lipoprotein links.

Medically Important Bacteria Not Seen in Gram Stain

• Mycobacteria (e.g., M. tuberculosis) have too much lipid, requiring an acid-fast stain.
• Treponema pallidum is too thin, needing dark-field microscopy or fluorescent antibodies.
• Mycoplasma pneumoniae lacks a cell wall and is very small.
• Legionella pneumoniae has poor counterstain uptake, requiring prolonged counterstaining.
• Chlamydiae (e.g., C. trachomatis) are intracellular and very small, visible as inclusion bodies.
• Rickettsiae is also intracellular and very small, requiring Giemsa or tissue stains.

The Plasma Membrane

• The cytoplasmic membrane is the most dynamic structure in prokaryotic cells.
• The main function is a selective barrier, regulating substance passage.
• The plasma membrane defining structure of a cell, separating its components.
• The bacterial membrane allows passage of water and uncharged molecules, but not larger or charged substances (except via specialized transport).
• Bacterial membranes are structurally similar to eukaryotic membranes, with saturated or monounsaturated fatty acids, but typically no sterols.

Functions of Prokaryotic Plasma Membrane

• Osmotic or permeability barrier
• Location of transport systems for solutes (nutrients and ions)
• Energy generation involving respiratory/photosynthetic electron transport, proton motive force, and ATP-synthesizing ATPase
• Membrane lipid synthesis, including lipopolysaccharide in Gram-negative cells
• Murein synthesis
• Assembly and secretion of extracytoplasmic proteins
• DNA replication and segregation with cell division
• Chemotaxis
• Location of specialized enzyme systems

Flagella

• Flagella are filamentous protein structures that aid in swimming movement in motile prkaryotes.
• Prokaryotic flagella are thinner than eukaryotic flagella and lack the "9 + 2" microtubule arrangement.
• A prokaryotic flagellum's diameter is about 20 nanometers.

Fimbriae and Pili

• Fimbriae and pili are short, hair-like protein structures on cell surfaces.
• Fimbriae are shorter, stiffer, and smaller than flagella.
• Fimbriae are unrelated to bacterial movement unless concerning twitching movement on Pseudomonas)
• Fimbriae are very common in Gram-negative bacteria.
• Fimbriae most often adhere bacteria to surfaces, substrates, and other cells or tissues.
• A specialized pilus called the sex pilus, found in E. coli, stabilizes mating bacteria during conjugation.
• Major determinants of bacterial virulence that allow pathogens to attach (colonize) tissues and/or resist attack by phagocytic white blood cells.
• Pathogenic Neisseria gonorrhoeae specifically adheres to human cervical or urethral epithelium.
• Enterotoxigenic strains of E. coli adhere to the mucosal epithelium.
• Streptococcus pyogenes uses M-protein and associated fimbriae for adherence and resistance to phagocytosis

Cytoplasm

• Cytosol: gelatinous mass of protein, amino acids, sugars, nucleotides, salts, vitamins, ions.
• Bacterial structures: ribosomes, inclusion bodies, nucleoid, plasmids.

Ribosomes

• Ribosomes are a universal cell structure.
• Built from RNA and protein with small and large subunits.
• Two subunits combine to form a functional ribosome for protein synthesis.
• Targets for antibiotics like Streptomycin and Tetracycline

Nucleoid

• Area in cytoplasm where aggregated DNA, but no ribosomes, are present.
• Lacks covering or a membrane.
• Chromosome is a closed DNA loop; hereditary information or genes.
• Contains up to 3500 genes in chromosome
• No mitosis nor meiosis

Plasmids

• Smaller, nonessential DNA molecules.
• Smaller than bacterial chromosome and consist of (5-100 genes) which are non-essential for bacterial growth.
• Can be one or more plasmids.
• Plasmids carry the genes for antibiotic resistance, mediated by various enzymes.
• Plasmids carry the genes for resistance to heavy metals such as mercury.
• Plasmids carry the genes for resistance to ultraviolet light, mediated by DNA repair enzymes.
• Plasmids carry the genes for Pili (fimbriae) which mediate the adherence of bacteria to epithelial cells.
• Plasmids carry the genes for exotoxins.

Endospores (Spores)

• Spore formation (sporulation) occurs when carbon/nitrogen nutrients deplete
• Examples of spore forming bacteria: Bacillus, Clostridium
• The important features of spores include high resistance to heating, resistance to many chemicals (keratinlike coat), survival for years (soil), metabolic inactivity.

Inclusion Bodies

• Aggregates or granules of polysaccharides (glycogen), sulfur, or lipids.
• Used as nutrients during starvation periods.

Generation Time

• Most bacteria reproduce by asexual processes such as binary fission.
• Two genetically identical cells are formed from the parent cell (exponential growth).
• Generation time (or doubling time) is the time between binary fissions of a cell or population.
• Some have very short generation times, while others its is quite long.
• Eschercia coli; 20 min
• Staphylococcus aureus: 30 min
• Mycobacterium tuberculosis : 15 hours
• Teponema pallidum : 33 hours

Bacterial Growth Curve

• Lag phase: Bacteria prepares for division and population numbers remain stable prior to binary fission.
• Logarithmic Phase (exponential): Reproduction and growth rates are the highest and double with each generation time.
• Stationary Phase: Reproduction rate equals the death rate, therefore stabilizing population.
• Population growth limits include exhaustion of nutrients, accumulation of waste, and exhaustion of space.
• Decline Phase: Exponential death phase in which cell death becomes substantial.

Effect of Pressure on Growth

• Halophilic: Requires high salt (NaCl) concentrations to grow.
• Osmophilic: Requires high osmotic pressure (high sugar concentration) to grow.

Growth Media

• Nutrient broth is liquid and contains water, beef extract, and peptone (nitrogen).
• Nutrient agar is solid and contains a polysaccharide derived from marine red algae.
• Complex media are undefined, or synthetic media are defined.
• Selective medium: Suppresses growth of some bacteria while allowing others (Manitol salts for staphylococci).
• Differential medium: Distinguishes organisms on the same plate (MacConkey agar).
• Enriched medium: Cultivates fastidious bacteria with special nutrients (blood or chocolate agar)

Energy Sources and Biological Systems

• Autotrophs (self-feeding): synthesize food from simple carbon sources.
• Photoautotrophs (cyanobacteria) use light energy and carbon dioxide gas (photosynthesis).
• Chemoautotrophs use carbon dioxide and chemical reactions from inorganic compounds (chemosynthesis).
• Heterotrophs are other-feeders
• Photoheterotrophs use light and organic compounds fatty acid or alcohol (carbon source:.
• Chemoheterotrophs use organic compounds for energy and carbon (glucose).
• Parasites eat living organic matter.

Metabolism

• The Greek metabole means change.
• The totality of an organism's chemical processes
• Catabolism
• Anabolism
• The metabolism revision includes the sum of catabolism and anabolism.
• Opposing chemical processes
• Catabolism releases energy (exergonic),
• Anabolism takes up energy (endergonic)

Bacterial Metabolism

• Bacteria have an adequate supply of enzymes for metabolic processes to take place
• Enzymes: are a group of organic molecules that increase the rate of chemical reactions while themselves remain unchanged
• Cellular respiration is when the making of ATP is done by harvesting energy from exergonic metabolic pathways
• Aerobic respiration is used for consuming oxygen when making ATP
• Anaerobic respiration is when ATP is made without any oxygen

Carbon Source

• The most common Carbon source of bacteria is glucose
• The major carbohydrate-metabolizing pathways are glycolysis, pentose phosphate (PP) pathway, tricarboxylic acid cycle, TCA cycle, Kreb's cycle, citric acid cycle

Glycolysis

• Occurred in cytosol of bacteria
• Glucose can convert to 3-carbon organic molecule known as Pyruvate

Krebs Cycle

• Occurs in the cell membrane
• 38 molecules of ATP result from metabolism of glucose in a bacterial cell
• NADH (3 ATP), FADH2 (2 ATP)

Fermentation of Sugars

• Fermentation is the breakdown of a sugar (such as glucose or maltose) to pyruvic acid and then, usually, to lactic acid.
• More specifically, it is the breakdown of a monosaccharide such as glucose, maltose, or galactose.
• Fermentation is the process by which facultative bacteria generate ATP in the absence of oxygen.
• Identification of human pathogens comes on the basis of fermentation of certain sugars.
• Neisseria gonorrhoeae and Neisseria meningitidis can be distinguished from each other on the basis of fermentation of either glucose or maltose.
• E. coli can be differentiated from Salmonella and Shigella on the basis of fermentation of lactose.
• The pyruvate produced by fermentation then enters the Krebs cycle and is metabolized to two final products if oxygen is present, CO2 and H2O.
• The Krebs cycle generates much more ATP than the glycolytic cycle which means facultative bacteria grow faster in the presence of oxygen.
• Facultative and anaerobic bacteria can ferment monosacharides, but aerobes which can grow only in the presence of oxygen, do not ferment.
• Aerobes, like Pseudomonas aeruginosa, produce metabolites that enter the Krebs cycle by processes other than fermentation such as the deamination of amino acids.
• The production of pyruvate and lactate in fermentation tests in the clinical laboratory will turn the medium acid, this is detected by a pH indicator that changes color upon changes in pH.
• Eg if a sugar is fermented in the presence of the indicator phenol red, the pH becomes acidic and medium turns yellow. If, however the sugar does not ferment, no acid is produced and the phenol red remains red.

Explore bacterial structure: shapes, sizes, and internal components. Learn about the cell envelope, capsules, and cytoplasmic regions. Understand how these features define and differentiate bacteria.