Bacterial Structure and Biofilms

Lecture 3 Outline

• Lecture will discuss the structure of bacteria and atypical forms of bacteria.

What are Biofilms?

• A biofilm is a group of microorganisms where cells adhere to each other and/or a surface.
• Cells within a biofilm are embedded in a self-produced matrix of extracellular polymeric substances (EPS).
• Biofilms can form on both living and non-living surfaces.
• Microbes create films in response to factors like specific or non-specific surface attachment sites, nutritional cues, or exposure to sub-inhibitory antibiotic concentrations.
• Shifting to biofilm growth results in phenotypic changes, providing bacteria with extra protection to resist environmental hazards.
• Common issues like urinary tract and middle-ear infections, catheter infections, dental plaque formation with gingivitis, and contact lens coatings can involve biofilms.
• Endocarditis, cystic fibrosis infections, and those associated with permanent indwelling devices like joint prostheses and heart valves, are all processes that can involve Biofilms.

The Cell Envelope

• The bacterial cell envelope encompasses the material around the cytoplasm.
• It comprises chemically and functionally distinct layers but mainly includes the cell wall and cell membrane.

Functions of the Cell Wall

• Provides overall bacterial strength, protects from injury and damage like an exoskeleton.
• Preserves cell shape and differentiates major types of bacteria.
• A platform for surface appendages like flagella and pili.
• Cell wall supports the cell envelope against the high intracellular water or osmotic pressure.
• It prevents the cell from rupturing/lysis.
• Contributes to the pathogenicity of certain bacteria.

Cell Wall Composition

• A rigid peptidoglycan layer exists in most bacterial cell walls, otherwise known as murein.
• Peptidoglycan consists of polysaccharide with two glucose derivatives: N-acetylglucosamine (NAG), and N-acetylmuramic acid (NAM), in long chains.
• Chains cross-link via a tetrapeptide extending from the NAM sugar unit.
• The tetrapeptide consists of L-alanine, D-glutamine, L-lysine, and D-alanine.
• In most bacteria, the peptide cross-links form between D-alanine (4th amino acid) on one tetrapeptide and L-lysine (3rd aa) on another tetrapeptide.
• The cross-linking increases the overall cell structure strength.
• Bacterial cells can be gram-positive or gram-negative, depending on the thickness of the peptidoglycan and presence of other components.
• The Gram stain identifies the two cell wall types, developed in 1884 by Hans Christian Gram.

Gram-Positive Cell Walls

• Thick and rigid peptidoglycan cell wall.
• Teichoic acid is an additional component that is a polymer of ribitol or glycerol and phosphate in peptidoglycan layers.
• Teichoic acids can covalently link to peptidoglycan or connect to the cell membrane called lipoteichoic.
• Cell net negative charge is due to teichoic acid.
• Provides rigidity, also involved in cell division.
• Teichoic acids play a role in resistance against adverse conditions such as high temperatures and high salt concentrations, and antibiotics resistance.

Gram-Negative Cell Walls

• The peptidoglycan layer is thinner than that of Gram-positive bacteria, and it doesn't have teichoic acid.
• Lipopolysaccharide(LPS) contains specialized types of phospholipids similar to cell membrane, and makes up the outer membrane (OM).
• LPS consists of polysaccharide attached to a unique lipid molecule i.e. lipid A and variable polysaccharide chain (O-antigen).
• O-polysaccharide can identify variants of a species.
• Lipid A release is a result of cell death, which represents an endotoxin that is toxic and can trigger fever and shock.
• The outer membrane has unique proteins called porins, which form pores through which small hydrophilic molecules pass into the periplasm.
• Large, hydrophobic molecules cannot pass, which accounts for the resistance of gram-negative cells to many antimicrobial agents, dyes and disinfectants.
• The periplasm is the resulting gap from the separation of the cell membrane by the outer membrane.
• Peptidoglycan coating is in the periplasm, which holds large amounts of digestive enzymes and transport proteins.

Non-Typical Bacteria

• Some bacterial groups lack the standard Gram-positive or negative cell wall structure, or they have no cell wall.
• Mycobacterium, Chlamydia, Mycoplasma, and Rickettsia species lack cell-wall structure.
• Mycobacterium has a peptidoglycan with a long-chain fatty acid called mycolic acid that makes cells waxy and resists decolorization. Staining known as acid-fast stain, Ex: Mycobacterium tuberculosis causes TB.
• Chlamydia contains an outer lipopolysaccharide membrane lacking typical peptidoglycan, but has cysteine-rich proteins in the OM that function like peptidoglycan. Chlamydia pneumoniae results in diseases such as pneumonia, trachomatis causes eye infection.
• Mycoplasma lacks a cell wall around its cell membranes, and absence peptidoglycan. They are among the smallest bacteria, which contain sterols that resist lysis. Mycoplasma pneumoniae causes pneumonia.
• Rickettsia is a tiny, gram-negative bacteria, they are obligate intracellular parasites alternating between a mammalian host and bloodsucking arthropods. Rickettsia typhi transmits endemic typhus through lice.

Cell Membrane

• A cell (or plasma/cytoplasmic) membrane is a universal biological structure that separates external from internal (cytoplasmic) environments.
• It has phospholipids and proteins.
• Described as a fluid-mosaic model because components move around.
• The membrane has two layers or a bilayer of phospholipids.
• A phospholipid is composed of a hydrophilic polar head and a hydrophobic non-polar tail (two fatty acids).
• To keep the polar regions in contact with the cytoplasm and aqueous environment, phospholipids come together.

Protein Molecule Position

• Membrane protein molecules are Peripheral or Integral types.
• Peripheral proteins are on the inner or outer membrane surfaces that help move substances and act as enzymes.
• Easily removed.
• Integral proteins extend fully through the membrane and are fixed, where some act as channels for substances moving in and out of the cell.
• Prokaryotic plasma membranes are less rigid than eukaryotic plasma membranes because they lack sterols like cholesterol and ergosterols.

Main Functions of the Cell Membrane

• Selective permeability by regulating transport of nutrients and waste, but keeps dangerous substances out.
• The membrane is selectively permeable with special carrier mechanisms, allows water and small uncharged molecules to diffuse unaided.
• Energy production as important site for metabolic activity with ATP in cell membrane.
• Synthesizes macromolecules with enzymes to incorporate structural macromolecules in cell envelope.
• Other enzymes and toxins secreted by the membrane into the extracellular environment.
• Communication via the binding or intake of small molecules, providing information to the cell.

Processes of Transport Through the Cell Membrane

• Cell membrane transport across it happens through passive or active transport:
  • Passive transport is with the concentration gradient and needs no energy. This includes simple diffusion, facilitated diffusion, and osmosis.
  • Active goes against the concentration gradient, needing energy such as carrier mediated, group translocation, and endocytosis.

Passive Transport Processes

• Substances cross the membrane from an area of high concentration to an area of low concentration, with the concentration gradient without energy use (ATP).
• Simple diffusion, facilitated diffusion, and osmosis are all Passive transport processes.

Simple Diffusion

• The net movement of molecules or ions down their gradients, from an area of high concentration to an area of low concentration until molecules are evenly distributed or reach equilibrium.
• Movement of small nonpolar or oxygen/lipid molecules works well with simple diffusion.

Facilitated Diffusion

• This passive transport mechanism uses a carrier protein in the membrane, binding a specific substance and facilitating its movement.
• Carrier protein resumes shape after substance transport, which helps transport a single molecule type.
• Process shows specificity and saturation. Specificity allows proteins to transport a single type of molecule. Saturation occurs when all transporters’ binding sites are occupied, and rate of transport reaches a steady state.

Osmosis

• The spontaneous net movement or diffusion of solvent (usually water) molecules through a selectively-permeable membrane.
• Movement goes from an area of high-water potential (lower solute concentration) to an area of low-water potential (higher solute concentration).
• Cell gains or loses water in comparison to the environment, conditions are labelled as isotonic, hypotonic, and hypertonic.
• Isotonic solution has no resulting net water movement.
• Hypotonic solution causes water to flow into the cell causing damage or lysis of the cell.
• Hypertonic solution results in water flowing out of the cell, resulting in shrinkage or plasmolysis of the cell.

Active Transport Processes

• Features include systems transporting nutrients against the diffusion gradient or in the same direction but faster than diffusion alone; the presence of membrane proteins; and the expenditure of cellular energy in the form of ATP-driven uptake.
• Transported substances are monosaccharides, amino acids, organic acids, phosphates, and metal ions.
• Includes carrier mediated active transport, group translocation, and endocytosis.

Carrier Mediated Active Transport

• Specific membrane proteins bind both ATP and molecules to be transported.
• Moves sugars, amino acids, vitamins, and phosphate into the cell.
• Pumps drugs out of the cell increasing resistance, and rapid transport of ions like K+, Na+, and H+ across the membrane.

Group Translocation

• A specific molecule actively gets captured, which by passing the membrane protein carrier is chemically altered and activated for use in the cell; with no way to exit the cell.
• By coupling transport with synthesis, the cell conserves energy.
• Example, adding a phosphate group to glucose or fructose during their transport.

Endocytosis

• Involves the cell enclosing a substance within its membrane, thus forming a vacuole and engulfing it.
• Examples are amoebas and certain white blood cells ingesting whole or large solid matter in a type of endocytosis called phagocytosis.
• Liquids enter the cell through pinocytosis.

The Cytoplasm and Internal Structures

• Cytoplasm is all of the contents within a cell membrane, and is composed of internal structures and a matrix or complex solution.
• The main component is water (70-80%), which provides solvents with nutrients including sugars, amino acids, and other organic molecules and salts.

Nucleoid

• Nucleoid has bacterial chromosomes.
• Bacterial chromosome consists of a single, extremely long, circular strand of DNA that is not enclosed and instead aggregated and tightly coiled in the cell.
• Carries genes that provide information needed for bacterial maintenance/growth.

Plasmid

• Plasmids are additional genetic structures that are small, circular, double-stranded DNA exist separately from the cytoplasm replicate independently.
• Non-essential for survival but carries genes that produce cell traits like toxin production or resistance to antibiotics known as R plasmid.

Ribosomes

• Ribosomes are sites of protein synthesis that exist in a prokaryotic cell tens, which make it visually granular.
• Chemically combined with ribosomal RNA (rRNA or 60%) and protein (40%).
• Bacteria grade with 70S, consist of small and large subunits (30S and 50S respectively).

Inclusions

• Inclusions are distinct structures in the cytoplasm or periplasm that do tasks.
• Examples:
  • Carbon storage contains condensed, energy-rich organic storages such as glycogen and polyhydroxybutyrate (PHB).
  • The gas vacuoles found in bacterial cells control flotation.
  • Polyphosphate granules as those in Corynebacterium, is important for building blocks in nucleic acid and ATP, also known as metachromatic granules as they stain differently with methylene blue dye.
  • Carboxysomes are storage of enzymes used for CO2 fixation, process of changing carbon dioxide for organic compounds like sugar.
  • Magnetosomes have elongated chains of iron oxide in bacteria found in oceans and swamps. Their orientation with the earth's magnetic field to locations provides favorable oxygen and nutrient levels.

Cytoskeleton

• Protein polymers traverse the cell and play a role in defining shapes, division, and cell wall integrity.
• Skeletal and intermediate filament proteins can be akin to those in Eukaryotes, whilst the rest are unique to bacteria.