Bacterial Cell Wall: Structure, Properties and Growth

• Students should be able to describe the structure and properties of bacteria
• Students should be able to classify bacteria into various groups based on some properties
• Students should be able to discuss the bacterial growth curve and factors that favour or disfavour growth

Bacteria

• Bacteria are simple, prokaryotic organisms lacking a nucleus or nuclear membrane
• Bacteria can appear as rods (bacilli), spheres (cocci), or spirals (spirilla or spirochetes)
• Bacteria reproduce via binary fission and they have unique cell walls

Bacterial Cell Wall

• All bacteria have a semi-rigid cell wall, except for mycoplasmas
• The cell wall gives shape to the organisms and prevents them from bursting due to osmotic pressures
• Peptidoglycan is the main component of the prokaryotic cell wall, a large polymer of N-acetylglucosamine and N-acetylmuramic acid
• The bacterial cell wall is a unique structure that surrounds the cell membrane
• There are two major types of bacterial cell walls: Gram-positive and Gram-negative

Structural Wall Necessity

• Maintaining the cell's characteristic shape
• Countering the effects of osmotic pressure
• Providing attachment sites for bacteriophages
• Providing a rigid platform for surface appendages like flagella, fimbriae, and pili

Gram-Positive Bacteria

• Gram-positive bacteria's cell wall consists of many polymer layers of peptidoglycan connected by amino acid bridges
• The peptidoglycan polymer consists of an alternating sequence of N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAMA)
• Each peptidoglycan layer is connected, or cross-linked, by a bridge made of amino acids and amino acid derivatives
• The particular amino acids vary among different species
• Cross-linked peptidoglycan molecules form a grid-like network covering the cell
• The Gram-positive cell wall is about 90% peptidoglycan
• Teichoic acid polymers are commonly associated with peptidoglycan in Gram-positive bacteria

Gram-Negative Bacteria

• Gram-negative bacteria's cell wall is thinner, at only 20% peptidoglycan
• Gram-negative bacteria have two unique regions surrounding the outer plasma membrane, the periplasmic space, and the lipopolysaccharide layer
• The periplasmic space separates the outer plasma membrane from the peptidoglycan layer and contains periplasm
• The lipopolysaccharide layer is a phospholipid bilayer attached to peptidoglycan by lipoproteins, it is similar to the cell membrane
• Lipid A, a toxic substance in the lipid portion of LPS, is responsible for most pathogenic effects of harmful Gram-negative bacteria
• Polysaccharides that extend out from the bilayer also contribute to the toxicity of LPS
• LPS, lipoprotein, and associated polysaccharides together form the outer membrane
• Lipopolysaccharides known as endotoxins are a part of the outer membrane

Bacterial Cell Appendages

• Flagella: flagellin proteins found in flagellum
• Pili (fimbria): pilin (fibrous proteins in pilus)
• Pilin is used to exchange genetic materials

Types of Flagella Arrangement

• Polar/Monotrichous: Single flagellum at one pole
• Lophotrichous: Tuft of flagella at one pole
• Amphitrichous: Flagella at both poles
• Peritrichous: Flagella all over
• Amphilophotrichous: Tuft of flagella at both ends

Bacterial Endospores

• Bacterial endospores are a resting phase of bacteria
• Bacteria can be resistant to heat, but can be killed at 121°C, chemicals, radiation, drying and organic solvents
• Examples of bacteria with spores are Bacillus and Clostridia

Methods of Classification

• Phenotypic classification
• Genotypic classification
• Combination of phenotypic and genotypic methods (Polyphasic approach)

Phenotypic Classification

• Morphological
• Anatomical
• Staining
• Nutrition
• Environmental factors
• Biochemical reactions
• Antigenic structures

Bacterial Shapes

• Bacillus (rod)
• Coccus (sphere)
• Spirillum (spiral)

Based on Cultural Characteristics

• Utilization of carbohydrates
• Oxidative eg., Neisseria
• Fermentative eg., E. coli
• Growth Rate
• Rapid growers -eg., Vibrio cholerae
• Slow growers – M. tuberculosis
• Pigment production - eg., S. aureus

Based on Nutrition

• Autotrophs obtain carbon from inorganic sources like carbon dioxide (CO₂)
• Heterotrophs obtain reduced carbon from other organisms

Based on Environmental Factors

• Temperature
• Oxygen dependence
• PH/Salt concentration
• Atmospheric pressure

Temperature

• Psychrophiles (15-20°C) eg., Pseudomonas fluorescens
• Mesophiles (20-40°C) eg., E. coli, S. aureus
• Thermophiles (50-60°C) eg. Bacillus stearothermophilus
• Extreme thermophiles (as high as 250°C). Eg. Thermococcus spp

Oxygen Dependence

• Aerobe: Grows in ambient temperature: requires 21% O₂ and 0.03% of CO₂
• Microaerophilic: Requires 5 -10% of CO₂, eg., C. jejuni, H. pylori, H. influenzae, N. gonorrhoeae
• Obligate aerobes: Strictly require O₂ for their growth, eg., Pseudomonas aeruginosa, M. tuberculosis
• Facultative anaerobe: Capable of growing either in the presence or absence of O₂. Eg., E. coli
• Obligate anaerobe: eg. Clostridium perfringens

pH/Salt Concentration

• Acidophiles: eg., Lactobacillus acidophilus, Lactobacillus fermentum
• Alkaliphiles: eg., Vibrio sp, Bacillus subtilis
• Halophiles/non-halophiles: eg. Halobacterium salinarum, Tetragenococcus halophilus
• The majority of medically important bacteria grow best at a pH ranging from 7.2-7.6

Phases of Bacterial Growth in a Closed System

• Lag phase: The number of cells does not increase, viable cell produce necessary enzymes for use of new medium
• Log phase – exponential phase: Cells divide at steady state, later limited by absence of nutrients or increase in harmful metabolites
• Stationary phase: The rate of growth decreases, very little cell division, there is a variation in cell morphology, G + ve may change to G-ve, spores may be produced, and toxins are produced
• Decline/death- cells die rapidly, marked difference between viable and total count, the number of dying cells exceed the number of new cells produced

Factors Affecting Growth

• Nutrients
• Right temperature
• pH
• Aerobic or anaerobic environment