Bacterial Characteristics and Cell Wall

Bacterial Characteristics

• Bacteria are simple prokaryotic organisms
• Bacteria cells lack a nucleus or nuclear membrane
• Bacteria may appear as rods (bacilli), spheres (cocci), or spirals (spirilla or spirochetes)
• Bacteria reproduce through binary fission
• Bacteria have unique constituents in their cell walls

Bacterial Cell Wall

• All bacteria, except mycoplasmas, possess a semi-rigid cell wall
• The cell wall provides shape and prevents bursting due to osmotic pressures
• The primary component of the prokaryotic cell wall is peptidoglycan
• Peptidoglycan is a large polymer made of N-acetylglucosamine and N-acetylmuramic acid
• The bacterial cell wall is a unique structure that surrounds the cell membrane
• The cell wall contributes to the cell's shape by compensating for the flexibility of the phospholipid membrane
• The cell wall prevents bursting by withstanding intracellular osmolarity
• The cell wall offers attachment sites for bacteriophages via teichoic acids
• The cell wall acts as a platform for flagella, fimbriae, and pili

Gram-Positive Bacterial Cell Walls

• The cell wall consists of multiple layers of peptidoglycan connected by amino acid bridges
• The peptidoglycan polymer comprises alternating N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAMA) sequences
• Each peptidoglycan layer is connected or cross-linked via amino acid bridges, with amino acids varying among species
• The cross-linked peptidoglycan molecules form a grid-like network
• Peptidoglycan makes up approximately 90% of the cell wall and teichoic acid polymers are associated with it

Gram-Negative Bacterial Cell Walls

• The cell wall is thinner, containing only 20% peptidoglycan
• Features two unique regions surrounding the outer plasma membrane: the periplasmic space and the lipopolysaccharide layer
• Separates the outer plasma membrane from the peptidoglycan layer
• Gram-negative cell walls feature a phospholipid bilayer construction similar to the cell membrane
• This bilayer is linked to the peptidoglycan via lipoproteins
• Lipid A is a toxic component within the LPS lipid part causing pathogenic effects in harmful Gram-negative bacteria
• Polysaccharides extending from the bilayer enhance LPS toxicity
• LPS, lipoprotein, and associated polysaccharides form the outer membrane
• Lipopolysaccharides, or endotoxins, are components of the outer membrane

Bacterial Cell Appendages

• Flagella are composed of flagellin proteins
• Pili (fimbriae) are composed of pilin, a fibrous protein
• Pilin aids in the exchange of genetic material

Flagella Arrangement Types

• 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

• Represent a resting phase for bacteria
• Exhibit resistance to heat but can be killed at 121°C
• Exhibit resistance to chemicals, radiation, drying, and organic solvents
• Bacillus and Clostridia are spore-forming bacteria

Methods of Bacteria Classification

• Phenotypic Classification
• Genotypic Classification
• Polyphasic Approach: Combination of Phenotypic and Genotypic Methods

Phenotypic Classification Factors

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

Phenotypic classification (Cultural Characteristics)

• Carbohydrate Utilization: oxidative (e.g., Neisseria) or fermentative (e.g., E. coli)
• Growth Rate: rapid (e.g., Vibrio cholerae) or slow (e.g., M. tuberculosis)
• Pigment Production: e.g., S. aureus

Phenotypic classification (Nutrition)

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

Phenotypic classification (Environmental Factors)

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

Temperature Classifications

• Psychrophiles: 15-20°C (e.g., Pseudomonas fluorescens)
• Mesophiles: 20-40°C (e.g., E. coli, S. aureus)
• Thermophiles: 50-60°C (e.g., Bacillus stearothermophilus)
• Extreme thermophiles: up to 250°C (e.g., Thermococcuss spp.)

Bacteria Classification (Oxygen Dependence)

• Aerobe: grows in ambient temperature, requires 21% O2 and 0.03% CO2
• Microaerophilic: 5-10% CO2 (e.g., C. jejuni, H. pylori, H. influenzae, N. gonorrhoeae)
• Obligate Aerobes: strictly require O2 (e.g., Pseudomonas aeruginosa, M. tuberculosis)
• Facultative Anaerobe: grows in either presence or absence of O2 (e.g., E. coli)
• Obligate Anaerobe: cannot grow in the presence of O2 (e.g., Clostridium perfringens)

Bacteria Classification (pH/Salt concentration)

• Acidophiles: e.g., Lactobacillus acidophilus, Lactobacillus fermentum
• Alkaliphiles: e.g., Vibrio sp, Bacillus subtilis
• Halophiles/non-halophiles: e.g., Halobacterium salinarum, Tetragenococcus halophilus
• Medically important bacteria typically grow best at pH 7.2-7.6

Bacterial Growth Conditions

• Bacterial growth is an orderly increase in cytoplasm and other cellular components
• Bacterial growth is influenced by certain conditions
• Reproduction occurs through binary fission

Batch Culture Growth

• Bacteria cultivated in liquid medium typically grow in batch cultures or closed systems
• Nutrients decline while waste accumulates during incubation
• Bacteria growth can be shown as the logarithm of cell number versus incubation time

Phases of Growth

• Lag Phase – Latent: No apparent cell division; viable cells produce enzymes for new medium use
• Log Phase – Exponential Phase: Cells divide at a steady state; absence of nutrients limit it or harmful metabolites increase.
• Stationary Phase: Growth rate decreases; finally very little cell division, variation in cell morphology, G+ve may change to G-ve, spores may be produced, toxins are produced and the number of dying cells equals the number of cells produced
• Decline/Death: Cells die rapidly; marked difference between viable and total count; dying cells exceed new cells produced

Factors Affecting Bacteria Growth

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