Introduction to Cellular Microorganisms

Introduction to Cellular Microorganisms

• Cellular microorganisms include eukaryotes and prokaryotes
• Viruses and prions are also considered microorganisms

Learning Objectives (ILOs)

• Students must differentiate between eukaryotic and prokaryotic cells.
• Students must describe bacterial cell structures and functions.
• Students must understand virulence factors in bacteria that cause disease.

Eukaryotic vs. Prokaryotic Cells

Bacterial Classification

• Classification arranges bacteria into taxonomic groups (orders, families, genera, and species).
• Species are defined by close phenotypic resemblance (determined by biochemical tests) or genetic similarity.
• Species can be divided into subspecies or types by other typing methods.

Nomenclature

• Nomenclature is the scientific naming of microorganisms using the genus and species names (e.g., Staphylococcus aureus).

Bacterial Morphology

• Morphological characteristics include size, shape, staining properties, arrangement, motility, sporulation, and capsule.
• Size is measured in microns.

Bacterial Shape and Arrangement

• Cocci are spherical and can be arranged in bunches, diplococci, chains, or tetrads.
• Bacilli are rod-shaped and can be single, diplobacilli, or square or round ended bacilli in chains.

Bacterial Structure

• Bacteria have a chromosomal DNA, ribosomes, a cytoplasmic membrane, plasmids, inclusions, a capsule, and a cell wall
• These components are seen in diagrams of bacterial cells.

Cell Membrane

• Semipermeable membrane formed of phospholipids and proteins.
• Functions include selective permeability, nutrient transport, cell wall synthesis, DNA replication, and respiration (energy).

Intracellular Structures

• Nuclear body: containing double-stranded DNA.
• Ribosomes: involved in protein synthesis (70S sedimentation rate, 30S and 50S subunits)
• Plasmids: circular extrachromosomal DNA.
• Intracytoplasmic inclusions: food reserves

Surface Structures (Capsule)

• Usually a polysaccharide layer, but in some cases a polypeptide.
• Examples include B. anthracis (polypeptide) and Streptococcus pyogenes (hyaluronic acid).
• Not stained by Gram stain but by special stains.
• Function in antiphagocytosis and antigenic identification.

Surface Structures (Glycocalyx)

• Polysaccharide layer outside the cell.
• Function: adherence to surfaces (e.g., S. mutans and dental caries).

Cell Wall

• Gram-positive bacteria have a thick peptidoglycan layer (50-60%).
• Gram-positive walls also contain teichoic and teichuronic acids and polysaccharides.
• Gram-negative bacteria have a thin peptidoglycan layer (5-10%) with an outer membrane containing lipopolysaccharide (LPS) and proteins.

Cell Wall Function

• Maintains cell shape.
• Protects against hypotonic environments
• Responsible for Gram staining
• Surface antigens
• Endotoxin activity in Gram-negative bacteria.

Bacteria with Deficient Cell Walls

• Mycoplasma, L-forms, protoplasts, and spheroplasts lack cell walls.
• These variations are influenced by environmental conditions and/or exposure to substances like penicillin.

Bacterial Enzymes

• Lysozyme is an enzyme that breaks down the peptidoglycan layer of bacterial cell walls, making the walls easily destroyed.

Flagella

• Motility organelle formed of flagellin protein.
• Composed of a filament, hook, and basal body.
• Anchored to the cell membrane.
• Different distributions include monotrichate, lophotrichate, amphitrichate, and peritrichate.

Pili (Fimbriae)

• Hair-like projections, thinner than flagella, formed of pilin protein.
• Found mainly in Gram-negative bacteria.
• Ordinary pili function in attachment.
• Sex pili facilitate DNA transfer (conjugation).

Endospores

• Highly resistant, resting forms of bacteria produced during unfavorable growth conditions.
• Found outside the host tissues.
• Made of the core material, a thick layer called cortex, and an outer coat.
• Resistant to chemicals, dryness, heat, and are killed by autoclaving.