Bacterial Classification: Prokaryotes vs. Eukaryotes

• Study notes from the text are below

Bacterial Classification: Prokaryotic vs. Eukaryotic Cells

• Eukaryotic cells possess a true nucleus with a nuclear membrane, while prokaryotic cells do not
• Eukaryotic DNA is organized with histones; prokaryotic DNA is not
• Eukaryotic cell walls are simple, while prokaryotic cell walls are complex and contain peptidoglycan
• Eukaryotes reproduce sexually via mitosis or asexually
• Prokaryotes reproduce asexually through binary fission
• Eukaryotes respire via mitochondria, prokaryotes respire via their cytoplasmic membrane
• Eukaryotic genomes have multiple chromosomes and are diploid
• Prokaryotic genomes consist of a single, circular DNA and are haploid
• Eukaryotes have membrane-bound organelles, prokaryotes do not

Medically Important Microorganisms

• Pathogenic microorganisms cause disease
• Bacteria and fungi are free-living organisms capable of surviving outside a host
• Viruses are obligate intracellular parasites and require a host to replicate

Cell Type and Nucleic Acids

• Bacteria are prokaryotic cells that contain both DNA and RNA
• Fungi are eukaryotic cells containing both DNA and RNA
• Viruses are not cells, but contain either DNA or RNA, never both

Ribosomes and Membranes

• Bacteria have 70S ribosomes while fungi have 80S ribosomes
• Neither possess membrane bound organelles
• Viruses lack ribosomes and membrane-bound organelles

Outer Surfaces

• Bacterial outer surface features peptidoglycan
• Fungi outer surface features chitin
• Viruses outer surface features a protein capsid

Bacterial Morphology (Shapes)

• Coccus (cocci) are round or spherical shaped bacteria
• Examples include Staphylococcus, Streptococcus, and Neisseria gonorrhoeae
• Bacillus (bacilli) are rod-shaped bacteria
• Examples include Escherichia coli, Shigella, Vibrio cholerae, and Mycobacterium tuberculosis
• Spirillum (spirilli/spirochetes) are spiral-shaped bacteria
• Treponema have fine, regular coils
• Leptospira have very fine coils with hooks
• Borrelia have coarse, irregular coils

Bacterial Staining Techniques

• Simple stains use a single dye, eg, crystal violet or methylene blue
• Primarily for observing bacterial shape and basic structure.
• Differential stains differentiate between types of bacteria

Gram Staining

• Gram staining differentiates Gram-positive from Gram-negative bacteria
• Gram-positive bacteria stain purple or blue
• Gram-negative bacteria stain red or pink
• Almost all cocci are Gram-positive, except Neisseria, Veillonella, and Branhamella.
• All bacilli are Gram-negative with a few exceptions
  • Corynebacterium, Clostridium, Bacillus and Mycobacterium

Acid-Fast Staining

• Acid-fast staining is used for bacteria with high lipid content in their cell walls such as Mycobacterium and Nocardia
• Ziehl-Neelsen stain is a hot method; Kinyoun stain is a cold method
• Acid-fast bacteria stain red
• Non-acid-fast bacteria stain blue or green

Special Stains

• Loeffler Alkaline Methylene Blue (LAMB) is used for metachromatic granules of Corynebacterium diphtheriae
• Hiss stain is used for capsule/slime layers
• Dorner, Schaeffer-Fulton, Wirtz-Conklin stains are used for spores

Bacterial Structure and Functions: Envelope Structures

• Glycocalyx (Capsule/Slime Layer)
• Composed of polysaccharides
• Functions in adherence, prevents desiccation, is antigenic and antiphagocytic, and increases virulence
• Cell Wall
• Gram-positive bacteria have a thick peptidoglycan layer containing teichoic acid
• Gram-negative bacteria have a thin peptidoglycan layer containing lipopolysaccharide (LPS or endotoxin)
• Acid-fast bacteria have a mycolic acid (waxy, lipid-rich layer)
• Mycoplasma & Chlamydia have no cell wall
• Outer membrane (Gram-Negative Bacteria Only)
• Contains LPS (endotoxin)
• Functions in structural integrity, acts as a molecular barrier, is antigenic and toxic to humans

Internal Structures

• Plasma Membrane is selectively permeable and contains electron transport enzymes
• Nucleoid contains circular, double-stranded DNA, but has no nuclear membrane
• Ribosomes are 70S (50S + 30S subunits) and are the site of protein synthesis
• Storage Granules store nutrients, such as metachromatic granules in Corynebacterium

Surface Appendages

• Flagella functions in locomotion and contain H antigen
• Pili (Fimbriae) facilitate adherence and genetic transfer (conjugation)

Endospores

• Endospores are highly resistant structures that can withstand heat, radiation, and disinfectants
• Produced by Clostridium and Bacillus species
• Composed of DNA, proteins, and calcium dipicolinate