Bacterial Structure & Morphology (Ditki Notes)

• The cell wall sits outside the plasmic/cytoplasmic membrane.
• An infolding exists in the cytoplasmic membrane.
• The cytoplasmic membrane has a lipid bilayer which serves to transport molecules, secrete toxins, and produce energy.
• Ribosomes are located in the cytoplasm
• Ribosomes are sites of protein synthesis.
• The nucleoid is the region with bacterial DNA.
• There is no nuclear membrane.
• Outer capsules usually comprise gelatinous polysaccharides.
• Capsules contribute to serologic type and enhances virulence, but are variable
• Short filaments on gram-negative bacteria are called pili (fimbriae).
• Pili participate in bacterial attachment to host cells but are variable.
• During conjugation, a sex pilus attaches donor and recipient bacteria.
• Flagella propels the cell and is variable.
• Different bacterial strains possess unique anatomical and physiological traits.
• These traits relate to infectious disease.

Staining

• Bacteria associated with infectious disease are categorized by gram stain.
• Gram stain depends on cell wall composition and the ability to retain crystal violet stain.

Gram-positive

• Gram-positive bacteria stain bright purple in a microscopic sample.
• A thick peptidoglycan layer, also known as murein and mucopeptide, exists in the cell wall.
• Peptidoglycan comprises a network of sugars and amino acids
• Peptidoglycan is the target of some antibiotics.
• Teichoic acids attach to the peptidoglycan in the cell wall.
• Lipoteichoic acid attaches to the cytoplasmic membrane.
• Acids on the surface contribute to cell wall structure and charge.

Gram-negative

• Gram-negative bacteria appear reddish-pink in a microscopic sample.
• A thin layer of peptidoglycan is covered by an outer membrane.
• Lipopolysaccharide, known as endotoxin, comprises the outer membrane of gram-negative bacteria.
• Endotoxin contributes to disease symptoms like fever and shock.

Other Stain Types

• Not all bacterial strains are appropriate for gram-staining.
• Mycobacteria does not have a cell wall and is visualized with acid-fast staining methods.
• Treponema pallidum has a thin cell wall and can be seen with dark-field microscopy or fluorescent antibodies
• Some strains of intracellular Chlamydiae can be seen with hematoxylin and eosin (H&E) staining.

Morphology

• Used for bacterial classification.
• The bacterial cell wall contributes to morphology.

Cocci

• Spherical shaped bacteria.
• Staphylococcus are an example of bacteria arranged in clusters
• Streptococcus pyogenes are an example of bacteria arranged in chains
• Streptococcus pneumoniae are an example of bacteria arranged in pointed pairs
• Neisseria gonorrhoeae are an example of coffee-bean shaped pairs

Bacilli

• Bacillus anthracis are bacteria with rectangular ends.
• Salmonella are bacteria with rounded ends.
• Corynebacterium diptheriae are club-shaped
• Fusobacterium nucleatum is fusiform-shaped and associated with periodontal disease.
• Desulfovibrio desulfuricans is bent/comma-shaped and associated with gut flora.

Coccobacilli

• Coxiella burnetii causes Q fever and are short, rounded rods

Spirochetes

• Treponema pallidum causes syphilis and is spiral shaped.
• Some authors describe more or fewer morphological types of bacteria.