Bacterial Cell Structure: Pili and Fimbriae

Pili and Fimbriae

• Fine hair-like filaments on the surface of many Gram-negative bacteria, not involved in motility
• Fimbriae composed of 20 kDa protein, while pili are generally longer
• Both involved in attachment of bacteria to host cells by binding to lectins on cell surfaces (e.g., sugar-binding proteins), giving tissue specificity

Sex Pili and Conjugation

• Used by conjugative plasmids to transfer copies of themselves to a new host (F+ and F- strains)
• Sometimes other genetic material is mobilized during conjugation
• After conjugation, the previously negative cell has a copy of the plasmid and can pass it on to another cell, efficiently transferring antimicrobial resistance
• The F plasmid of E. coli can also mobilize the chromosome to be transferred through cell-to-cell contact, leading to extensive genetic recombination (Hfr strain)

Cytoplasmic Components

• Glycogen is used as a storage product, a starch-like polymer of glucose subunits
• Smaller than PHB granules
• Genetic elements:
• Plasmids: circular and autonomously replicating
• Transposons: "jumping" genetic cassettes

Pili and Fimbriae

• Bacteria within a human host constantly lose and reform fimbriae, which are fragile structures
• Fimbriae aid in evasion of the immune system
• Bacteria (e.g., N. gonorrhoeae) can change the antigenic structure of their pili/fimbriae

Bacterial Conjugation and Recombination

• Donor cell attaches to a recipient cell with its pilus, drawing the cells together
• One strand of plasmid DNA transfers to the recipient, which synthesizes a complementary strand to become an F+ cell
• The donor synthesizes a complementary strand, restoring its complete plasmid

Hfr-specific Recombination

• Hfr cells contain genes that allow them to transfer some or all of their chromosome to another cell
• Conjugation tube connects Hfr cell to normal cell, and a copy of Hfr chromosome begins to move to the recipient cell
• Homologous sections of chromosome synapse, and cells separate
• Section of Hfr chromosome integrates into recipient chromosome by crossing over

Capsules and Slime Layers

• Consist of polysaccharide or sometimes protein, with a general term of glycocalyx
• Capsule: a thick, viscous gel outside and attached to the cell wall
• Slime: a capsule loosely attached to the cell wall and easily washed off
• Both are hydrophilic, appearing as halos around cells in a Gram stain

Composition of Capsules

• Mainly acidic polysaccharide, with acidic groups being glucuronic acid or phosphate
• Most are immunogenic, stimulating host antibody response
• Exceptions:
• Group B meningococci: capsule contains N-acetylneuraminic acid found on host cells
• Hyaluronic acid of S. pyogenes capsule can lead to host cell damage because it is found on host cells

Capsules and Slime Layers

• Variable thickness: up to 10µm thick in Klebsiella spp. and Streptococcus pneumoniae
• Microcapsule of E. coli and Salmonella spp. is too thin to be seen on a light microscope and is detected by antisera
• Capsule not always expressed under lab conditions

Advantage to Bacteria

• Protect cells from desiccation and toxic material
• Attachment to host cells
• Soluble material released into solution blocks opsonizing antibodies
• Resist complement alternative pathway
• Resist phagocytosis by neutrophils

Bacterial Chromosomes

• Single circular chromosome of double-stranded DNA, called a nucleoid
• 300 to 400 µm in length and supercoiled like a rubber band
• Genes arranged linearly along the chromosome
• Size range from 600 to 9500kb (9.5Mb)
• Composed of nucleotides: adenine, guanine, cytosine, and thymine (uracil in RNA)

Prokaryotic Protein Synthesis

• Protein translation via tRNA and mRNA
• 30S subunit initiation complex + 50S subunit
• Location of protein-encoding genes determined by looking for Open Reading Frames (ORF):
• Start codon
• Codon (triple sequences)
• Stop codon

Prokaryotic Protein Synthesis

• Takes place on ribosomes, 70S (30S+50S), in the cytoplasm
• 30S contains 16S RNA (small subunit)
• 50S contains 23S RNA (large subunit)
• Up to 10,000 ribosomes per cell
• Up to 35% of dry weight is RNA in growing cells
• Bacterial mRNA is polycistronic (multiple genes on one mRNA), which can be translated by several ribosomes simultaneously