Lecture 2 Notes

A differential stain used to differentiate Gram-negative and Gram-positive bacteria.
Bacteria stain differently due to differences in their cell envelope structure.
Crystal Violet (CV) penetrates the cytoplasm of both Gram-positive (Gm+) and Gram-negative (Gm-) bacteria.
Iodine (I) forms complexes with CV (CV-I).
Alcohol strips off the outer membrane (OM) of Gram-negative bacteria and pulls CV-I across the thin cell wall.
CV-I is trapped in the Gram-positive cell.
Safranin is used as a counterstain.
Gram-negative cells turn pink, while Gram-positive cells remain purple.

Brownian motion:
- Not true motility.
- Bacteria appear to wiggle or vibrate.
- Due to the dynamics of fluid in the environment or invisible molecules striking the bacteria.
Swimming motility:
- Due to flagellar rotation.
- Movement of single cells through fluid.
Swarming motility:
- Due to flagellar rotation.
- Movement of groups of cells across solid surfaces or in liquid.
- Swarmers move very fast.
- Why swarm instead of swim? To move more cells at once, helps whole populations survive and helps bacteria establish infection.

Flagellum (singular), flagella (plural).
Acts as a “propeller” that provides motility.
Long tail-like structures.
Flagella can have various arrangements:
- Monotrichous
- Lophotrichous
- Amphitrichous
- Peritrichous
Flagella are very thin and must be stained in order to be seen with a light microscope.

Movement in response to chemical signals.
Chemical attractant:
- Usually a favorable stimulus, like a nutrient.
- Causes positive chemotaxis – the bacterium moves toward the chemical.
Chemical repellant:
- Usually an unfavorable stimulus, like a toxin.
- Causes negative chemotaxis – the bacterium moves away from the chemical.
Chemosensors send signals to cause flagellar rotation.

Short, flexible hairlike structures.
Promote adhesion of bacteria to inanimate surfaces, host cells, and other bacteria.
Composed of protein called fimbrin.

Pilus (singular) / Pili (plural).
Gram-negative and some Gram-positives.
Slightly longer than fimbriae.
Adhesion to host cells and to other bacteria.
Sex pilus:
- Special pilus possessed by many Gram-negative bacteria.
- Rigid, hollow tubular structure.
- Promotes conjugation.
- Genes for antibiotic resistance or virulence are often transferred via a sex pilus.

Glycocalyces differ in thickness, organization, and chemical composition.
- Slime layer - a loose, thin shield made of polysaccharides.
- Capsule - a thick, sticky layer of polysaccharides and protein.
Purpose: Protection, adhesion, biofilm formation.

A complex aggregation of microbes growing together on a surface.
Biofilms are everywhere, and can be good and bad.
- Healthy humans – teeth, gut.
- Diseased humans – lungs of cystic fibrosis patients.
- Hospitals – indwelling medical devices.
- Food industry – food prep surfaces, drains.
- Environment – streams.
Fimbriae, pili, and glycocalyces allow bacteria to attach to the surface and to each other.

Small, circular pieces of dsDNA.
Contain nonessential genes
Often encode for protective traits like virulence factors, antibiotic-resistance, etc.
Usually exist outside the chromosome, although in some cases the plasmid may integrate into the chromosome.
During replication, the plasmid is duplicated and passed on to offspring.

Dormant bodies produced by 3 genera of bacteria – Bacillus, Clostridium, Sporosarcina.
Facilitate survival when environmental conditions are unfavorable.
Resistant to desiccation, extreme heat, chemicals, etc.
Most bacterial endospores do not facilitate reproduction.
Spore-formers have two-phase life cycle:
- Vegetative cell
- Spore – inert, resting phase that is resistant to harsh conditions.

Spore coats
Cellular material (DNA, ribosomes, proteins)
Low pH (keeps enzymes inactive)
Dipicolinic acid (DPA), calcium and DNA-binding proteins
Core
Cortex
DPA and $Ca^{++}$ remove water from the core. This dehydration (along with DNA binding proteins) stabilizes and protects bacterial DNA & proteins from damage. Both protect against radiation and chemicals
Cortex contains peptidoglycan

Transformation of dormant spores into vegetative cells.
Triggers: amino acids, sugars, gentle heat
Some water must be present.
Major events:
- Internal core pH increases
- Spore swells and ruptures to release vegetative cell
- DPA and calcium are released from the core

Caused by C. botulinum – sporeformer
Germination and neurotoxin production occur at pH > 4.5 and in absence of oxygen (obligate anaerobe)