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Lecture 5

MICB 202 Lecture Notes - Lecture 5: Gram Staining, Teichoic Acid, Cell MembranePremium

8 pages94 viewsSummer 2017

Department
Microbiology
Course Code
MICB 202
Professor
Tracy Kion
Lecture
5

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Topic 1: Introduction to Bacterial Diseases and Vocabulary
Disease-causing microbes: only a very small fraction of the microbes on Earth
Vast majority of microbes (including viruses): serve critical and beneficial roles in the biosphere
Worldwide, ~30% of deaths are from an infectious disease.
DALYs (Disability-adjusted life year)
expresses impact of infectious disease
= 1 year of healthy life lost due to combined effects of disability and premature death
Example of diseases/infections caused only by bacteria: pertussis, tuberculosis
Bacteria and archaea are prokaryotic.
There are no known archaeal diseases.
Topic 2: Bacterial Background for MICB 202
I. Nomenclature
e.g. Escherichia coli or Escherichia coli
Genus: Escherichia
Species: coli
Strain: O157:H7 (O-polysaccharide type 157, flagella structure type 7)
Strain: a member of a species with a particular genotype
Serovar or serotype: a strain differentiates from other strains by immunological (serological)
methods i.e. whether certain cell components bind with certain antibodies
detected with agglutination tests, ELISA, Western blots (immunology and virology
segments)
bacterial surface structures (antigens) often used for serotyping
no rules about how strains can be designated different systems for different species of
bacteria
Usually, but not always, referred by:
K = capsule
O = LPS
H = flagella
II. The Bacterial Species Concept (definition of species)
For higher eukaryotes (e.g. macroscopic plants and animals): an interbreeding group of
diploid reproductively isolated organisms that produce fertile offspring
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Not applicable to bacteria
haploid (most genes)
reproduce asexually
soe of their geeti aterial ay ot oe fro their paret horizotal gee
transfer)
For ateria: a olletio of orgaiss strais haraterized y a high degree of siilarity i
the nucleotide sequences of their genomes (genotypes) and phenotypic properties
High – varies from species to species depending on what microbiologists decide
e.g. Docile E. coli strain K- lies i itestie ad deadly haurger disease E. coli strain
O157:H7 are the same species
Comparison of their circular chromosomes:
E. coli K-12 E. coli O157:H7
~ 4,600,000 nucleotide pairs ~5,500,000 nucleotide pairs (20% more DNA)
The definition of species is a human construction: humans decide what a species is.
tight aterial speies – similar strains
loose aterial speies – strains exhibit clear differences e.g. E. coli.
III. Structure
Cell envelope = cytoplasmic membrane + cell wall
Cell shapes
Cocci
Rods (bacillus)
Spirilli
Vibrios (bent rod)
Often used in nomenclature, e.g. Vibrio cholera, Staphylococcus aureus
Cytoplasmic membrane
surrounds cytoplasm
similar to eukaryotic plasma membrane
Cytoplasm
water-filled compartment
dense suspension of ribosomes
concentrated solution of tRNA, mRNA enzymes, small organic molecules, inorganic ions
Prokaryotic nucleoids usually contain 1 circular dsDNA oleule = the orgais’s chromosome
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Chromosome
very large relative to overall cellular dimensions
compacted with other molecules to form the nucleoid
Nucleoid
DNA of a bacterium
in direct contact with the cytoplasm
Bacterial cell walls
1. Types and Gram stain
Two most common cell wall types: Gram positive & Gram negative
Cells with Gram +ve cell walls: stain purple in Gram-stain procedure
Cells with Gram -ve cell walls: stain red/pink in Gram-stain procedure
2. Peptidoglycan (PG)
one huge, mesh-like molecule in the shape of the cell
Structure
Molecule found in both Gm +ve and Gm -ve cell walls
Glycan chain: 2 covalently linked alternating NAM and NAG sugars
X-linking peptide: NAM sugars covalently X-linked by amino acids joined by covalent
bonds
3. Gram-positive cell walls
many layers of PG and any associated proteins and teichoic acids
Teichoic acids: chains of phosphorylated sugars (negatively-charged) anchored to PG
(TA) or in membrane (Lipoteichoic acid, LTA)
Includes: PG, periplasm, cytoplasmic membrane (CM)
4. Gram-negative cell walls: the outer membrane and LPS
a few layers of PG and an additional membrane (outer membrane)
Includes: OM, periplasm (thin PG layer), CM (aka inner membrane)
Outer membrane
lipid bilayer with embedded proteins
outer leaflet dominated by lipopolysaccharide
Lipopolysaccharide (LPS)
hybrid molecule of lipid and polysaccharide
only found on OM of Gm-ve bacteria
O-polysaccharide: long chain of uncharged sugars
R-core region: composed of sugars with negatively charged chemical groups
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