Introduction to Microbiology and bacterial cells

Question 1

Prokaryotes

Answer 1

Bacteria
Archaea

1-5µm
Haploid
Asexual reproduction
Unicellular
No true nucleus
Cell wall*

Question 2

Eukaryotes

Answer 2

Fungi (yeasts)
Protozoa, Helminths
Slime moulds, Algae

Diploid *
Sexual reproduction*
Unicellular /multicellular
True nucleus

Question 3

Viruses

Answer 3

No cellular
Structure.
(Genetic material surrounded by protein)

Question 4

Prions

Answer 4

No nucleic acid
(Pieces of infectious protein)

Question 5

Bacterial classification

Answer 5

Morphology: cell shape
size
motility
spore forming

Metabolism: use of energy sources/nutrients

Molecular characteristics:
protein, lipid structure
sequence of gene encoding 16S RNA

Question 6

16S ribosomal RNA Gene sequencing

Answer 6

Essential Gene-all bacteria have ribosomes
16S gene highly conserved ( it is similar in lots of different bacteria)
but contains hypervariable regions (short section of DNA which are different)
BIG variation in the hypervariable regions between species
Organisms identified by their sequence.

Question 6

Classification: Bacterial shape

Answer 6

Cocci
rods
spirals

Question 7

bacterial cell structure

Answer 7

ribosomes
DNA- nucleoid
plasmids
cell wall
plasma membrane
cytoplasm

Question 8

Plasma (cytoplasmic) membrane- bacteria

Answer 8

Selectively semi-permeable barrier
Mediates nutrient transport
Site of secretion & respiration
Site of environmental response regulators

Question 9

cytoplasm- bacteria

Answer 9

Between plasma membrane & nucleoid
* Ribosomes – protein translation (70S: 50S & 30S subunits)
Inclusion bodies: Storage bodies? Contain important nutrients (C,P,N,S)
No mitochondria! – (Energy production occurs on the cytoplasmic membrane)
No True Nucleus

Question 10

The Nucleoid

Answer 10

Double-stranded DNA (dsDNA)
- Not enclosed by a nuclear membrane
- Usually a single closed circular chromosome
- DNA is supercoiled
- 1 copy of each gene

Plasmids
- extrachromosomal
small, circular dsDNA
- replicate independently
of chromosome (multiple copies)
- Encode auxiliary functions
e.g. antibiotic resistance

Question 11

Prokaryotic cell wall

Answer 11

An extremely important structure
-Protects from the environment
Provides rigidity, strength
Unique to bacteria ( makes an excellent drug target)
Composed of peptidoglycan cross-linked to form a mesh

A glycan backbone made of a chain of sugar residues-
Each sugar is joined by a strong glycosidic bond.
Every other sugar is linked to a short peptide (chain of amino acids)
These peptide sidechains crosslink to a peptide on an adjacent glycan backbone.
A strong mesh like structure is formed- It is NOT a solid barrier

Mesh like structure allows molecules to pass in and out.
2 types of bonds make it strong and rigid
Glycosidic bonds and peptide bonds
Contains other components that contribute to pathogenicity
(lipopolysaccharide, lipoteichoic acids)

Question 12

Differences in peptidoglycan (PG) structure
divide MOST pathogenic bacteria into 2 Groups

Answer 12

Gram positive: stain PURPLE
-THICK (20-80nm) PG layer which constitutes 60-80% cell wall
-X-linked to form thick mesh
Eg- Staphylococcus aureus
Gram positive bacteria:
Staphylococcus e.g. S. aureus
Streptococcus e.g. S. pneumoniae
Clostridioides e.g. C. difficile

Gram negative: stain PINK
-THIN (1-3nm) PG layer which
constitutes 10-20% cell wall
- Surrounded by an outer membrane
Eg. E.coli
Gram negative bacteria:
Escherichia coli (E. coli)
Pseudomonas e.g. P. aeruginosa
Salmonella e.g. S. enterica

Question 13

Gram-positive cell walls

Answer 13

Lipoteichoic acids-
Teichoic acids bonded
to membrane lipids
Released by killed
bacteria during infection.
Trigger inflammatory
response

Teichoic acids-
Acidic polysaccharides
bonded to peptidoglycan
Transport metal cations

peptidoglycan
plasma membrane

Question 14

Gram-negative cell walls

Answer 14

Lipopolysaccharide-
Stabilises membrane
structure

Porins-
Water filled channels

peptidoglycan
plasma membrane

Periplasm
Gel-like consistency
Contains proteins

Outer membrane
Impermeable to large molecules

Question 15

Lipopolysaccharide (endotoxin)

Answer 15

Strongly immunogenic
Extremely toxic to animal cells
Heat resistant

Question 16

Pathogenic bacteria NOT classified by
Gram stain: ‘atypicals’

Answer 16

Acid-fast bacilli
e.g. Mycobacteria tuberculosis
Cell wall a Thick waxy lipidic layer

Bacteria lacking peptidoglycan
Often cause intracellular infections
e.g. Mycoplasma pneumoniae
atypical pneumonia
problems for tissue culture
Chlamydia spp.

Question 17

Bacterial endospores- specialized structures

Answer 17

Highly resistant dormant structures

A structure which forms inside a bacteria in response to adverse conditions. (nutrients/ oygen)

Dormant: Survive without water and nutrients (for 10 years +)

Non reproductive: A single endospore forms within a bacteria. Reactivates in response to more favourable conditions

Tough:
Resistant to: UV radiation, Desiccation, Freezing, High temperature and
MOST disinfectants (highly impermeable)

Question 18

Capsules & slime layers- specialized structures

Answer 18

A Network of polysaccharides secreted outside of the peptidoglycan cell wall
Function:
Give protection
Aid attachment
Contribute to virulence
- Evade body defences

e.g. capsule of Streptococcus pneumoniae

Question 19

P- pili (Fimbriae) and F-pili- specialized structures

Answer 19

Morphologically & chemically similar
Hair-like structures composed of protein subunits

Bacterial conjugation is the transfer of genetic material between cells.
This can occur within species and between species ( Eg Plasmids- Antibiotic resistance)