Intro to bacteria

Question 1

Describe the bacterial cell surface

Answer 1

¨ The Cytoplasmic membrane (aka plasma membrane) is a very
thin, elastic and semi-permeable membrane around the bacterial
cell contents.
¨ Outside this and closely covering is the cell wall -rigid, giving the
cell strength and shape but it is permeable.
¨ The Outer membrane is a component of the Gram negative cell
wall

Question 2

What is meant by gram negative and gram positive?

Answer 2

gram positive- thick wall (peptidoglycan)
gram negative- thin wall (peptidoglycan)

Question 3

How is a gram stain carried out?

Answer 3

growth of colonies of bacteria growing on an agar plate after incubation.
Smear a small amount on glass slide using glass loop and leave to dry.
Then apply a primary stain called crystal violet (purple in colour) that is taken up by peptidoglycan cell wall.
Then an iodine stain that binds the crystal violet.
Next, a decolouriser such as acetone- removes the gram negative outer membrane and crystal violet from the thin peptidoglycan, however the thick gram positive peptidoglycan traps the crystal violet.
A counterstain is added to give gram negatives a red colour.

Question 4

Describe the shape of cocci?

Answer 4

small and round

Question 5

Describe the shape of bacilli?

Answer 5

rod shaped

Question 6

Describe the cytoplasmic membrane?

Answer 6

¨5-10 nm thick
¨Made of phospholipids (40%) and
proteins (60%)
¨Allows passage of water and uncharged
small molecules but otherwise forms a
barrier

Question 7

What are the functions of proteins in the cytoplasmic reticulum?

Answer 7

– Transport systems for specific nutrients
and ions

– As enzymes involved in processes such as
energy generation, cell wall synthesis and
DNA replication

– Sensing proteins that measure
concentrations of molecules in the
environment

Question 8

Describe the cell wall?

Answer 8

¨10-25 nm thick
¨Made of peptidoglycan
– N-acetylglucosamine acid and N-acetylmuramic acid molecules linked
alternately in a chain, with short peptides
forming cross-links
¨Thick, strong and relatively rigid-maintains shape
¨Several antibiotics interfere with the
construction of cell wall peptidoglycan
The cell wall plays an important part in
cell division ie cross-walls form prior to
separating into 2 daughter cells

Question 9

What does the term lysis mean?

Answer 9

If wall is weakened or ruptured,
osmosis means the cell will swell and
burst –termed lysis

Question 10

Where is Lipopolysaccharide (LPS)
outer membrane present?

Answer 10

only present in gram negative bacteria

Question 11

Describe the Lipopolysaccharide (LPS)
outer membrane ?

Answer 11

¨Only present in Gram negative bacteria
¨Protects peptidoglycan from bile salts
(in the gut), also from lysozyme
¨Blocks many antibiotics getting into the
cell
¨ Lipid A (within LPS) forms endotoxin,
which, when released into the
bloodstream (when multiplying or when
lysed), may give rise to endotoxic shock
–fever and low blood pressure

Question 12

Why do some bacteria have rigid capsule?

Answer 12

prevent phagocytosis

Question 13

Why do some bacteria have less defined capsule?

Answer 13

form strong biofilms.
Attach to a surface, produce slime, divide and produce microcolonies within slime layer and forms protective environment

Question 14

Describe flagellum?

Answer 14

long, thin filament- twisted spirally in an open regular way form
gives bacterial cell motility

Question 15

What are the different classes of flagella?

Answer 15

monotrichous- single flagellum
lophotricous- more than one flagellum comes from a single point
amphitricous-flagella come from both sides
peritrichous-come from all over the cell
poar- originate from one or both ends of the cell

Question 16

Why is motility important for bacterial cells?

Answer 16

serves to place cells in environments favourable to growth - conyributes to pathogenesis of disease

Question 17

What are fimbriae?

Answer 17

many bacteria possess filaments termed fimbriae, far more numerous than flagella and are much shorter and thinner

important in mediating adhesion between bacterial cells and host cells

Question 18

How do bacterial cells replicate?

Answer 18

replicate by binary fission to produce two daughter cells

Question 19

How do bacteria show genetic variation?

Answer 19

contain one or more additional fragments of DNA- plasmids
genetic variation by plasmid transfer between bacterial cells and spontaneous mutation in chromosome

Question 20

Describe the steps of bacterial conjugation?

Answer 20

F plus cells carry the F plasmid- enabling it make an f Pillus (protein appendage that attaches donor to recipient cell) and act as a genetic donor. Recipient cells that lack an F plasmid are referred to as F minus.
First step is contact between donor and recipient
The F pilus recognizes and binds to specific receptor sites on cell wall of recipient cell. Plasmid then becomes mobilised for transfer when plasmid encoded endonuclease cleaves one strand of plasmid at a specific nucleotide sequence (origin of transfer). Single strand of F plasmid beginning at origin of transfer enters the f minus cell. A complimentary strand to single strand of plasmid remaining in donor is synthesized via rolling circle mechanism. Once inside F minus a complimentary strand to single stranded DNA is sythesized.
All cells become F plus

Question 21

What is a bacteriophage?

Answer 21

virus that infects bacteria
bacteria eater
all composed of nucleic acid mol that is surrounded by protein structure

Question 22

Describe action of a bacteriophage?

Answer 22

attaches itself to a susceptible bacteria and infects it
following infection, the bacteriophage hijacks the bacterium cellular machinery to prevent it from producing bacterial components and instead forces cell to produce viral components.
Eventually new bacteriophages assemble and burst out of bacteria in process called lysis

Question 23

Describe ribosomes?

Answer 23

¨Consist of RNA and associated proteins
¨Function is to synthesise proteins
¨Divided into subunits
– Measurement is by the Svedberg unit, a
measure of sedimentation rather than size
¨Bacteria have a 70S ribosomes, with
large (50S) and small (30S) subunits
¨The small 30S subunit has itself a
subunit, called 16S (used to identify bacteria)

Question 24

In bacillus and clostridium, what is benefit of being endospore?

Answer 24

highly resistant resting phase
organism can survive in a dormant state through long period of starvation or other adverse environmental conditions.

Question 25

Describe spore formation?

Answer 25

spore forming bacteria grow as vegetative cells and divide by binary fission when nutrients are availlible and environment not adverse.
However if the opposite, spore formation is initiated. DNA condenses into centre of the cell (mother cell) and DNA divides into two complete copies.
Mother cell membrane invaginates to form the developing forespore.
Mother cell membrane continues to grow and engulfs developing spore. Developing spore now surrounded by two membrane layers.
Peptidoglycan is laid down between two membranes of developing spore to form cortex, dipicolinic acid is formed inside developing spore and calcium enters from outside.
As calcium enters, water is removed. A protein coat forms exterior to the cortex and spore becomes mature. Some form exospore
Finally, lytic enzymes destroy mother cell and mature spore is released.

Question 26

Detection and culture?

Answer 26

¨individual bacteria
– can only be seen with a microscope
» Overall x1000, using oil immersion lens
– stain (eg Gram, fluorescent dyes)
¨colonies of individual bacteria
– cultured on solid medium (agar)
– bacterial colonies can be seen by eye