Selective Culturing

Question 1

What are the general nutritional requirements of bacteria?

Answer 1

• Sodium (Na+) : transporters - Phosphorus (PO43-) : nucleotides & ATP - Sulphur (SO42-; H2S): aminoacids - Iron (Fe2+): cytochromes, catalase & other enzymes - Trace Elements: Zn; Cu, Mn, required for a range of enzyme activities at low levels (mg l-1)

Question 2

Define: Defined media

Answer 2

Mixture of chemicals in precisely controlled amounts (a thorough understanding of cell metabolism)

Question 3

Define: General purpose media

Answer 3

Media that are prepared as a mixture of extracts including: yeast extract, peptones and meat infusions

Question 4

What are peptones?

Answer 4

A soluble protein formed in the early stage of protein breakdown during digestion. providing amino acids and other minerals which favour growth of different organisms, E.g. casein peptones

Question 5

Define: Selective agar

Answer 5

Contains selective agents which prevent growth of some organisms but allows others to grow. *Requires an understanding of the growth parameters of the whole population

Question 6

Define: Differential medium

Answer 6

A particular organism has a characteristic appearance on the medium

Question 7

What does S. Aureus look like on Baird parker agar

Answer 7

Shiny, grey/ black colonies with a clear halo.

Question 8

What do Staphylococci look like on Baird Parker agar

Answer 8

Shiny, grey/ black colonies with no halo

Question 9

What do Proteus look like on Baird Parker agar

Answer 9

Brown/ black colonies

Question 10

What are the differential elements of MacConkey agar?

Answer 10

Lactose, polyphenol red

Question 11

What are the selective elements in MacConkey agar

Answer 11

Bile salts, sodium chloride, crystal violet

Question 12

What does E. Coli look like on MacConkey agar

Answer 12

Pink Colour chance due to ability to ferment lactose (lactose +ve)

Question 13

What does Salmonella look like on MacConkey agar

Answer 13

Yellow No lactose fermentation occurs so no pH change

Question 14

What does do Bacillus look like on Bacillus Selective agar

Answer 14

Precipiate. Medium colour stay blue

Question 15

What does S. Aureus look like on Mannitol Salt agar?

Answer 15

Medium colour change from red to yellow.

*S. Aureus grows in the presence of high salt (S) Ferments mannitol to acid (D)

Question 16

What are the 3 general modes of action for antibiotics

Answer 16

1) prevent synthesis of bacterial cell wall
2) damage cytoplasmic membrane
3) interfere with biochemical synthesis
(e. g. Chloramphenicol, Streptomycin, Tetracycline – inhibit ribosome function

Question 17

Why are some microbes resistant to antibiotics?

Answer 17

1) Absence of a target site

–e.g. Mycobacteria are resistant to all β-lactams as they use different enzymes to synthesise the cell wall

2) Variants/point mutations in target sites that prevents the antibiotic binding

–e.g. mutations in ribosomal subunit RpsL prevents Streptomycin binding

3) Enzymes to detoxify the antibiotics

–e.g. beta lactamases attack the lactam ring and inactivate the β-lactam antibiotics

4) Efflux pumps that remove the antibiotic before it damages the cell

–e.g. fluroquinoline

Question 18

Define: Prototroph

Answer 18

Organism that is nutritionally able to synthesise all its required growth factors from simple substances. Substances vary depending on the nutritional classification of the organism. Other physical conditions must also be met