Lecture 4

Question 1

thermal death point

Answer 1

the lowest temperature required to kill all cells in a standard suspension of bacteria in a liquid culture at a given temperature

Question 2

decimal reduction time (d value)

Answer 2

the LENGTH OF TIME taken to obtain a TEN-FOLD REDUCTION in the number of bacteria in a standard suspension of bacteria in a liquid culture

ten-fold reduction = 90% killed = 1 log difference (on a log graph)

Question 3

K is…

Answer 3

the death rate constant which is also the SLOPE OF THE CURVE

K = 2.3/t x log10 (Nt/No)

Question 4

z value

Answer 4

he temperature required for one log10 reduction in the D value

Question 5

filtration

Answer 5

physically removing cells

through filters with pores too small for microorganisms, but large enough to allow the liquid or air to pass through

Question 6

what are the 3 types of filters

Answer 6

biggest to smallest pore

depth filter
membrane filter
nucleopore filter

Question 7

what are the advantages/disadvantages of each filter?

Answer 7

depth
A: high dirt handling capacity
D: bacteria trapped randomly - retains some liquid

Membrane
A: definite pore size, wont retain liquid
D: can block filter

Nucleopore FIlter
A: super thin - rigorous pore size
D: slow flow rate
- used for prep for electron microscopy work

Question 8

what are the 2 types of radiation

Answer 8

non-ionising and ionising

Question 9

non-ionising radiation

Answer 9

UV
damages the DNA in the cell by forming pyrimidine dimers, or direct protein damage - sterilisation of benches and air

Question 10

disadvantage of non-ionising radiation

Answer 10

not used to sterilise large volumes of liquids as it cannot penetrate these to a significant depth + does not kill bacterial endospores

Question 11

how do bacteria deal with non-ionising radiation

Answer 11

in nucleotide excision repair, UvrABC endonuclease enzyme removes damaged nucleotides

direct repair uses photoreactivation ) visible light and photolyase

recombinational repair corrects damaged DNA using Rec A

in SOS repair a transcriptional repressor protein (LexA) is destroyed.

ERROR PRONE - if damage is greater thatn repair, cell death results

Question 12

Different bacteria have different reactions to UV….

Answer 12

Gram negative = highly suseptable to UV light (eg E.coli)

Gram positive= large peptidoglycan wall protects them
(eg B. Subtilis = endospore former)

Question 13

Ionising radiation

Answer 13

gamma/xray (short wavelengths, higher energy)

kills indirectly by inducing reactive chemical radicals (free radicals) by breaking individual molecules into ions

kills bacterial endospores

Question 14

disadvantage of ionising radiation

Answer 14

expensive
requires elaborate safety precautions (lead shielding of operators)