Killing Bacteria

Question 1

define ‘sterilisation’

Answer 1

killing all microorganisms

Question 2

define ‘disinfection ‘

Answer 2

killing (most) contaminating microbes

Question 3

define ‘decontamination’

Answer 3

physical removal of (most) contaminating microbes

Question 4

list the main methods of killing bacteria

Answer 4

• heating
• chilling
• radiation
• filtration
• use of chemicals

Question 5

describe how heating is used to kill bacteria

Answer 5

• heat death is exponential
• higher temperature = shorter time to reach D
  to ensure sterility, the temperature must be sustained above 100C
• achieved by use of autoclave, oven or pasteurisation

Question 6

what is meant by ‘D’?

Answer 6

decimal reduction time; the time taken for the population to be reduced to 10%

Question 7

what does autoclaving involve?

Answer 7

121C, 15 psi, >15 mins

moist heat denatures enzymes, nucleic acids and membranes

Question 8

what does oven heating involve?

Answer 8

160C, 2-3 hours

dry heat kills microbes, mainly by oxidation

Question 9

what does Pasteurisation involve?

Answer 9

moderate heating; reduced microbial number without excessive product damage
UHT milk is heated for 1-2 seconds at >135C

Question 10

describe how chilling is used to kill bacteria

Answer 10

• slows down/stops growth
• very little killing effect
• refrigerator at 4C - bacteria grow very slowly unless psychrophilic
• deep freeze at -18 or below - no bacterial growth (this requires liquid water); kills some bacteria

Question 11

describe how radiation is used to kill bacteria

Answer 11

UV light (220-300 nm):

• kills microbes by damaging DNA
• stopped by most glass and by opaque materials

Gamma radiation (60Co or 137Cs source)

• very short wavelength electromagnetic radiation
• produces lethal free radicals
• used to sterilise heat-sensitive products
• sterilisation requires 12 x D

Question 12

describe how filtration is used to kill bacteria

Answer 12

depth filters:

• fibrous sheet/mat
• pre-filter for liquids/gases
• HEPA = high efficiency particulate air filter

membrane filters:

• sterilisation of heat-sensitive liquids in the lab
• 0.45 μm or 0.22 μm cut-off
• most bacterial cells are >1 μm in width

Question 13

what does HEPA stand for?

Answer 13

high efficiency particulate air filter

Question 14

what are the three effects of chemicals on bacteria?

Answer 14

bacteriostatic = stops growth but bacteria are still alive
bacteriocidal = irreversibly inhibits growth to kill bacteria
bacteriolytic = bacteria are killed by lysis

Question 15

what does MIC stand for?

Answer 15

Made In Chelsea
also minimum inhibitory concentration = lowest concentration of an antimicrobial that will inhibit the visible growth of the organism after overnight incubation

Question 16

what chemicals are suitable for external use?

Answer 16

• sterilants - cold-sterilisation of heat-sensitive items eg. formaldehyde, sodium chlorite
• disinfectants - toxic chemicals for killing microbes on inanimate objects eg. hypochlorite (bleach), phenolic compounds (eg. carbolic soap)
• antiseptics - mildly toxic; suitable for contact with skin eg. ethanol, iodine