Microbiology Lecture 7

Question 1

Define sterilisation.

Answer 1

Killing of all viable organisms, including endospores.

Question 2

Define disinfection. Does it kill endospores?

Answer 2

Killing of all vegetative microbes. Typically doesnt kill endospores.
High level disinfection may kill endospores.

Question 3

What is the most important environmental factor in microbe growth?

Answer 3

Temperature.

Question 4

Do all microbes grow poorly in cold temperatures?

Answer 4

No, some grow well at 4*C, refrigerated foods are particularly susceptible to them.

Question 5

What temperature foods should pregnant women avoid and why?

Answer 5

Cold foods, some bacteria grow well in cold conditions, and these foods are susceptible to their growth.

Question 6

Define pasteurisation.

Answer 6

71*C for 15 seconds.

Question 7

Define tyndallisation.

Answer 7

80*C on 3 successive days.

Question 8

BDfine boiling.

Answer 8

100*C.

Question 9

Define hot air oven temperatures.

Answer 9

160-180*C

Question 10

Define incineration temperatures.

Answer 10

> 800*C.

Question 11

Define autoclaving.

Answer 11

Steam at >100*C, pressurised.

Question 12

What is superior for sterilisation, hot air oven, or autoclaving, and why?

Answer 12

Autoclaving, despite higher temperatures in the oven. This is because water is a better conductor of heat versus air.

Question 13

Does boiling sterilise water?

Answer 13

No, boiling doesnt kill endospores. May also cause the release of endotoxins from appropriate bacteria like gram negative.

Question 14

What temperatures can endospores resist up to? Are they susceptible to UV light and chemical disinfectants?

Answer 14

Up to 150*C. Resistant to UV light and most chemical disinfectants.

Question 15

Where are endospores commonly found?

Answer 15

Soil.

Question 16

What method is used to kill endospores, at what temperatures, and for how long?

Answer 16

Typically 121*C for 10-15 minutes.

Question 17

What is especially important when autoclaving regarding the presence of air?

Answer 17

Some items wont sterilise properly as they may have trapped air pockets. Air must be allowed to flush out for correct sterilisation.

Question 18

Define precleaning, and why itsimportant.

Answer 18

Precleaning is the removal of visible soiling from items to be autoclaved. This is so that the item may have full contact with the pressurised steam, so it can reach high enough temperatures and be sterile.

Question 19

Define decimal reduction time.

Answer 19

Time required for a 10-fold reduction in viable cells - 90% dead.

Question 20

What kind of curve does the decimal reduction time exhibit? How is it manipulated?

Answer 20

Exponential - logged to linearise for accurate extrapolation.

Question 21

Give an example of decimal reduction time beginning with 1k microbes, and microbe count per minute, until 99.9% chance of death.

Answer 21

0 minutes - 1k
1 minute - 100
2 minutes - 10
3 minutes - 1
4 minutes - 0.1
5 minutes - 0.01
6 minutes - 0.001

Question 22

In practice, what chance of death of all microbes is needed? Give both the decimal form and the scientific notation required.

Answer 22

99.9999% or 0.000001 - 10^-6 on the curve.

Question 23

What are the three checks that can be done to assess correct autoclaving?

Answer 23

Printout of the cycle - physical check
Heat sensitive paper/tape - chemical
Spore test - biological

Question 24

How often should a spore test for correct autoclave function be performed?

Answer 24

Once weekly.

Question 25

Why is it important for containers to have their lids on loosely when autoclaved?

Answer 25

To allow steam inside to displace air.

Question 26

What is used to sterilise heat sensitive items?

Answer 26

Gamma irradiation or ethylene oxide.

Question 27

What is UV light suitable for?

Answer 27

Doesnt penetrate glass or get around corners.

Only usable for flat surfaces, air, or clear water.

Question 28

How can heat/UV sensitive liquids be sterilised?

Answer 28

Filtration.

Question 29

Define filtration.

Answer 29

Membrane filters commonly used, has specific pore sizes. Bacteria cannot pass.

Question 30

When pore filtering, are the pore sizes the size of the bacteria, or less?

Answer 30

Less, if its the size of the bacteria, then bacteria will get stuck in the pores, and clog them.

Question 31

How else can filtering be used aside from sterilisation?

Answer 31

Concentrating the bacteria for culturing. Filtering doesnt kill them.

Question 32

On a culture, what does one colony represent? What does this allow for?

Answer 32

One bacteria from the original inoculation culture. Allows counting bacteria in the original solution.

Question 33

What are HEPA filters? Where are they used, and what for?

Answer 33

High efficiency particulate air filters - used to filter air, especially biological safety cabinets and ORs, removes down to 0.2um.

Question 34

Can cloudy liquids be filtered?

Answer 34

No, pores are blocked, Pressure will tear them.

Question 35

What is used to sterilise biological safety cabinets before and after use?

Answer 35

UV light before and after, but never during.

Question 36

How is the air within biological safety cabinets kept sterile? Explain the movement of air as well.

Answer 36

Using HEPA filters, air travels downwards from above and enters a grate within the cabinet, so contaminants dont leave it.