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You may do a sterility test, and conclude no growth is present, therefore product is sterile, however, you must remember that there are some bacteria that are impossible to grow, as we do not understand the conditions they require, therefore we may not pick up on them.

Remember nothing is 100% bacteria free!
The Sterilisation process may be effective for some, but ineffective for other microorganisms.


What does a bactericidal agent do?

Kills bacteria, so it will reduce the current number back down the the minimum. Bacteria are killed by these agents.


What does a bacteriostatic agent?

These just inhibit growth of bacteria (don't kill). They won't reduce the current number of bacteria, they just result in no further increase in number.


What sterilisation method should you use for anything heat sensitive?

Esp if you have a clear solution that's completely soluble


What is a good reference to use for validation of sterilisation processes?

Spores of bacillus stearothermophilus
The most resistant type of organism you can use is a bacterial endospore
If these have been killed, everything will have been killed


What are VBNCs?

Viable but not culturable microorganisms
Ie they are living but won't grow when put on media that normally supports growth.
These can produce endotoxins which can damage a drug and human.
Got to watch out for these!


Microbial inactivation follows ______ kinetics (ie killing microbes off)

First order kinetics
Surviving fraction curve decreases exponentially


What is the definition of the D value?

The TIME (or dose of radiation) required to reduce the number of viable cells by a factor of 10
The TIME (or dose of radiation) required to reduce the number of viable cells by 90%
The TIME (or dose of radiation) required to achieve a 1 log reduction in the number of viable cells


The higher the D value of a particular microorganism, the ______ the resistance.

As this means it takes longer to reduce the number of viable cells by a factor of 10


Less resistant organisms will have a _____ line as their survivor curves, as their D value is a _____ time.

Steeper line
Shorter time


Definition of Z value?

The temperature increase required to give a ten-fold reduction in the D value
The thermal destruction value


What graph do you plot to determine Z value?

Log of D value (y axis) against temperature ( x axis)


What is the usual range of Z values?

9-12 degrees
If you got an answer of say 20, you've probs gone wrong!!


D and Z values tell you about the resistance of a population but don't tell you about sterility

SAL and PNSU's are probabilitys of sterility.
They have to be expressed as a probability as there is no way we can tell if we have killed off every microorganism


Difference between SAL and PNSU?

SAL refers to the whole solution / whole batch, so it needs to be less than 1 microorganism per every 1 million (10^6) microorganisms
The PNSU refers to number of units in a batch. So this much be no more than 1 unit per 1 million units can be non sterile.


How do we calculate the D value of non linear curves?

The method changes with non linear curves
We work out the gradient of the straight part of the line using gradient=-k/ 2.303
We then use: D value = 1/ gradient To work out D value


How would you go about ensuring critical items, semi critical items aNd non critical were clean enough? I.e What method would you use for each?

Can't use disinfectants
You need to clean and sterilise using chemical sterilants and heat
Semi critical:
Clean and use High level disinfectants
Non critical:
these are items that are going to come into contact with the patient, but not in contact with mucous membranes 
Clean these and use a low to medium level disinfectant


Why is it important to clean a surface before disinfecting it?

This is because any Dirt, blood, pus, food (all organic matter)present on the surface can actually deactivate your disinfectant, so cleaning is important to make your disinfectant work! 


What will require high concentrations of disinfectants and longer exposure times?

High bio burdens
BIOFILMS: these are really resistant, the extracellular matrix is difficult to penetrate by the disinfectant. We need high level disinfectants for biofilms an long exposure times


What are the most resistant type of bacteria?

Spores of bacillus stearothermophilus are also very resistant


Which are more resistant to disinfection: Gram positive or gram negative bacteria? Why?

Gram negative more resistant,
They have an outer membrane that is impermeable to outer chemicals, making it harder for a disinfectant to get in!


Apart from organic matter, what other thing can decrease activity of disinfectants?

Divalent cations such as Mg2+ and Ca2+.
These stabilise the cell wall of organisms, and block disinfectant absorption sites on the outer cell.
Things are added to disinfectants to remove these cations and stop them being a problem.


What properties do you need to think about when using a disinfectant, (hint; microbial killing is essentially a chemical reaction!)?

Dilution (some disinfectants are much more susceptible to being inactivated by dilution than others)
Exposure time (disinfectants will only work when they're wet,☂as water helps the disinfectant to absorb onto the cell surface and be taken into the cell) need to consider exposure time; don't want disinfectant to dry out)


What will a higher concentration of disinfectant increase and decrease?

Increases efficacy
Decreases exposure time


How do we work out dilution coefficient of a disinfectant ?

✎plot a graph of Log D value(y axis) against Log concentration of disinfectant.
✍ then work out gradient of the line, the gradient will be your dilution coefficient.
✎✍ remember gradient is change in Y axis so change in D values, over change in X axis so change in concentrations


How can we tell how effected a disinfectant is by dilution?

Calculate the dilution coefficient
The bigger the number, the more effected it is by dilution.
This is because D values must be quite large, ie it takes longer to kill 90% of organisms as the disinfectant isn't as good on dilution!
The higher the dilution coefficient, the larger the reduction in potency, the more effected the disinfectant is by the dilution! 


What does a small dilution coefficient indicate?

The chemical isn't that effected by dilution
Dilution does not increase the D value
Dilution does not inactivate it or reduce its potency by much ☺


How does temperature effect disinfectants?

Increasing temperature will increase cidal activity of the disinfectant (it's killing ability) ☠☠☠
Increasing temp will increase killing Ability of different disinfectants by different amounts.
Be careful: if temp is too high your chemical may evaporate into a gas and you'll need an extractor fan!!☢☢


What is the infection control committee in hospitals? 

Nurse, microbiologist, and a pharmacist, they draw up a policy staff must stick to to keep hospital contamination down.


When applying disinfectant, what is the double bucket technique?

You have a clean bucket and a dirty bucket , both have disinfectant in, use clean bucket to apply disinfectant and dirty bucket to squeeze your mop in.

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