Lecture 5 - chemical control methods

Question 1

Disinfectant

Answer 1

chemicals that are chemicals that are capable of killing microorganisms. Cannot be applied to tissues. In the laboratory they are applied to inanimate surfaces and used in discard jars and buckets

Question 2

Antiseptics

Answer 2

chemical agents that kill or inhibit growth of microoorganisms and that are sufficiently non-toxic to be applied to living tissues

Question 3

Preservatives

Answer 3

included in pharmaceutical preparations or foods to prevent microbial spoilage of the product

Question 4

Chemical control methods are used for…

Answer 4

Antisepsis of the skin
Disinfection of equipment which cannot be sterilised by other methods
Cleaning up spills of cultures or infected clinical material
Disinfection of surfaces
As preservatives

Question 5

List the conditions that could influence the effectiveness of antimicrobial agents

Answer 5

Population size
Properties of the chemical agent - dilution and pH
Type of microbe - phase of growth, polymer, capsule or lipid production, altered cell wall/membrane/or modified sites like enzymes, cellular aggregation/biofilms. resistant structures, microbial interactions, contact time
Environmental factors
Toxicity of the agent

Question 6

Population size

Answer 6

Only a fraction of the microorganisms die during a given time interval

Death curve is logarithmic, line going down at a constant rate means that death is occurring at a constant rate

For example if 90% of a bacterial population is killed during the first minute, then approximately 90% of those remaining will be killed during the next minute and so on (the decimal reduction time for this bacteria is therefore one minute)

Therefore the time it takes to achieve sterility, or to reduce microbial numbers to an acceptable level, depends in part on the number of organisms present at the beginning of sterilisation
Try to minimise initial bacterial load

If the rate of killing is the same then it will take longer to kill all members of a larger population than a smaller one

Question 7

Properties of the chemical agent

Answer 7

Dilution
If we use a chemical that is in a diluted state then it may not be as effective but if we use the chemical at a very concentrated state (higher than recommended) again it won’t be effective - chemicals have a concentration that they should be used at and this is the most effective one
For example, ethanol is not as effective at 100% therefore use 70% ethanol and 30% water and this is because activity of ethanol is enhanced by water

pH
pH rises/becomes more basic, the killing time in minutes takes longer therefore the pH of the solution can also affect the killing power

Question 8

Type of microbe

Answer 8

Big thing to remember here is that the disinfectant needs to be in contact with the microorganism and allowed the correct time to act!

Type of microbe - phase of growth, polymer, capsule or lipid production, altered cell wall/membrane/or modified sites like enzymes, cellular aggregation/biofilms. resistant structures, microbial interactions, contact time

Question 9

Environmental factors

Answer 9

Neutralisation by organic material
Microbiology often involves pus, blood, faecal material etc from samples from the human body and all of these are very high in organic matter and organic matter can have a drastic effect on antimicrobial capacity
It either absorbs the chemicals or interact with them in a way that inactivates them therefore some chemicals are not good to use when there is a high amount of organic material

Temperature
Disinfecting agent must be used at the recommended temperature and generally microbial death rate is higher when chemical agents are used at higher temperatures

Question 10

Toxicity of the agent

Answer 10

It needs to be toxic enough to kill the bacteria which is easy to do when talking about inanimate surfaces but if you are talking about antiseptics that you use on the skin then you have to balance the toxicity so that it is not toxic to the human it might be applied to/user - so it is dependent on the microbe and the situation

Question 11

Type of microbe - phase of growth

Answer 11

If microbes are in their exponential phase of growth where they are dividing the most rapidly, they are going to be taking up chemicals from their environment a lot more quickly which is therefore going to make it the most effective time to kill microbes compared to the stationary phase population which is going to take longer to kill

Question 12

Type of microbe - polymer, capsule or lipid production

Answer 12

Surrounds the outside of them, a lot of the medically important microbes do have something like this on their cell wall which increases their virulence and makes them a better pathogen

Question 13

Type of microbe - altered cell wall or membrane or modified sensitive sites (Like enzymes)

Answer 13

Can change this as it grows and therefore exposure to chemicals whilst they are growing can lead to chemical resistance in that microbe

Question 14

Type of microbe - cellular aggregation/biofilms

Answer 14

Aggregation means that you have a mass of microbial cells all together and bacteria do tend to clump naturally and this provides a lot of protection to the bacterial cell in the middle

Question 15

Type of microbe - resistant structures

Answer 15

For example bacterial endospores - any bacteria that can form endospores are usually more resistant

Question 16

Type of microbe - microbial interactions (competition, antagonism, mutualism)

Answer 16

Microbes can act together in a symbiotic way, against each other (antagonism) or can be competing and when you throw this into the mixture it can affect the microbes that are in your mixture and how they react to chemicals being introduced into the environment

Question 17

Type of microbe - contact time

Answer 17

the longer the population is exposed to a microbicidal agent, the more organisms that are killed. To achieve sterilisation, contact time should be long enough to reduce the probability of survival by at least 6 logs.

Question 18

Phenol coefficient test

Answer 18

The phenol coefficient test is frequently used to evaluate the effectiveness of antimicrobial agents. However it does so using conditions that do not replicate real life use

Question 19

List the ideal characteristics of chemical control agents

Answer 19

High antimicrobial activity- Has to be able to kill microbes
Broad spectrum of antimicrobial activity- Has to be able to kill bacteria, fungi, viruses
Stability - Often store chemical disinfectants for a long period of time therefore it has to retain its potency after storage
Homogeneity - Again, if it is stored, it is often a liquid and we have to make sure that the active ingredient does not settle out and it remains in a homogenous solution
Adequate solubility - Often use these chemical disinfection agents in a variety of other liquids such as fat, oil and water so it must be soluble with each of these
Minimum toxicity - For both the user and the environment
Detergent activity - When we use chemical agents it is often in situations where there is a lot of dirt and debris around so detergent activity is a chemical that can get rid of this as well
Minimum material effects- Can be used on a variety of surfaces such as wood, plastic, paint, metal and have to make sure that the chemical agent won’t corrode the surface
Minimum inactivation by organic material - If there is a lot of organic material around, make sure that the chemical agent is not inactivated by organic material
Activity at ordinary temperatures - Don’t want to have to heat it up or cool it down - want it working at room temperature
Deodorising ability
Low cost - Especially when you are thinking about a research facility or a health board they might go through a huge volume

NOTE - Last 5 are optional extras/not essential

There is no ideal chemical control agent - there is no one thing that will work in every situation

Question 20

Alcohol

Answer 20

Compound that has a hydroxyl group on them

Kill microbial cells by disrupting the lipids in the cell membrane and denaturing proteins

Common alcohols used = ethanol and isopropanol
These alcohols are often used to enhance the effectiveness of other chemical agents and this is called a tincture. The bactericidal properties of the alcohols increases with the length of the carbon chain until the solubility in water becomes an issue

Alcohol does not kill endospores but it is not toxic to humans (bactericidal and fungicidal but not sporicidal)

Question 21

Aldehydes

Answer 21

Kills endospores but is toxic to humans

Called an alkalating agent because they attach a short chain of carbon atoms/an alkyl group into the bacterial proteins which denatures them, inactivation gate enzymes and killing the bacterial cell. Aldehydes are highly reactive e.g. you only embalm tissues with a 37% solution which just shows how toxic it is

Question 22

Quaternary ammonium compounds

Answer 22

Long hydrocarbon chain, four groups attached to a charged nitrogen atom (N+)

Kill microbes by penetrating and destroying their cytoplasmic membrane (lots of the chemical agents act on the outside of the bacteria i.e. the cell wall and the cytoplasmic membrane) and they may also denature proteins

QACs are an example of surfactants which are ‘surface active agent(s)’ which is to do with the positively charged and the negatively charged portions of the molecule

Quaternary comes from the fact that the nitrogen has 4 hydrophobic groups attached to it

Large use of surfactant in the home, factory and hospital settings and it is good to know that one particular group of bacteria known as pseudomonas will not be killed by QACs and they will actually actively grow in this disinfectant - do not forget to target the microbe and chemical that you need

Known as a detergent as they have both polar hydrophilic components and non-polar hydrophobic components

Question 23

Halogens and hydrogen peroxide

Answer 23

Halogens such as iodine and chlorine act as an oxidising agent and they inactivate bacteria by oxidising their functional groups leading to destruction of vegetative bacteria and fungi

Iodine is used as an antiseptic before surgery

Chlorine is used as a disinfectant and for example when chlorine is added to water it forms hypochlorous acid which is what we use to sterilise out drinking water or in swimming pools to disinfect

Something to remember about chlorine is that the activity of chlorine is decreased by any organic matter in the environment which is why it is often used with water because drinking water and swimming pools should have very little organic matter there

Hydrogen peroxide is not a halogen but it works in the same way - it is an oxidising agent and kills microbes the same way that halogens do and it is often used as a weak solution 3% for disinfecting medical items

Question 24

Heavy metals

Answer 24

For example mercury, lead, silver, copper

The salts of these heavy metals react with the sulphydral groups (SH) of bacterial proteins and by reacting it poisons the enzymes in the microbial cell and kills them

Silver dressings
Silver ions are biocidal at very low concentrations
In the cell the silver ions bind to and denature proteins, inhibiting cell replication
E.g. acticoat

Question 25

Phenols and phenolic derivatives

Answer 25

Phenolic derivatives are a development from phenol as phenols are toxic - they have a benzene ring with a hydroxyl group attached

They kill microbes by denaturing the vital proteins including enzymes and phenol is not often used today because it is so toxic but you can sometimes find it as one of the ingredients for sore throats which is why you have to spit it out

Acts by denaturing proteins and disrupting cell membranes

Question 26

Sterilising gases

Answer 26

Disadvantage is that it is explosive therefore it is difficult to use and any items that are put in there have to sit for a while for the gas to go off them

Ethylene oxide, 700ppm, 40-50% relative humidity, temperature 38 degrees for 5-8 hours (or 54 degrees for 3-4 hours)

Question 27

Trigene

Answer 27

mixture of chemicals from the table which means that you have several barriers and ways of killing the microbes - better chance of killing the microbes by having a mixture

Question 28

Better chance of killing the microbes is by having…

Answer 28

a mixture

Question 29

Trigene - high antimicrobial activity?

Answer 29

Yes

Question 30

Trigene - broad spectrum of antimicrobial activity?

Answer 30

yes

Question 31

Trigene - stability?

Answer 31

Yes

Question 32

Trigene - homogeneity?

Answer 32

Yes

Question 33

Trigene - adequate solubility?

Answer 33

Yes

Question 34

Trigene - minimum toxicity?

Answer 34

Ok

Question 35

Trigene - detergent activity?

Answer 35

Yes

Question 36

Trigene - minimum material effects?

Answer 36

ok

Question 37

Trigene - minimum inactivation by organic material ?

Answer 37

ok

Question 38

Trigene - activity at ordinary temperatures?

Answer 38

yes

Question 39

Trigene - deodorising ability?

Answer 39

yes

Question 40

Trigene - low cost?

Answer 40

?

Question 41

Advantages, disadvantages, major use and mode of action - alcohols

Answer 41

non-toxic

but endospores are not killed, poor penetration

used for skin swab prior to injection

denature proteins and membrane

Question 42

Advantages, disadvantages, major use and mode of action - aldehydes

Answer 42

kills endospores and vegetative cells

toxic

preserving tissues

inactivates enzymes by adding alkyl group

Question 43

Advantages, disadvantages, major use and mode of action - quaternary ammonium compounds

Answer 43

Non-irritant, detergent action

Endospores not killed, inactivated by organic matter

Soaps, detergents, skin antiseptics

Physical removal, disrupts membranes

Question 44

Advantages, disadvantages, major use and mode of action - halogens and hydrogen peroxide

Answer 44

Wide activity

inactivated by organic matter, short life

water chlorination, skin antiseptic

oxidises vital biochemicals

Question 45

Advantages, disadvantages, major use and mode of action - heavy metals

Answer 45

no major advantages

Toxic

Drops in eyes of newborns, silver dressings

Reacts with sulphydral groups

Question 46

Advantages, disadvantages, major use and mode of action -phenols and phenolic derivatives

Answer 46

Wide spectrum of activity

endospores not killed, some toxic, skin irritant

Home and hospital use, mouthwashes

Denatures proteins, disrupts membranes

Question 47

Advantages, disadvantages, major use and mode of action -sterilising gases

Answer 47

Kills endospores

Explosive and toxic to humans

Pre-wrapped disposable items

Strong alkylating agent