Lecture 5 - chemical control methods Flashcards
Disinfectant
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
Antiseptics
chemical agents that kill or inhibit growth of microoorganisms and that are sufficiently non-toxic to be applied to living tissues
Preservatives
included in pharmaceutical preparations or foods to prevent microbial spoilage of the product
Chemical control methods are used for…
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
List the conditions that could influence the effectiveness of antimicrobial agents
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
Population size
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
Properties of the chemical agent
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
Type of microbe
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
Environmental factors
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
Toxicity of the agent
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
Type of microbe - phase of growth
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
Type of microbe - polymer, capsule or lipid production
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
Type of microbe - altered cell wall or membrane or modified sensitive sites (Like enzymes)
Can change this as it grows and therefore exposure to chemicals whilst they are growing can lead to chemical resistance in that microbe
Type of microbe - cellular aggregation/biofilms
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
Type of microbe - resistant structures
For example bacterial endospores - any bacteria that can form endospores are usually more resistant
Type of microbe - microbial interactions (competition, antagonism, mutualism)
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
Type of microbe - contact time
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.
Phenol coefficient test
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
List the ideal characteristics of chemical control agents
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
Alcohol
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)
Aldehydes
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
Quaternary ammonium compounds
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
Halogens and hydrogen peroxide
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
Heavy metals
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
Phenols and phenolic derivatives
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
Sterilising gases
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)
Trigene
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
Better chance of killing the microbes is by having…
a mixture
Trigene - high antimicrobial activity?
Yes
Trigene - broad spectrum of antimicrobial activity?
yes
Trigene - stability?
Yes
Trigene - homogeneity?
Yes
Trigene - adequate solubility?
Yes
Trigene - minimum toxicity?
Ok
Trigene - detergent activity?
Yes
Trigene - minimum material effects?
ok
Trigene - minimum inactivation by organic material ?
ok
Trigene - activity at ordinary temperatures?
yes
Trigene - deodorising ability?
yes
Trigene - low cost?
?
Advantages, disadvantages, major use and mode of action - alcohols
non-toxic
but endospores are not killed, poor penetration
used for skin swab prior to injection
denature proteins and membrane
Advantages, disadvantages, major use and mode of action - aldehydes
kills endospores and vegetative cells
toxic
preserving tissues
inactivates enzymes by adding alkyl group
Advantages, disadvantages, major use and mode of action - quaternary ammonium compounds
Non-irritant, detergent action
Endospores not killed, inactivated by organic matter
Soaps, detergents, skin antiseptics
Physical removal, disrupts membranes
Advantages, disadvantages, major use and mode of action - halogens and hydrogen peroxide
Wide activity
inactivated by organic matter, short life
water chlorination, skin antiseptic
oxidises vital biochemicals
Advantages, disadvantages, major use and mode of action - heavy metals
no major advantages
Toxic
Drops in eyes of newborns, silver dressings
Reacts with sulphydral groups
Advantages, disadvantages, major use and mode of action -phenols and phenolic derivatives
Wide spectrum of activity
endospores not killed, some toxic, skin irritant
Home and hospital use, mouthwashes
Denatures proteins, disrupts membranes
Advantages, disadvantages, major use and mode of action -sterilising gases
Kills endospores
Explosive and toxic to humans
Pre-wrapped disposable items
Strong alkylating agent
