16 - Microbial Death Flashcards
Sterilisation
Process that removes or kills ALL living organisms. Physical or chemical agents (heat, electromagnetic radiation, filters and sterilants)
Disinfection
Treatment of materials with disinfectants to kill, inhibit or remove disease causing microorganisms. May be residual living organisms after treatment.
Disinfectants
Chemicals employed to kill, inhibit or remove microorganisms present on inanimate objects. Harsh, not used to decontaminate plants or animals
Sanitisation
Reduction of microbial population on inanimate object to low level (safe by public health standards). Physical or chemical agents (e.g. cleaning cutlery)
Antiseptics
Chemical agents applied to tissue/body surfaces to prevent infection by killing pathogens. Mild to avoid tissue damage (alcohol)
Aseptic technique
Procedures to prevent contamination of previously uncontaminated materials to obtain pure cultures of microorganisms and to prevent infection
Chemotherapy
Use of chemical agents to kill or inhibit growth of microorganisms within host tissue
Bactericidal
Kills bacteria
Bacteriostatic
Inhibits bacterial growth
Viricidal
Kills viruses
Fungicidal
Kills fungi
Fungistatic
Inhibits fungal growth
Algicidal
Kills algae
Antimicrobial
Kills microbes
Selective toxicity
Activity against microbial pathogen but damages the host as little as possible
Chemotherapeutic
Antimicrobial agent with selective toxicity. Compound used in treatment of disease
Decimal reduction time (D value)
The time value required to kill 90% of the microbes in a sample under specified conditions
6 factors influencing the effectiveness of control agents
- Population size
- Contact time
- Population susceptibility
- Antimicrobial concentration
- Temperature
- Local environment
Population size
The larger the size of the population, the longer it takes to achieve sterility
Contact time
The longer a population is exposed to a microbicidal agent, the more organisms are killed
Population susceptibility
- Endospores are more resistant than vegetative forms of their organism
- Actively growing log phase bacteria are more susceptible than stationary phase cells due to increased uptake of agent
- Acid fast bacteria are more resistant
- Gram negative are more resistant than gram positive
Antimicrobial concentration
- The more concentrated a chemical agent or intense a physical agent, the more rapidly organisms are destroyed
- Some agents more effective at lower concentrations (70% ethanol more effective than 95% ethanol)
Temperature
- Mild heat enhances activity of chemical disinfectants by promoting chemical reactions
- 10ºC rise in temp increases disinfectant activity 2 to 3 fold
Local environment
Some environmental factors are protective, others contribute to killing. (e.g. organic matter can protect microorganisms)
Examples of physical methods in control
- Heat
- Radiation
- Filtration
- Low temperatures
Heat
Kills by:
- Denaturing proteins and nucleic acid degradation
- Membrane becomes more fluid so contents leak out, membrane potential lost preventing nutrient transport
Types of heat
- Dry heat (dehydrates the cell, greater amount of heat required than moist) e.g. Incineration (flaming) and Hot air ovens
- Moist heat (Steam sterilisation, boiling, cooking and pasteurisation)- more effective
Hot air ovens (dry heat)
For glassware and metal instruments.
- Advantages: Does not corrode so can be used for glassware and metal
- Disadvantages: Slow, not suitable for heat sensitive materials like rubber
Steam sterilisation (moist heat)
- Carried out by autoclave.
- Increase in steam temp is proportional to pressure
- Used for media, liquids, bandages
- Advantages: fast, convenient and reliable
- Disadvantages: corrodes metal, leaves glassware wet, damage to heat sensitive items
boiling (moist heat)
Kills vegetative cells and eukaryotic spores. Will not destroy bacterial endospores therefore not a sterilant
Cooking (moist heat)
Kills all but spores
Pasteurisation (moist heat)
- Reduces microbial population of liquid.
- Extends shelf life of products (reduces spoilage) but does not sterilise (except UHT)
- heat liquids under 100C killing disease causing microbes without cooking the liquids
Types of pasteurisation
- Batch pasteurisation: in large tanks, used for beer and fruit juice
- Ultrahigh temp: ~140ºC for 3 seconds. No refrigeration needed and product is stored at room temp. Used for milk
- high temperature short time (continuous flow): Liquid forced through metal plates or pipes. Used for milk and dairy products to be stored in fridge
Types of radiation
- ionising radiation
- UV light
Ionising radiation
- e.g. X rays and gamma rays
- Damage cell by reacting with proteins and nucleic acids
- used for heat sensitive medical supplies (plastic syringes, ABs, drugs, food)
- Advantages: penetrates deeply, kills all cells (including endospores), very effective
- Disadvantages: Expensive and dangerous
UV light
- Electromagnetic radiation absorbed by nucleic acids altering DNA structure (thymine dimers)
- T dimer distorts DNA, preventing replication
- Used to disinfect air in rooms, water treatment and biological safety cabinets
- Advantages: Easy to use, can disinfect an area
- Disadvantages: Does not penetrate glass, liquid or opaque solids
filtration
- used for culture media, oil, pharmaceuticals (e.g. safety cabinet, N95 masks)
Types of filtration
- Depth filters: Fibrous material excludes bacteria and large microorganisms
- Membrane filters: Solution forced through filter why vacuum or syringe. 0.2 and 0.45 micrometers common size. made with cellulose acetate
Factors influencing effectiveness of chemical control
- concentration and nature of disinfectant
- length of treatment
- kind of microorganisms present (TB, colds and polio resistant)
Types of chemical control agents that denature proteins and disrupt cells membranes
- phenols
- alcohols
- quarternary ammonium compounds
Phenols
- Lysol and triclosan
- Advantages: Effectives in presence of organic matter, long lasting, good for disinfecting pus, saliva, faeces. Tuberculocidal
- Disadvantages: Strong odour, irritating to skin, not sporadical
Alcohols
- Ethanol and isopropanol
- Advantages: Antiseptic, cheap, stable, nontoxic
- Disadvantages: not sporadical
Why is 70% ethanol more effective than 100%
100% is dehydrating agent, can extract cellular water and preserve cells
Halogens
- Chlorine:
D: high affinity for organic matter, too harsh for antiseptic.
A: Very effective in water supplies and food industries - iodine:
A: Sporadical at high concentrations
D: May damage skin, leave stain and cause allergic response - Betadine: water soluble, stable, nonstaining, releases iodine slowly to minimise skin burns and irritation
Heavy metals
- Silver nitrate: antiseptic on newborns, prevents eye infection
- Copper sulfate: algicidal and fungicidial
Quaternary ammonium compounds
- Cation detergents
- Detergents: have both hydrophilic and phobic ends.
- e.g. Zephiran
- Advantages: stable and nontoxic used and food and antiseptics
- Disadvantages: inactivated by hard water and soap. Not sporadical
Aldehydes
- Formaldehyde and Glutaraldehyde (chemical sterilant)
- Sporicidal, tuberculocidal, virucidal, bactericidal, fungicidal
Sterilising gases
- Ethylene oxide: Sporicidal, tuberculocidal, virucidal, bactericidal, fungicidal. Penetrates packing materials, suitable for heat sensitive items (spacecraft). D: Expensive, toxic and flammable
- Vaporised hydrogen peroxide: Decontaminate operating rooms and large facilities
Advantages of filtration
effective and economical for
liquids and gases which cannot be autoclaved: e.g. thermolabile liquids
Disadvantages of Filtration
Mycoplasma - small, can
squeeze through pores; VIRUSES can pass
also through 0.2µm pores, so solutions are
not necessarily “sterile”.
Chemical control agents that chemically modify cell constituents
- aldehydes
- halogens
- sterilising agents
- metals
