7: Microbiology 2 Flashcards
Sterilisation
Killing all organisms
Disinfection
Reducing the number of microorganisms to an “acceptable” level
Inhibiting Growth
Bacteriostatic, virustatic, fungistatic agents
Factors Affecting Rate of Death
- Type of microorganism present
- Size of the microbial population
- Growth phase (Exponential, stationary, endospores)
- Conc. of antimicrobial agent (up to a threshold)
- Duration of exposure (Heat and chemical)
- Local environment (pH & presence of organic matter)
Concentration of Biocidal Agent
- Correct concentration of biocidal agent must be present
- Provides sufficient molecules of the chemical to interact with microbe
- Weaker or stronger solution and insufficient time will compromise the process – 70% ethanol is more effective than 95%
Heat Sterilisation
- Heat (moist or dry)
- Incineration
- Radiation
- Must establish TDP and TDT prior to use
Mechanical Sterilisation
Filtration of air and fluids
Chemical Sterilisation
- Acids
- Oxides
- Peroxides
Thermal Death Point (TDP)
Lowest temp which kills all microbes in 10 mins
Thermal Death Time (TDT)
Minimum time taken to kill all microbes in a liquid at a given temp
Heat Sterilisation: Moist Heat
- The most reliable method used in clinical environment
- Some microbes can withstand 100°C
- An autoclave used to produce steam at higher temperature by increasing pressure:
- 2 x atmospheric pressure raises boiling point to 121°C
- 3 x atmospheric pressure raises boiling point to 134°C
- 15 mins at 121°C or 3 mins at 134°C will kill all microorganisms
Heat Sterilisation: Dry Heat
- Temperature raised to 180°C for 60 mins
- Only suitable for materials resistant to high temperatures (Metals, glass, etc.)
- Does not cause corrosion
- Kills by oxidation (removes electrons from cellular components)
Gamma Radiation
- Ionising radiation (-rays) has high penetrative power and kills most bacteria
- Endospores and many viruses resistant
- Kills by disrupting DNA chains
- Can penetrate packaging:
- Used to sterilise syringes, dressings etc.
- Uses radioactive cobalt-60 as a source
Mechanical Sterilisation: Filtration
Uses filtration membranes to remove microorganisms. Solutions for surgical irrigation, peritoneal dialysis, IV therapy, ophthalmic solutions etc.
Disinfection
- Killing or inactivation of large numbers of microorganisms
- Reduction of microorganisms to an ‘acceptable’ level (dependent on clinical environment)
- Different methods of disinfection used (physical, mechanical, and chemical)
Physical Disinfection: Heat
- Boiling at 100° for 5 mins kills most microbes
- Pasteurisation: 63C for 30 mins – food or milk
- Flash pasteurisation: 72 degrees C for 15 secs
- UHT pasteurisation: 140 degrees C for <5 secs, then cooled rapidly
UV Radiation
- Short wavelength ultraviolet (UV) light lethal to many microbes (causes breaks in DNA)
- Specialised use of UV lights to disinfect surfaces and air
Filtration
- HEPA – high-efficiency particulate air filters
- Used in ventilation systems to filter air in burns wards, operating theatres, isolation wards, ICU and high dependency
- Air filtration in biological safety hoods
- Filters changed regularly and decontaminated prior to disposal
Koch’s Postulates
- The microorganism must be present in all cases of the disease
- The microorganism must be isolated from the host and grow in pure culture
- The pure culture, when inoculated into a new susceptible host, must produce the same symptoms of disease
- Microorganism must be recovered from new host
Morbidity
The number of individuals who develop serious illness when exposed to a disease
Mortality
The number of deaths caused by a disease
Incidence
- Number of new cases of disease in a population during a specific period
- Indicates whether cases are increasing or decreasing
Prevalence
- Total number of people infected with the disease at any one time
- Can indicate duration of disease
Primary
Maintaining good health, nutrition, hygiene and vaccination programs
