7: Microbiology 2

Question 1

Sterilisation

Answer 1

Killing all organisms

Question 2

Disinfection

Answer 2

Reducing the number of microorganisms to an “acceptable” level

Question 3

Inhibiting Growth

Answer 3

Bacteriostatic, virustatic, fungistatic agents

Question 4

Factors Affecting Rate of Death

Answer 4

• 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)

Question 5

Concentration of Biocidal Agent

Answer 5

• 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%

Question 6

Heat Sterilisation

Answer 6

• Heat (moist or dry)
• Incineration
• Radiation
• Must establish TDP and TDT prior to use

Question 7

Mechanical Sterilisation

Answer 7

Filtration of air and fluids

Question 8

Chemical Sterilisation

Answer 8

• Acids
• Oxides
• Peroxides

Question 9

Thermal Death Point (TDP)

Answer 9

Lowest temp which kills all microbes in 10 mins

Question 10

Thermal Death Time (TDT)

Answer 10

Minimum time taken to kill all microbes in a liquid at a given temp

Question 11

Heat Sterilisation: Moist Heat

Answer 11

• 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

Question 12

Heat Sterilisation: Dry Heat

Answer 12

• 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)

Question 13

Gamma Radiation

Answer 13

• 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

Question 14

Mechanical Sterilisation: Filtration

Answer 14

Uses filtration membranes to remove microorganisms. Solutions for surgical irrigation, peritoneal dialysis, IV therapy, ophthalmic solutions etc.

Question 15

Disinfection

Answer 15

• 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)

Question 16

Physical Disinfection: Heat

Answer 16

• Boiling at 100° for 5 mins kills most microbes
• Pasteurisation: 63C 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

Question 17

UV Radiation

Answer 17

• Short wavelength ultraviolet (UV) light lethal to many microbes (causes breaks in DNA)
• Specialised use of UV lights to disinfect surfaces and air

Question 18

Filtration

Answer 18

• 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

Question 19

Koch’s Postulates

Answer 19

• 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

Question 20

Morbidity

Answer 20

The number of individuals who develop serious illness when exposed to a disease

Question 21

Mortality

Answer 21

The number of deaths caused by a disease

Question 22

Incidence

Answer 22

• Number of new cases of disease in a population during a specific period
• Indicates whether cases are increasing or decreasing

Question 23

Prevalence

Answer 23

• Total number of people infected with the disease at any one time
• Can indicate duration of disease

Question 24

Primary

Answer 24

Maintaining good health, nutrition, hygiene and vaccination programs

Question 25

Secondary

Answer 25

Tests for early detection and screening programs

Question 26

Tertiary

Answer 26

Preventing further complications of disease