Lecture 8 (Linda Stewart) - Control Of Microbial Growth

Question 1

Compare and contrast actions of disinfection, antisepsis, chemotherapy, and sterilisation

Answer 1

• Disinfection destroys vegetative pathogens but not bacterial endospores. It is usually used only on inanimate objects.
• Antisepsis are chemicals applied to body surfaces to destroy or inhibit growth of vegetative pathogens.
• Chemotheraphy is the internal use of chemicals to kill or inhibit growth of microorganisms within host tissues.
• Sterilisation is the complete destruction of all viable microorganisms. It is used on inanimate objects.

Question 2

Distinguish between cidal and static agents

Answer 2

• cidal agents kill microorganisms e.g. germicides and bactericides.
• static agents inhibit growth of microorganisms e.g. bacteria static and fungistatic.

Question 3

Explain the mechanism by which filtration removes microorganisms

Answer 3

• Liquid filtration is done by using membrane filters. These are porous membranes with defined pore sizes that remove microorganisms primarily by physical screening. Pores are too small for organisms to pass through.
• Some mechanisms by which air is filtered includes surgical masks, cotton plugs on culture vessels, and high-efficiency particulate (HEPA) filters ( used in laminar biological safety cabinets).

Question 4

Describe the application of heat and radiation to control microorganisms

Answer 4

• Moist heat - Destroys viruses, fungi, and bacteria. Boiling will not destroy endospores. Degrades nucleic acids, denatures proteins, and disrupts membranes.
• Steam sterilisation - Above 100 degrees Celsius which requires saturated steam under pressure. Uses an autoclave.n Effective against all microorganisms including spores. Quality control is carried out using strips with Geobacillus stearothermophilus spores.
• Pasteurisation - Involves controlled heating at temperatures well below boiling (50-70 degrees Celsius). Used for milk and beer. Does not sterilise but does kill pathogens present and slow spoilage by reducing the total load of organisms present.
• Dry heat sterilisation - Less effective than moist heat sterilisation, requiring higher temperatures and longer exposure times. Oxidises cell constituents and denatures proteins.
• Dry heat incineration - used to sterilise inoculating loops.
• UV radiation - Wavelength of 260nm is most bactericides as this is the optimum wavelength at which DNA absorbs UV light. Causes thymine dimers which prevent replication and transcription. UV limited to surface sterilisation as it does not penetrate glass, dirt films, or water. Used for water treatment.
• Ionising radiation - Gamma radiation penetrates deep into objects. Destroys bacterial endospores but not always effective against viruses. Used for sterilisation and pasteurisation of antibiotics, hormones, and food.

Question 5

Predict the effects of microbial population size and composition, temperature, exposure time, and local environmental conditions on antimicrobial agent effectiveness

Answer 5

• Population size and composition - Larger populations take longer to kill than smaller populations. Microorganisms less sensitive to antimicrobial agents will take longer to kill than those that are more sensitive.
• Temperature - Higher temperatures usually kill microorganisms quicker than lower temperatures.
• Exposure time - Longer exposure time results in more organisms killed.
  Local environmental conditions - pH, viscosity, and concentration of organic matter can profoundly impact the effectiveness of antimicrobial agents. Organisms in biofilms are less susceptible to many microbial agents.