MicroBiology- Control of Microbial Growth

Question 1

WHAT ARE CONTROLS OF MICROBIAL GROWTH?

Answer 1

• Any measures taken to greatly reduce number of microorganisms

Processes used in microbial control (3 types)

• Sterilisation→ removal/destruction of all living cells (including spores) and other ineffective material (viruses) from an object
• Disinfection→ Killing/ inactivation of large number of organisms (agent is disinfectant, e.g. chlorine in pools)
• Antisepsis→ Prevention of tissue infection: Agent used is antiseptic (e.g. dettol)

Question 2

PHYSICAL AGENTS (1) HEAT: Incineration

Answer 2

• Total destruction of contaminated material by fire

- Only for single- use objects (needles, syringes, dressings)

Question 3

PHYSICAL AGENTS (1) HEAT: Dry Heat Sterilisation (“Baking”)

Answer 3

• Heating items at 160℃ for 1 hour→ kills all microorganism forms (including endospores)
• However many items needing to be sterilised wont tolerate this condition: e.g. plastic ware

Question 4

PHYSICAL AGENTS (1) HEAT: Boiling

Answer 4

• Putting item in water at 100℃ for 10 mins is disinfection only→ won’t kill endospores
• Effectiveness as a microbial control agent relies on thorough cleaning beforehand
• Remains a simple means of microbial control available to everyone
• “Boiled water alert” issued when disinfection at water supplier is compromised

Question 5

PHYSICAL AGENTS (1) HEAT: Wet Heat Sterilisation: Autoclaving

Answer 5

• Autoclave: Water in autoclave chamber is subjected to pressure 2x that of atmospheric levels→ water must be heated to 121℃ to boil and produce steam
• Steam must be able to penetrate whatever is being sterilised
• When performed for 15 mins→ can kill vegetative cells and endospores

Question 6

PHYSICAL AGENTS (2) RADIATION: Ionising Radiation

Answer 6

• Gamma rays from radioactive source will disrupt DNA→ sufficient to sterilise objects
• Common method in commercial suppliers of clinical and surgical disposables which wouldn’t withstand heat sterilisation

Question 7

PHYSICAL AGENTS (2) RADIATION: Ultraviolet Radiation

Answer 7

• UV rays disrupt DNA but poor level of penetration of UV light→ means this is primarily used to disinfect surfaces
• E.g. communal kitchens in nursing homes may have UV lights left on when not in use

Question 8

PHYSICAL AGENTS (3) FILTRATION

Answer 8

• Processes which rely on physical barrier that excludes particles of a specific size or greater
• Ideal process for liquids sensitive to other agents (e.g. water can be treated in this way)
• Size of holes (pores) in barrier determines level of sterility

Question 9

CHEMICAL AGENTS

Answer 9

• Agents work via restrictions between agent and components of infection agent
• Reaction modifies the component so organisms is no longer viable
• Hypochlorous acid→ “bleach” and is chemical disinfectant
• 70% v/v alcohol swabs and hand washes
• Chemical sterilants and disinfectants work on dosage basis
• Dirtier an item; the more agent you would need
• In a clinical situation; important that where it’s safe to the handler→ equipment cleaned prior to sterilization or disinfection

Question 10

ANTIBIOTICS AND ANTIVIRALS

Answer 10

• Drugs with selective toxicity
• They target chemical/metabolic pathways unique to pathogen which are critical for growth/replication
• E.g. Penicillin (antibacterial drug) blocks cell wall synthesis
• Zidovudine→ Anti HIV drug; targets a unique viral enzyme