Microbial Control: Approaches, Principles, and Methods - Prof. Luzhou Xing, Study notes of Microbiology

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Control of Microbial Growth

Chapter 5

Approaches to Control

• (^) Control mechanisms either physical or chemical
  • (^) May be a combination of both
  • (^) Physical methods
    • (^) Heat
    • (^) Irradiation
    • (^) Filtration
    • (^) Mechanical removal
  • (^) Chemical methods
    • (^) Use a variety of antimicrobial chemicals
    • (^) Chemical depends on circumstances and degree of control required

• (^) Decontamination
  • (^) Treatment to reduce pathogens to level considered safe
• (^) Degerming
  • (^) Mechanism uses to decrease number of microbes in an area - Particularly the skin
• (^) Sanitized
  • (^) Implies a substantially reduced microbial population
    • (^) This is not a specific level of control
• (^) Preservation
  • (^) Process used to delay spoilage of perishable items
    • (^) Often includes the addition of growth-inhibiting ingredients

Principles of control continued

Microbial control methods are highly variable

• (^) Depends on situation and degree of control required - (^) Daily life - (^) Hospital - (^) Microbiology laboratories - (^) Food and food production facilities - (^) Water treatment

Situational considerations

2) Hospitals

• (^) Minimizing microbial population very important
  • (^) Due to danger of nosocomial infections
    • (^) Patients are more susceptible to infection
    • (^) Pathogens more likely found in hospital setting » (^) Numerous organisms develop antimicrobial resistance due to high concentrations of antibiotics
  • (^) Instruments must be sterilized to avoid introducing infection to deep tissues

Situation continued

3) Microbiology laboratories

• (^) Use rigorous methods of control
  • (^) To eliminate microbial contamination to experimental samples and environment - (^) Aseptic technique and sterile media used for growth » (^) Eliminates unwanted organisms - (^) Contaminated material treated for disposal

Situation continued

5) Water treatment facilities

• (^) Ensures drinking water is safe
• (^) Chlorine generally used to disinfect water
  • (^) Can react with naturally occurring chemicals
    • (^) Form disinfection by-products (DBP)

» Some DBP linked to long-term health risks

• (^) Some organisms resistant to chemical disinfectants

Situation continued

Selection of Antimicrobial Procedure

• (^) Selection of effective procedure is complicated
  • Each has drawbacks and procedural parameters
• (^) Choice of procedure depends on numerous factors
  • (^) Type of microbe
  • (^) Extent of contamination
    • (^) Numbers of organisms initially present
  • (^) Environment
  • (^) Risk of infection
  • (^) Composition of infected item

2) Number of organisms initially

present

• (^) Time it takes to kill it directly affected by population size
• (^) Commercial effectiveness is gauged by decimal reduction time - (^) D value - (^) Time required to kill 90% of population under specific conditions

Procedure factors continued

3) Environmental conditions

• (^) Environmental conditions strongly influence

effectiveness

• (^) pH, temperature and presence of organic materials can increase or decrease effectiveness - (^) Most chemicals are more effective at higher temperatures and lower pH

Procedure factors continued

5) Composition of the item

• (^) Some sterilization and disinfection methods

inappropriate for certain items

• (^) Heat inappropriate for plastics and other heat

sensitive items

Procedure factors continued

Physical methods to control

• (^) Moist heat
  • (^) Destroys through irreversible coagulation of

proteins

• (^) Moist heat includes

Boiling

Pasteurization

Pressurized steam

i) Boiling (100° C)

• (^) Destroys most microorganisms and viruses
• (^) Not effective means of sterilization
  • (^) Does not destroy endospores

ii) Pasteurization

• (^) Pasteur developed to avoid spoilage of wine
• (^) Does not sterilize but significantly reduces organisms
• (^) Used to increase shelf life of food
• (^) Most protocols employ HTST method
  • (^) Heated to 72°C and held for 15 seconds (milk)
• (^) Other protocol UHT
  • (^) Heated to 140°C - 150°C, held for several seconds then rapidly cooled