Microbiology Chapter 7: The Control of Microbial Growth

The Terminology of Microbial Control

• Sepsis refers to bacterial contamination
• Asepsis is the absence of significant contamination
• Sterilization removes and destroys all microbial life
• Commercial sterilization kills C.botulinum endospores from canned goods
• Disinfection destroys harmful microorganisms
• Antisepsis destroys harmful microorganisms from living tissue
• Degerming is the mechanical removal of microbes from a limited area
• Sanitization lowers microbial counts on eating utensils to safe levels
• Biocide (germicide) treatments kill microbes
• Bacteriostasis inhibits, but does not kill, microbes

Actions for Microbial Control

• There are three main mechanisms of action:
  • Alteration of membrane permeability (Targeting the cell wall structure)
  • Damage to proteins (Affecting the metabolic pathways)
  • Damage to nucleic acids

Physical Methods of Microbial Control

• Heat denatures enzymes
• Thermal death point (TDP) is the lowest temperature at which all cells in a liquid culture are killed in a certain time (e.g., 10 min)
• Thermal death time (TDT) is the minimal time for all bacteria in a liquid culture to be killed at a particular temperature
• Decimal reduction time (DRT) is the number of minutes of exposure to a defined temperature to reduce viable bacteria by 90%

Moist Heat Sterilization

• Moist heat denatures proteins
• Boiling, free-flowing steam, and autoclaving (steam under pressure at 121°C for 15 min) kill all organisms and endospores
• Steam must contact the item's surface
• Large containers require longer sterilization times
• Test strips are used to indicate sterility
• Not all objects can withstand moist heat sterilization
• Example sterilization times: test tube (15 min), Erlenmeyer flask (2000ml) (30 min), fermentation bottle (9000 ml) (70 min)

Heat

• Pasteurization reduces spoilage organisms and pathogens using HTST (72°C for 15 sec) or UHT (140°C for 4 sec)
• Thermoduric organisms survive pasteurization

Dry Heat Sterilization

• Kills by oxidation using flaming, incineration, or hot-air sterilization

Filtration

• Passage of substance through a screen-like material
• Used for heat-sensitive materials
• High-efficiency particulate air (HEPA) filters remove microbes >0.3 µm
• Membrane filters remove microbes >0.22 µm

Other Physical Methods

• Low temperature has a bacteriostatic effect using refrigeration, deep-freezing, or lyophilization
• High pressure denatures proteins
• Desiccation prevents metabolism due to the absence of water
• Osmotic pressure uses salts and sugars to create a hypertonic environment, causing plasmolysis

Radiation

• Ionizing radiation (X-rays, gamma rays, electron beams) ionizes water, creating reactive hydroxyl radicals that damage DNA, causing lethal mutations
• Nonionizing radiation (UV, 260 nm) damages DNA by creating thymine dimers
• Microwaves kill by heat, not especially antimicrobial

Chemical Methods

• Concentration of disinfectant is important: different disinfectants work better at different concentrations
• Organic matter interferes with disinfection
• pH affects disinfection: chlorine is most effective between pH 5.5-7.5
• Time is a factor in disinfection