630 Microbiology for Teachers

Microbial Growth Control

Definitions

• Sterilize - to kill or remove all living organisms, as well as viruses ... sterility is the complete absence of living organisms
• Disinfect - to kill the harmful living microogranisms
• -cidal - having the effect of killing (i.e., bacteriocidal)
• -static - having the effect of retarding growth, but not killing (i.e., bacteriostatic)

Radiation

• Ultraviolet (UV) light
  • Formation of thymine dimers in DNA blocks ability of microbe to replicate
  • Effective only on thin films or surfaces because UV rays (low energy) do not penetrate well
  • Used as a means of disinfection, not sterilization
• Ionizing radiation (X-rays, gamma rays)
  • Generation of free radicals, such as °OH, which damage macromolecules, especially DNA, blocking microbes' ability to replicate
  • Effective for many substances (plastics, cloth, etc.) because these high energy rays penetrate well
  • Good means of sterilization

Heating and Cooling

• Heating
  • Dry heat (oven)
    • good only for items that are not themselves destroyed by heat
    • requires high temperatures and long exposure times for sterilization (140C for 2 hr)
  • Moist heat
    • Pasteurization (e.g., 71C for 15 sec) kills harmful organisms
    • Canning (115C for 25-100 min) sterilizes
    • Autoclaving (121C at 15 psi for 15 min sterilizes)
• Cooling retards microbial growth and extends holding times, but is not an effective way to sterilize anything

Filtration

• Removes bacteria and larger microbes
• Effective for fluids only ... liquids or gasses
• Used as a means of disinfection, not sterilization, because it does not remove viruses

Dehydration

• Limits water availability to retard microbial growth
• Not an effective means of disinfection or sterilization

Growth Control in Food Microbiology

• Food spoilage
  • Perishability
    • Highly perishable foods - fish poultry, eggs, milk, most fruits and vegetables
    • Semi-perishible foods - potatoes, some apples, nuts
    • Non-perishable (stable) foods - sugar, flour, rice, dry beans
  • Factors underlying perishability
    • Moisture content (high)
    • pH (above 5, below 8)
    • Salinity (low)
• Food preservation
  • Control of perishability factors
    • Moisture content (lower)
    • pH (below 5, above 8)
    • Salinity (high)
  • Storage temperature (below 4C, above 60C)
  • Chemical preservatives retard microbial growth
    • sodium or calcium propionate
    • sodium benzoate
    • sorbic acid
    • formaldehyde (generated during "smoking" process)
    • sodium nitrite or nitrate
    • sulfites, sulfur dioxide (SO₂)

Germicides - Antiseptics and Disinfectants

• Germicide
  • Definition - a chemical agent that kills microbes
  • Examples - disinfectants, antiseptics
• Antiseptic
  • Definition - a chemical agent that kills harmful microbes and is selectively toxic enough that it can be used on human skin
  • Examples - organic mercurials (merthiolate), silver nitrate, iodine solution, alcohol, phenols, cationic detergents, hydrogen peroxide
• Disinfectant
  • Definition - a chemical agent that kills harmful microbes, but is too harsh to be used on human skin
  • Examples - mercuric dichloride, copper sulfate, iodine solution, chlorine gas, chlorine compounds (chlorox, etc.), phenolic compounds, cationic detergents, ethylene oxide gas, ozone

Antimicrobials - Antibiotics and Antimetabolites

• Origin
  • Antibiotics are naturally occurring substances made by certain bacteria and fungi (these can be modified chemically, as well)
  • Antimetabolites are synthetic substances made from chemicals by humans in factories
• Principles
  • Spectrum
    • Definition - measure of the variety of microbes killed
    • Broad-spectrum antimicrobials kill a wide variety of microbes
  • Effectiveness - CTAs must be administered by an appropriate route so they can reach infected sites in an appropriate concentration to act against the organism(s) within their spectrum
  • Selective toxicity - exhibiting a high degree of selectivity or specificity in toxicity; a selectively toxic agent is toxic (harmful, life-threatening) to one living thing (e.g., microbe) but not to another (e.g., humans)
  • Resistance
    • Acquired lack of sensitivity (resistance) to an antimicrobial agent should not be exhibited by successive generations of microbes
    • Sensitivity testing of bacterial and fungal cultures is important for proper therapy of infected persons
• Mechanisms of Action and Examples
  • Antimetabolites block metabolic reactions - sulfa drugs, INH (isoniazid)
  • Antibiotics interfere with:
    • Membrane function ... both outer and cytoplasmic - nystatin, polymyxin B
    • Cell wall synthesis - penicillin, bacitracin, vancomycin
    • Protein synthesis - streptomycin, tetracycline, chloramphenicol
    • Nucleic acid synthesis - naladixic acid, rifampin