Control of Microbial Growth

Sterilization

• 100% destruction of all living microorganisms, including spores

• Exception:

  • Commercial sterilization

    • Only eradication of spoilage and pathogenic microorganisms

    • Limited heat treatment → otherwise, food will lose taste and texture

    • Canned foods → treated for C.botulinum spores but not all spores

Disinfection

• Control directed at destroying harmful microorganisms, especially pathogens

  • Use of chemical and physical methods

Antisepsis

• Disinfection directed at living tissue

  • “Antiseptics”

Degerming

• Physical removal of microorganisms

  • Handwashing, babywipes

Sanitization

• Lowering # of microorganisms to a safe levels → public health issue

  • Glassware and dishwashers in bars

Bacteriostatic

• Stops bacterial growth, once removed the bacteria can start to grow again

Bactericidal

• Causing bacterial death

What factors affect bacterial death rate?

• The number of microbes

• Environmental conditions

  • In media or environmental samples

  • Temperature

  • pH

• Time of exposure

• Microbial characteristics

Microbial Characteristic

• Many bactericides are more effective against gram-positive bacteria

• Resistance of gram-negative bacteria is likely due to lipopolysaccharides in their outer membrane

• Pseudomonas and Burkholderia are unusually resistant to many bactericides and antibiotics

  • Porins → selectively allow molecules into cells

Modes of Action

• Protein denaturation

  • Enzymes and their tertiary structure

• Nucleic acid damage

• Damage to lipids and proteins in cell membrane

  • Disruption of the membrane

Physical Methods of Control

• Temperature

  • Heat

  • Cold

• Desiccation

• Filtration

• Osmotic pressure

• Radiation

Temperature

• Heat:

  • Moist heat

    • Boiling - 100 degrees celsius for 10 minutes

      • Kills vegetative cells (bacteria, fungi, protozoa, some viruses) Doesn’t kill endospores

    • Autoclave/steaming -

      • Heat & steam, 100 degrees celsius, 20-30 minutes. Kills everything except prions

    • Pasteurization

  • Dry heat

    • Ovens

    • Incineration

      • Flaming

• Cold:

  • Refrigeration -

    • 0 to 7 degrees celsius

    • Bacteriostatic, psychrophilic organisms can survive

  • Freezing -

    • < o degrees celsius

    • Slow freezing → crystals that form disrupt cellular structure

    • Freezing and thawing is more damaging

Osmotic Pressure

• High concentrations of salt & sugar

• Hypertonic environment

  • Water leaves microbial cells

• Used for food preservation

  • NOT antifungal (mold)

Dessication

• Drying

• Bacteriostatic

  • No growth/reproduction - can remain viable

• Spores and viruses remain viable

• Resistance to desiccation varies from organism to organism

  • Neisseria gonorrhoeae (1 hour)

  • Mycobacterium Tuberculosis (Months)

Radiation

• Ionizing radiation:

  • y-rays, x-rays

  • Used to treat food (Like produce)

  • Pharmaceuticals, dental supplies, and plastic medical supplies

• Nonionizing UV radiation:

  • Strongly bactericidal

  • Damages DNA

  • Kills organisms in the air

  • Harmful to our skin + eyes

Chemical Methods of Control

• Living tissue/inanimate objects

• Most reduce the # of microorganisms to safe levels

  • Or remove vegetative forms of pathogens

• No single disinfectant is good for all things

  • Label specifies what organisms it can combat

  • Concentration

• Nature of the material being disinfected

  • Organic matter, blood and vomit, may interfere w/ the effectiveness of the disinfectant

• Phenols, alcohols, halogens, metals, aldehydes, and oxidizers

Phenols

• Denatures proteins and disrupts lipid membranes

• Effective in dense organic material

• Active for a long time

  • Throat sprays, Lysol

• Cons:

  • Corrosive at high concentration

  • Irritant

  • Mutative

• Bisphenols - Derivatives of phenols

  • Hexachlorophene

    • Surgical lotion

  • Triclosan

Alcohols

• Denature proteins and dissolve lipids

• Effective against:

  • Bacteria, fungi, and viruses

• Ineffective against:

  • Endospores and fungal spores

• Effective concentration = 60-70%

Halogens

• Destabilize cellular macromolecules

• Effective against:

  • Vegetative bacteria, bacterial + fungal spores, and protozoan cysts

• Examples -

  • Iodine -

    • Oldest and most effective antiseptic

    • Skin wounds

    • Impairs cell membrane synthesis

  • Chlorine -

    • Bleach - NaOCl

    • Can clean drinking water w/ 1-2 drops after 30 min

Metals

• Denatures proteins

• Examples:

  • Ag - 1% silver nitrate - eyes of newborns to prevent infection

  • Hg -

    • Mercuric chloride (very toxic)

    • Merbromin was used as a topical antiseptic → not used anymore because of its mercury content

  • Cu -

    • Water based treatments to get rid of alagae

    • Xgel hand sanitizers

Aldehydes

• Denatures proteins

• Damages DNA + RNA

• Examples:

  • Formaldehyde -

    • Formalin (37%) preserves biological samples

    • Not used on living tissue → irritant and carcinogenic

  • Glutaraldehyde -

    • Less irritating and more effective than formaldehyde

    • Disinfectant in hospitals (surgical equipment)

    • 3-10 hours kill endospores

Oxidizers

• Oxidizing agents

• Effective against anaerobes

  • Deep wounds

• High conc. can kill spores on surfaces

• Examples:

  • Peroxides, H2O2

  • Peracetic Acid

Evaluating disinfectants

• Dilution test -

  • Meta/glass beads dipped in bacteria and then dried

  • Dried cultures are placed in disinfectant for 10-20 minutes

  • Beads were then transferred to culture media to see if bacteria survived

• Disk diffusion method -

  • Disk filters soaked w/ the chemical being tested

  • Placed on agar plates already inoculated w/ bacteria tested

  • Incubated and analyzed for bacterial growth