Topic 6: Control of Microbial Growth

Question 1

Why do we need microbial control?

Answer 1

1. Prevent cross contamination of cultures in a lab
2. Prevent contamination of equipment used in medical and food prep procedures.
3. To increase the shelf life of food and beverages.
4. To limit the detrimental effect the microbes or their metabolic products would have on us and our food.

Question 2

Microbial Growth Control Terms

Answer 2

Sterilisation: Killing or removal of all microoganisms
Commercial Sterilisation: Killing C. botulinum ensospores (no pathogens left, growth stopped).
Disinfection: Removing pathogens (inanimate objects).
Antisepsis: Removing pathogens from living tissue
Degerming: Removing microbes in a limited area
Sanitisation: Lowering microbial counts, e.g. on eating utensils.
Biocide/germicide: Kills microbes
Bateriostasis: inhibiting, not killing, microbes
Sepsis: refers to microbial contamination.
Asepsis: is the absence of significant contamination
Aseptic surgery: techniques prevent microbial contamination of wounds.

Question 3

Effectiveness of Treatment

Answer 3

Depends on

• Number of microbes
• Environment (organic matter, temperature, biofilms)
• Time of exposure
• Microbial characteristics

Question 4

Actions of Microbial Control Agents

Answer 4

1. Alteration of membrane permeability
2. Damage to proteins
3. Damage to nucleic acids

Question 5

Physical control of mechanisms

Answer 5

• Temerature +/- Moisture
• Filtration
• Desiccation
• Osmotic Pressure
• High Pressure
• Radiation

Question 6

Heat

Answer 6

• Denatures and coagulates proteins
• Microbes differ in their resistance to the affects of temperature.
• Thermal death point (TDP): Lowest temperature at which all cells in a culture are killed in 10 min.
• Thermal death time (TDT): Time during which all cells in a culture are killed.

Question 7

Decimal Reduction Time (DRT)

Answer 7

DRT = Time in (MInutes) to kill 90% of a population at a given temperature.

Question 8

Autoclaving

Answer 8

• Moist heat (steam) under pressure
• temperature acting on items is raised by increasing the pressure around the item
• 15 psi, 121 degrees Celsius @ 15min - kills all (except prions).
• Sterilisation depends on volume
• 10ml- 15min
• 6L- 70min

Question 9

Steam Sterilisation

Answer 9

-Steam must contact the item’s surface.

Question 10

Dry Heat Sterilisation

Answer 10

• Kills by oxidation
• Flaming
• Incineration
• Hot air sterilisation

Question 11

Pasteurisation

Answer 11

• Kills pathogens and reduces spoilage organisms
• Equivalent treatments
• 63 degrees Celsius for 30 minutes
• High temperature short time: 72 degrees Celsius @15sec
• Ultra High Temperature: 140 degrees Celsius @<1sec

*Thermoduric (heat resistant) organisms can survive and cause spoilage

Question 12

Low Temperatures

Answer 12

• Refrigeration:
  1. Slows metabolic activity
  2. Generally bacteriostactic, rather than bacteriocidal.
• Freezing
  1. Slow Freezing more effective than flash freezing as forming ice crystals disrupts the cells.
  2. Thawing more damaging than freezing.
• Freeze drying:
  1. Long term preservation
  2. Decrease water availability.

Question 13

Filtration

Answer 13

• HEPA: removes microbes >0.3um
• Membrane filtration removes microbes > 0.22um
• Does not inactivate any metabolic products e.g. toxins.
• Will not remove viruses.

Question 14

Disk diffusion method

Answer 14

• Will determine the most strongest antibiotic to combat a microbe.
  e. g. Streptococcus pyogenes grown on HBA plate, with antibiotic discs.

Question 15

Phenol and Phenolics

Answer 15

• Disrupt plasma membranes

e. g. Phenol and O-Phenylphenol

Question 16

Bisphenols

Answer 16

• Hexachlorophene, triclosan

- Disrupt plasma membranes.

Question 17

Biguanides

Answer 17

• Biguanide can refer to a a molecule, or to a class of drugs based upon this molecule.
• Acts on membranes by blocking lipid synthesis and disrupting plasma membranes.
• Often used in combination with other agents
• Antiseptic uses
• Disinfectant uses
• Preservative uses
• Example - Chlorhexidine

Question 18

Surface-Active Agents or Surfactants

Answer 18

-Soap: Degerming (cleaning/washing)
-Acid-anionic detergents: Sanitizing
-Quarternary ammonium compunds (cationic detergents):
Batericidal, denature proteins, disrupt plasma membrane.

Question 19

Quats

Answer 19

• Quaternary ammonium compounds
• Effective against bacteria, fungi, viruses (enveloped and non) and amoebae.
• Do not kill endospores or mycobacteria
• Widely used in both home and industry
• Examples- Benzalkonium chloride (Lysol), Cetylpyridinium chloride (Cepacol).

Question 20

Microbial Resistance

Answer 20

-Most Resistant (Decreasing Order)
Prions
Endospores of Bacteria
Cysts of Protozoa 
Vegetative protozoa 
Gram Negative Bacteria 
Fungi, including most fungal spores 
Viruses without envelopes
Gram Positive Bacteria
Viruses with lipid envelopes
Least resistant

Question 21

Aseptic Technique

Answer 21

-A set of practices and procedures performed under carefully controlled conditions with the aim of minimising contamination by unwanted pathogens or cells.

e.g.
Culturing Microoraganisms 
Cell structure 
Sterile liquid transfers 
Surgery
Wound care.

Question 22

Ehrlich and Fleming’s contribution to antibiotics

Answer 22

• Paul Ehrlich, discovered Salvarsan
• Ehrlich came up with the concept that different organisms have different types of cells and therefore should have unique drug targets within those cells. “Magic Bullet” idea - kill an invading pathogen, but leave us alive (Salvarsan, 1910).
• Fleming Discovered the most famous of all magic bullets, penicillin in 1928. Florey and Chain were able to develop this for use in the second world war. Modern Antibiotic era was born.