Growth Control

1
Growth Control

• Chapter 7, part 1

2
The Control of Microbial Growth

• Sepsis refers to microbial contamination.
• Asepsis is the absence of significant
  contamination.
• Aseptic technique is what we used in the lab to
  prevent contamination with unwanted
  microorganisms.
• Aseptic surgery techniques prevent microbial
  contamination of wounds.

3
TERMINOLOGY
4
TERMINOLOGY

• Biocide/Germicide
• Cide kill
• Homocide, suicide
• Kills microorganisms
• Bacteriostatic
• Inhibits growth of microorganisms
• Why do we need to take a full course of
  antibiotics?

5

• Bacterial populations die at a constant
  logarithmic rate.

For each minute of treatment, 90 of the
remaining population is killed Time is plotted
on a linear scale The number of cells is
plotted on a log scale (logarithmic scale).
Figure 7.1a
6
Effectiveness of Antimicrobial Treatment

• Depends on
• Number of containing microbes more microbes, the
  longer it takes to kill them
• Environmental influence (organic matter, pH,
  temperature)
• Length or time of exposure
• More time for endospores or more resistant
  microbes
• Microbial characteristics

Figure 7.1b
7
Actions of Microbial Control Agents

• Alteration of plasma membrane
• Loss of permeability barrier
• Damage to proteins (enzymes)
• Metabolism and transport disrupted
• Damage to nucleic acids (DNA)
• Cell cannot replicate or produce new enzymes
• Cell Wall
• Lysis

8
Physical Methods of Microbial Control

• Heat
• Kills microorganisms by denaturing their enzymes
• Change the 3D structure
• Heat resistance varies between organisms
• Thermal death point (TDP) Lowest temperature at
  which all cells in a culture are killed in 10
  minutes
• Thermal death time (TDT) Time to kill all cells
  in a culture at a given temperature

9
Decimal Reduction Time (DRT)

• Time, in minutes, to kill 90 of a population at
  a given temperature

Table 7.2
10
Physical Methods of Control Heat

• Bacteriocidal
• Heat resistance by a population varies
• Factors that effect sterilization by heat
• Time
• Organism
• Temperature
• Concentration
• Presence of organic material
• pH
• Spores or no spores
• Type of heat
• Osmotic pressure

11
Moist Heat

• Denaturing enzymes
• Boiling (100C water)
• Usually 10 minutes for vegetative cells
• Virus and spore problems

12
Moist Heat

• Autoclave
• Steam under pressure
• Raising the pressure in the chamber raises the
  temperature of the steam
• 121ºC at 15 psi
• Can kill endospores and hepatitis
• 15 minutes
• Most popular sterilization technique in
  laboratory
• Our used cultures and plates are sterilized in
  this way.

13
Table 7.4
14
Different indicators used to show if autoclave
worked effectively
Figure 7.3
15
Look at pictures of autoclave (and video)
16
Moist Heat

• Pasteurization
• Reduces spoilage organisms and pathogens
• Killing food pathogens without affecting flavor
• Different times for different consistencies
• Fats can have a protective effect on
  microorganisms
• Phosphatase test
• Phosphatase is an enzyme present in milk
• If the product has been pasteurized, phosphatase
  will have been inactivated
• Classic pasteurization experiment 63C for 30
  min
• Today High temperature, short time 72C for 15
  sec
• Ultra high temperature 140C for lt1 sec
• Can be stored without refrigeration
• As temperature is increased, less time is needed
  to kill the same number of organisms

17
Physical Methods of Microbial Control

• Dry heat sterilization kills by oxidation
• Flaming
• Used during aseptic technique
• Hot-air sterilization
• Heat not transferred well in air
• More time than moist heat
• 170 C air for 2 hours
• Not used very often in labs