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Chapter 5: Control of Microbial Growth. Important Point:. If you are having trouble understanding lecture material: Try reading your text before attending lectures. And take the time to read it well!. Methods are employed to destroy, remove, and/or inhibit the growth of microorganisms.

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Chapter 5: Control of Microbial Growth

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important point
Important Point:

If you are having trouble

understanding lecture material:

Try reading your text

before attending lectures.

And take the time to read it well!

control of microbial growth

Methods are employed to destroy, remove, and/or inhibit the growth of microorganisms.

  • This is either on inanimate objects or on body surfaces.
  • Most of these approaches are non-selective meaning that they can negatively impact a diversity of microbial (and non-microbial) life forms.
  • We can distinguish these microbial control methods into Physical and Chemical methods.
  • “…simple hand washing with plain soap and water is considered the simple most important step in preventing the spread of many infectious diseases. Plain soap itself generally does not destroy many organisms; it simply aids in the mechanical removal of transient microbes, including most pathogens...”
Control of Microbial Growth
important terms5
Important Terms

I’m not sure why these Ss are missing!

nosocomial infections

“Minimizing the numbers of microorganisms in a hospital is particularly important because of the danger

of hospital-acquired, or nosocomial, infections. Hospitalized patients are often more susceptible to

infectious agents because of their weakened condition. In addition, patients may be subject to invasive

procedures such as surgery, which breaches the intact skin that would otherwise help prevent infection.

Finally, pathogens are more likely to be found in hospitals because of the high concentration of patients

with infectious disease. These patients may shed pathogens in their feces, urine, respiratory droplets, or

other body secretions. Thus, hospitals must be scrupulous in their control of microorganisms. Nowhere

is this more important than in the operating room, where instruments used in invasive procedures must

be sterile to avoid introducing even normally benign microbes into deep body tissue where they could easily establish infection.”

Nosocomial Infections
aseptic technique

“To work with pure cultures, all media and instruments that contact the culture must first be rendered sterile to avoid contaminating the culture with environmental bacteria.”

  • “All materials used to grow microorganisms must again be treated before disposal to avoid contamination of workers and the environment.”
  • “The use of specific methods to exclude contaminating microorganisms from an environment is called aseptic technique.”
  • “Although all microbiology laboratory personnel must use these prudent measures, those who work with known disease-causing microbes must be even more diligent.”
Aseptic Technique
killing resistant microorganisms
Killing-Resistant Microorganisms
  • Endospore formers (endospores): Bacillus spp. & Clostridium spp. are highly heat- and chemical-treatment resistant.
  • Mycobacterium spp. are highly chemical-treatment resistant.
  • Certain Pseudomonas spp. can actually grow in certain disinfectants (i.e., in quats and iodophores).
  • Naked viruses (i.e., those lacking lipid envelopes) often are resistant to chemical disinfection.
exponential death

“It takes more time to kill a large population of bacteria than it does to kill a small population, because only a fraction of organisms die during a given time interval.

Exponential Death
more terms concepts

Decimal Reduction Time (D value) = the time it takes to kill 90% of a bacterial population (1 log reduction) under specific conditions.

  • Note importance of temperature: higher temperatures can result in dramatically faster disinfection.
  • Dirt and organic compounds can interfere with disinfection—if possible it consequently is important to clean items well before disinfecting or sterilizing (which also will reduced organism number).
  • Obviously the above is not practical if one is disinfecting in order to avoid contacting an item.
  • Critical Items penetrate into body tissues.
  • Semicritical Items come into contact with but don’t penetrate mucous membranes.
  • Non-Critical Items come into contact only with unbroken skin.
More Terms & Concepts
using heat to control microbes12
Using Heat to Control Microbes

10 min boiling (at sea level).

121°C for 15 min to kill endospores (but must be penetrating, moist heat, and 15 min at 121°C)

categories of germicides
Categories of Germicides
  • “To perform properly, germicides must be used strictly according to the manufacturer’s directions, especially as they relate to dilution, temperature, and the amount of time they must be in contact with the object being treated.”
  • “It is extremely important that the object be thoroughly cleaned and free of organic material before the germicidal procedure is begun.”
categories of germicides15
Categories of Germicides

But don’t use scented chlorine bleach to purify water!

Note that temperature can affect rate of water purification (i.e., cold water purifies slowly).

categories of germicides16
Categories of Germicides

Today iodine is more often used as an iodophore which is a less-irritating, slow-release form of iodine.

Iodine typically is used as a tincture (i.e., dissolved in alcohol).

Watch out because Pseudomonas spp. can live and grow in some iodophores!


“Formulae: Fresh juice of Organic Habanero peppers, New Mexico Jalapeno, African Bird peppers and Hatch Chili peppers.

  • Dosage: Five to thirty drops, three times daily.  CAUTION ~ EXTREMELY HOT!!
  • Therapeutic Action: Cayenne is the greatest herbal aid to circulation and should be used on a regular basis.  The extract is very concentrated and gets into the bloodstream quickly and makes it a perfect first aid remedy for heart attacks, stroke, fainting, shock, dizziness, hemorrhage, internal and external bleeding.  Use a few drops to 10 droppers full.  It has saved many lives.
categories of germicides20
Categories of Germicides

However, HOOH is still useful for supplying oxygen to otherwise anaerobic environments.

irradiation uv and gamma
Irradiation: UV and Gamma
  • Gamma radiation is penetrating and effective but not always practical (or legal) to use.
  • UV radiation is not terribly penetrating but generally is safe (and legal) to employ.
  • UV disinfection generally is limited to surfaces and shallow (and UV transparent) aqueous solutions (e.g., water).
  • Neither is terribly effective against bacterial endospores.
  • Contact with either is hazardous to human tissues.
food preservation

Chemical preservatives.

  • Low-temperature storage (freezing works better than above freezing).
  • Removing water by submerging in highly osmotic solutions (usually high salt or high sugar).
  • Desiccation = drying.
  • Lyophilization = freeze drying.
  • All of above generally are better at preventing microbial growth than they are at outright killing microbes, so some care must be taken upon reversal of these food-preservation conditions.
  • One also must consider that removing microorganisms from foods can make food more susceptible to subsequent microbial contamination.
  • An important example of the latter is staphylococcal food poisoning which can result when Staphylococcus aureus is allowed to incubate in only moderately salt-preserved foods such as potato salad.
Food Preservation