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Control of Microbial Growth

Prepared by Dr. Jhason John J. Cabigon

A. Factors Influencing Microbial Growth

1. Availability of Nutrients

2. Moisture- water is essential to life; endospores and cysts can resist desiccation

3. Temperature

- every microbe has an optimum growth temperature- Thermophiles: heat loving; hyperthermophiles: extreme thermophile

- Mesophiles: grow at moderate temperatures

- Psychrophiles: prefer cold temperatures

- Psychoduric organisms: prefer warm temperatures but can toleratevery cold temperatures

4. pH

- most microbes prefer neutral or slightly alkaline growth medium

- acidophiles: can live in stomach and pickled foods; includes fungi- alkaliphiles: prefer alkaline environment, like intestines; include

Vibrio cholerae5. Osmotic Pressure and Salinity

- Osmotic Pressure: the pressure that is exerted on a cell membrane by

solutions both inside and outside the cell- Isotonic solution: equal concentration of solutes inside and outside the

cell; no movement of water

- Hypertonic solution: the concentration of solutes in the environment is

greater than the concentration of solutes inside the cell; water willleave the cell by osmosis to equalize the concentrations; can cause

crenation (cell shrinkage) and plasmolysis (cell membrane andcytoplasm shrink away from the cell wall)- Hypotonic solution: the concentration of solutes inside the cell is

greater than the concentration of solutes in the environment; water

enters the cell; can cause lysis (cell bursting) and plasmoptysis (escapeof cytoplasm from the cell)

- Halophiles: salt loving

- Haloduric organisms: do not prefer salty environment but are capable

of surviving there6. Barometric Pressure

- Barophiles: pressure loving

7. Gaseous Atmosphere- aerobes vs anaerobes

B. Physical Methods to Inhibit Microbial Growth

1. Heat most practical, efficient and inexpensive method of sterilization;thermal death point (TDP) and thermal death time (TDT)

a. Dry Heat baking and incineration; can also use electrical

heating device

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b. Moist Heat faster and more effective; boiling and steaming;

can use autoclaves

2. Cold refrigeration slows the growth of microbes; slow freezing causesice crystals to form within cells and may rupture the cell membranes and

cell walls; rapid freezing puts bacteria into suspended animation

3. Desiccation drying and lyophilization (freeze-drying)4. Radiation use of UV lamp/Germicidal lamp; gamma (cobalt-60), beta

and X-rays are also used

5. Ultrasonic Waves sound waves are used to mechanically dislodgeorganic debris on instruments and glassware

6. Filtration use Millipore filters to filter microorganisms from liquids;

High-efficiency Particulate Air (HEPA) filters to prevent air

contamination7. Gaseous Atmosphere aerobes vs anaerobes; use of hyperbaric oxygen

chamber in treatment of Gas gangrene

C. Chemical Methods to Inhibit Microbial Growth

1. Disinfectants chemical agents to eliminate pathogens* Factors affecting effectiveness of disinfectant:

a. Prior cleaning of the object to be disinfectedb. Organic load

c. Bioburden number of microbes and presence of spores

d. Concentration of disinfectante. Contact time

f. Physical nature of Object being disinfected

g. Temperature and pH

2. Antiseptics used to disinfect skin and other living tissues

D. Aseptic Technique

1. Asepsis absence of pathogens

2. Aseptic technique employed to eliminate and exclude pathogens;handwashing, use of sterile gloves, masks and gowns, sterilization of

surgical instruments, use of disinfectants and antiseptics

3. Antiseptic technique type of aseptic technique; use of antiseptics