Question

Microbial Growth Control- Bacteriology

-Briefly describe factors affecting optimal growth temperatures

-Compare & contrast ways in which microbial cells are enumerated.

-Describe/define both LD and MIC

Answer 1

Microorganisms are ubiquitous. Since microorganisms are responsible for contamination, infection and decay it becomes necessary to destroy them from materials or areas. The control of microbial growth is achieved either by killing of microorganisms or by inhibiting the growth of microorganisms.

Microorganisms are controlled by Physical or Chemical Agent.

Physical agents include sunlight, drying, dry heat ( flaming, incineration, hot air), moist heat (pasteurization, boiling, steam), filtration, radiation, Ultrasonic and sonic vibration.

Chemical agents include alcohols, aldehydes, dyes, halogen, phenol, surface active agent, metallic salts and gases.

Factors affecting optimal growth temperature of microorganisms: Environmental temperature is one of the most important factor affecting the growth of microorganisms.

Each microorganism have a minimum temperature below which no growth occurs, an optimum temperature at which growth is rapid and a maximum temperature above which growth is not possible.

Increase in temperature, the chemical and enzymatic reactions proceed in faster rate and further increase leads to protein damage. Decrease in temperature leads to inactivity of microbial cell.

Microorganisms are classified on the basis of growth temperature as follows : a) Psychrophiles - 0 to 15 degree Celsius.

b) Psychrotrophs - 20 to 30 degree Celsius.

c) Mesophiles -20 to 40 degree Celsius.

d) Thermophiles-45 to 80 degree Celsius.

e) Hyperthermophiles- 89 to 120 degree Celsius.  

Enumeration of microorganisms: The number of viable microbial cells. Various methods of enumeration are

1) Viable count or plate count :

The sample is serially diluted and plated on agar surface . The number of colonies counted.

Colony forming units ( cfu) : The number of colonies ( 30-300) × dilution factor of the plate counted.

2) Direct Microscopic Count

3) Turbidity