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The Control of Microbial Growth

©2000 Kenneth Todar, University of Wisconsin-Madison

Introduction

The control of microbial growth is necessary in many practical situations, and significant advances in agriculture, medicine, and food science have been made through study of this area of microbiology.

"Control of growth", as used here, means to prevent growth of microorganisms. This control is effected in two basic ways:

  • By killing microorganisms or

  • By inhibiting the growth of microorganisms.

Control of growth usually involves the use of physical or chemical agents which either kill or prevent the growth of microorganisms. Agents which kill cells are called cidal agents; agents which inhibit the growth of cells (without killing them) are referred to as static agents. Thus the term bactericidal refers to killing bacteria and bacteriostatic refers to inhibiting the growth of bacterial cells. A bactericide kills bacteria, a fungicide kills fungi, and so on.

Sterilization is the complete destruction or elimination of all viable organisms (in or on an object being sterilized). There are no degrees of sterilization: an object is either sterile or not. Sterilization procedures involve the use of heat, radiation or chemicals, or physical removal of cells.

Methods of Sterilization

Heat: most important and widely used. For sterilization always consider type of heat, time of application and temperature to ensure destruction of all microorganisms. Endospores of bacteria are considered the most thermoduric of all cells so their destruction guarantees sterility.

  1. Incineration: burns organisms and physically destroys them. Used for needles , inoculating wires, glassware, etc. and objects not destroyed in the incineration process.
  2. Boiling: 100°C for 30 minutes. Kills everything except some endospores (Actually, for the purposes of purifying drinking water 100°C for five minutes is probably adequate though there have been some reports that Giardia cysts can survive this process). To kill endospores, and therefore sterilize the solution, very long or intermittent boiling is required.
  3. Autoclaving (steam under pressure or pressure cooker): 121°C for 15 minutes (15#/in2 pressure). Good for sterilizing almost anything, but heat-labile substances will be denatured or destroyed.
  4. Dry heat (hot air oven): 160°C/2hours or 170°C/1hour. Used for glassware, metal, and objects that won't melt.

The protocol and recommendations for the use of heat to control microbial growth are given in Table 1.

Table 1. Recommended use of heat to control bacterial growth
TreatmentTemperatureEffectiveness
Incineration>500°CVaporizes organic material on nonflammable surfaces but may destroy many substances in the process
Boiling100°C30 minutes of boiling kills microbial pathogens and vegetative forms of bacteria but may not kill bacterial endospores
Intermittent boiling100°CThree 30-minute intervals of boiling, followed by periods of cooling kills bacterial endospores
Autoclave and pressure cooker (steam under pressure)121°C/15 minutes at 15# pressurekills all forms of life including bacterial endospores. The substance being sterilized must be maintained at the effective T for the full time
Dry heat (hot air oven)160°C/2 hoursFor materials that must remain dry and which are not destroyed at T between 121°C and 170°C Good for glassware, metal, not plastic or rubber items
Dry heat (hot air oven) 170°C/1 hourSame as above. Note increasing T by 10 degrees shortens the sterilizing time by 50 percent
Pasteurization (batch method)63°C/30 minuteskills most vegetative bacterial cells including pathogens such as streptococci, staphylococci and Mycobacterium tuberculosis
Pasteurization (flash method)72°C/15 secondsEffect on bacterial cells similar to batch method; for milk, this method is more conducive to industry and has fewer undesirable effects on quality or taste

Irradiation: usually destroys or distorts nucleic acids. Ultraviolet light is usually used (commonly used to sterilize the surfaces of objects), although x-rays and microwaves are possibly useful.

Filtration: involvres the physical removal (exclusion) of all cells in a liquid or gas, especially important to sterilize solutions which would be denatured by heat (e.g. antibiotics, injectable drugs, amino acids, vitamins, etc.)

Chemical and gas: (formaldehyde, glutaraldehyde, ethylene oxide) toxic chemicals kill all forms of life in a specialized gas chamber.

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