Microbial Control - Physical Means

Nestor T. Hilvano, M.D., M.P.H.

Learning Objectives You should be able to: 1. Contrast sterilization, disinfection, antisepsis, sanitization

, and pasteurization . 2. Compare the effects of "-static" versus "-cidal" control

agents on microbial growth. 3. Define microbial death rate, and describe its significance

in microbial control. 4. Describe how antimicrobial agents act against cell

boundaries, proteins, and nucleic acids. 5. Identify the three most-resistant groups of microbes, and

explain why they are resistant to many antimicrobial agents.

6. Describe types of physical methods of microbial control and discuss their advantages and disadvantages.

Microbial Control• Antisepsis – reduction in number of microorganisms and

viruses on living tissue; ex: alcohol, iodine • Disinfection – destruction of most microorganisms and

viruses on non-living tissue; ex: alcohol, phenol • Pasteurization – use of heat to destroy pathogen; ex:

pasteurized milk and fruit juice• Sanitation – removal of pathogens from objects to meet

public health standards; ex: washing table wares in restaurants

• Sterilization – destruction of all microorganisms and viruses in or on an object; by autoclave, incineration, or ethylene oxide gas; ex: preparing culture media

Microbial Death Rates• Microbial death – permanent loss of

reproductive ability • MDR is constant over time for any microbes

under a particular set of conditions • Ex in graph: 90%/min.; requires 9 mins. to

achieve sterilization

Physical Methods of Microbial Control

1. Heat – moist; dry

2. Low temperature – refrigeration; freezing

3. Filtration

4. Desiccation

5. Osmotic pressure

6. radiation

Heat

• Heat – oldest and common means• Thermal death point -lowest temp. that kills

all cells in10 mins.• Thermal death time – minimum time

required to kill all organisms at given temp. (ex: 22.5 mins)

• Decimal reduction time (D) - time required to kill 90%of organisms; useful in food processing to eliminate endospores of C. botulinum

Moist HeatMoist Heat – used to disinfect, sanitize, sterilize, & pasteurize:

1.Boiling – vegetative bacteria & fungi, trophozoites of protozoas,and most viruses in 10 mins.

2. Autoclave – uses steam heat under pressure (121˚C, 15 psi at 15 mins.) to sterilize chemicals and objects.

3.Pasteurization – heating food/beverages to kill pathogens and control spoilage organisms.

Dry Heat

• Dry Heat – at higher temp. for longer times:

1. Oven - 160˚C for 2 hrs., for powders and oils

2. bunsen burner

3. complete incineration

Refrigeration and Freezing

• Refrigeration = between 0˚C and 7˚C, halts growth of most pathogens

• Slow Freezing = below 0˚C , inhibits microbial metabolism; but many vegetitive bacterial cells, bacterial endospores, and viruses can survive for years

Desiccation & Lyophilization

• Desiccation – drying to preserve foods; inhibits growth because metabolism requires liquid water

• Lyophilization – freeze-drying; preserve microbes and other cells for many years; freeze culture in liquid nitrogen or frozen CO2, then remove water via a vacuum

Filtration & Osmotic Pressure

• Filtration – passage of air or liquid through material that traps and removes microbes; 0.3 µm for O.R.; 0.22 µm for bacteria; 0.01 µm for viruses

• Osmotic pressure – use of high concentration of salt/sugar; drawing out of water; ex. honey, jams, salted fish, and pickles

Radiation1. Ionizing radiation – wavelengths shorter than 1

nm; electron beams, gamma rays, X-rays; effects leading to denaturation (destroy DNA); medical/lab equipment sterilization

2. Nonionizing radiation – wavelengths longer than 1 nm; UV light, visible light, infrared light, radio waves; UV causes pyrimidine dimers which kills cells; sterilization of surfaces and of transparent fluids and gases

3. Microwaves – heat kills microbes; Trichinella spiralis in pork resist heating

Homework 1. Define terms: sterilization, disinfection, antisepsis,

sanitation, bacteriostatic, and bactericidal.2. Compare moist heat and dry heat method of microbial

control and discuss advantages and disadvantages. 3. Compare uses/advantages of microbial control of

desiccation and lyophilization; and filtration and osmotic pressure.

4. Compare ionizing and non-ionizing radiation. 5. What is microbial death rate? Decimal reduction time?