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Physical Control of Microbial Growth

Control With Heat, Filtration, Ultraviolet light, radiation, Preservation methods, and Ultrasonic vibrations


Physical Control of Microbial Growth:

1. complete destruction of all life forms, including spores; but does not necessarily destroy bacterial toxins
2.usually physical (not chemical) agents are used, especially for spores

Thermal death time & point

Physical Control of Microbial Growth:

1.depends on type of organisms and the material to be treated
2.each organism has a specific time @ a given temperature

Direct flame:

Physical Control of Microbial Growth:

1. Time for sterilization: most rapid method
2. Disposable objects: used for gowns, plastic, cadavers (in micro. we flame loops & test tubes)

Hot air oven: dry heat

Physical Control of Microbial Growth:

1. useful for water-free oily substances, dry powders, glassware & syringes
2. disadvantage: penetration may take a long time
3. effects are like baking
4. organic matter acts as an insulator so must be removed
5. requires 2 hours @ 160ºC to destroy spores

Boiling water: moist heat

Physical Control of Microbial Growth:

1. effects: penetrates better since water is a better conductor of heat than air
2. kills by denaturing proteins
3. not considered a sterilizing agent especially against spores or certain viruses (spores may require 2 hours to destroy)
4. again, organic matter must be removed

Autoclave: pressurized steam of 121ºC for 15-20 min.

Physical Control of Microbial Growth:

1. most dependable method for destruction of all life-forms, including spores
2. disadvantage: cannot be used for some plasticware (those that melt) and dulls instruments
3. useful for blankets, bedding, utensils
4. to check effectiveness, a culture strip containing Bacillus spores is included (later cultured)
5. prevacuum ........: similar method but air is drawn out to decrease processing time

Fractional sterilization

Physical Control of Microbial Growth:

1. employs 30 minutes of steam on three successive days
2. alternate names include intermittent sterilization or tyndalization
3. limited use: some spores fail to germinate or must be in growth media ( cannot guarantee)


Physical Control of Microbial Growth:

1. not considered sterilization since only reduce bacteria (not spores)
2. holding method employs 62.9ºC for 30 min.
3. flash method employs 71.6º C for 15 sec.
4. objectives: to kill tuberculosis & Coxiella burnetii (Q fever) bacteria
5. ultra..........: 82ºC for 3 secs

Hot Oil 160ºC for 1 hour

Physical Control of Microbial Growth:

1. useful for surgical & dental instruments since doesn't rust/dull instruments
2. disadvantage: when ......... is removed, object may be recontaminated


Physical Control of Microbial Growth:

used to remove microorganisms from solution

Ultraviolet light: uses light waves in the 265nm range to destroy microbes

Physical Control of Microbial Growth:

1. effect is on the DNA of the bacterial cell
2. can reduce airborne contamination effectively
3. can be used to sterilize an environmental surface or hospital room
4. disadvantages: does not penetrate liquids or solids and requires direct contact with object

types of radiation

Physical Control of Microbial Growth:

1. Laser beam: can destroy microorganisms in less than a second
2. Microwave oven: microwave energy causes friction between molecules to generate intense heat

Ultrasonic vibrations: high frequency sound waves

Physical Control of Microbial Growth:

1. cause cavities (like bubbles) in fluids that collapse & send out shock waves (called cavitation)
2. microorganisms destroyed by pressure this creates
3. disadvantage: must be a liquid medium

Preservation methods

Physical Control of Microbial Growth:

1. Drying: lack of water inhibits life
2. Salting: causes microorganisms to lose water by osmosis (causes them to crenate)
3. Low temperatures: reduces growth, but does not eliminate microorganisms


a) filter is made of porcelain or glass
b) useful for intravenous solutions, culture media, toxoids, pharmaceuticals, & beverages


a) filter is made of organic substance like diatomaceous earth (from marine algae)
b) advantage: organic material attracts other organic material (in microbes)
c) used in aquarium filters and laboratories


a) filter is a pad made of organic material
b) bacteria trapped on pad by filtration and form colonies on its surface after culturing
c) useful for water testing

Laser beam

type of radiation,
can destroy microorganisms in less than a second

Microwave oven

type of radiation,
.......... energy causes friction between molecules to generate intense heat


Preservation method
lack of water inhibits life


Preservation method
causes microorganisms to lose water by osmosis (causes them to crenate)

Low temperatures

Preservation method
reduces growth, but does not eliminate microorganisms


Chemical Control of Microorganisms
refers to removal/destruction of pathogens


Chemical Control of Microorganisms
destruction/removal of all life forms (including spores); rarely done with chemicals


Chemical Control of Microorganisms
Disinfectants vs. antiseptics:
living tissue


Chemical Control of Microorganisms
Disinfectants vs. antiseptics:
inanimate object


Chemical Control of Microorganisms
kills microorganisms


Chemical Control of Microorganisms
retards growth but doesn't necessarily kill


Chemical Control of Microorganisms
means contaminated with microorganisms


Chemical Control of Microorganisms
decreasing a microbial population to safe levels (based on local health standards)


Chemical Control of Microorganisms
removing microorganisms from the surface without penetrating (as in skin)

antiseptics and disinfectants

1. Desirable properties of......
a) must kill microorganisms
b) long shelf life
c) non-toxic
d) water soluble
e) quick acting
f) effective in low concentrations
g) good penetration
h) readily available
i) inexpensive

Phenol coefficient test

Evaluation of antiseptics and disinfectants
a numerical value assigned to an antiseptic or disinfectant based on a comparison with the effectiveness of phenol (the standard) under the same conditions

In-use test:

Evaluation of antiseptics and disinfectants
the antiseptic or disinfectant is used on a surface, the surface is swabbed and a culture is grown

Important Chemical Agents

Phenol and phenolic compounds
Heavy metals


Chemical Agent
commonly used for disinfection
(Chlorine and Iodine)


a) used in municipal water supplies for disinfection
b) sodium hypochlorite: bleach (5% solution)
c) chloramines: used in wound antisepsis & root canals


a) often found in tincture form
b) may be used to disinfect water
c) broad range of activity when used as a skin antiseptic
d) iodophors


non-staining iodine/detergent compounds with time-release action (Betadine, Ioprep & others)

Phenol and phenolic compounds :

B. the standard used to determine the effectiveness of a chemical agent
1. spectrum of activity: active against Gram-positive bacteria
2. action: probably by coagulating proteins
3. disadvantages: organic matter must be removed, caustic to skin, & expensive
4. Cresols
5. Bisphenols
a) Hexachlorophene
b) Chlorhexidine
c) Hexylresorcinol


popular in '50s and '60s in personal hygiene products


used as surgical scrub & superficial skin wound cleanser: may be purchased as Hibiclens


found in mouthwashes, topical antiseptics & throat lozenges


Heavy metal
a) disadvantage: highly toxic and requires removal of organic matter
b) combined with less toxic substances to produce Merthiolate, Mercurochrome & Metaphen
c) Merthiolate is used to preserve vaccines & sera


Heavy metal
a) acts against organisms containing chlorophyll
b) useful as algaecide in pools and water supplies


Heavy metals

a) silver nitrate is useful antiseptic & disinfectant
b) used to prevent gonorrhea in the eyes of newborns & to disinfect suturing threads


used as skin antiseptic & instrument disinfectant



a) not effective against spore, only vegetative cells
b) dentures proteins, dissolves lipids & dehydrates
c) used in 50-80% solution to prevent rapid evaporation
d) organic matter must be removed



a) often used in 70% solution (found in stores as such)
b) removes skin oils (cleanses) and bacteria

Formaldehyde: solid at room temperature

1. Formalin: 37g of solid dissolved in 100ml of water
2. used for embalming & as disinfectant
3. used to weaken viruses to produce vaccines and to produce toxoids
4. acts by inserting an alkyl group into certain regions on nucleic acids & proteins (called an alkylating agent)

Ethylene oxide

1. chemical counterpart of the autoclave for sterilization
2. is useful for disinfecting plastics which melt in the autoclave
3. penetrates well and is a good sporicidal
4. disadvantage: is toxic & explosive: may cause "cold burns'" of the skin


1. used in 2% solution
2. advantage: active in the presence of organic matter & does not damage objects being treated
3. disadvantage: emits irritating fumes, requires rinsing of treated instruments
4. effective against vegetative cells & spores

Hydrogen peroxide

1. used on wounds, scrapes, & abrasions
2. six hours exposure in a six percent solution may effect sterilization
3. catalase in tissues encourages its digestion to end products
4. foaming action assists in mechanical removal of microorganisms
5. may inhibit anaerobic bacteria in wounds

Soaps & Detergents

1. aid in mechanical removal of microorganisms through activity as wetting agents (emulsify particles & remove skin oils)
2. anionic (negatively charges): somewhat useful against Gram-positives
a) use limited to laundry products since negative charges in bacterial cells repel them
3. cationic(positively charges): compounds often referred to as quats (quaternary ammonium compounds)
a) bacteriostatic against wide range of bacteria, especially Gram-positives
b) used for utensil, skin antiseptics, mouthwashes, hospital floors & walls and so forth


malachite green & crystal violet
acridine dyes:
gentian violet:

malachite green & crystal violet

useful against members of the genera Bacillus and Staphylococcus

acridine dyes

used for staphylococcal infections in wounds

gentian violet

used against trench mouth & thrush


1. benzoic, salicylic & undecylenic are used for fungal infections

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