Fundamentals of Control

Control--elimination, inhibition, or killing microbial cells


Importance of Microbial control

            Principal reasons for control

1)      to prevent transmission of disease and infection

2)      to eradicate microbes from an infected host

3)      to prevent deterioration and spoilage

Microorganisms can be removed, inhibited, or killed by physical agents, physical processes or chemical agents

            Physical agent--condition or property which causes change

                        (temp., pressure, radiation, filters)

            Physical processes--procedure causing a change

                        (sterilization, incineration, and sanitation)

            Chemical agent--molecular substance which causes a reaction

                        (phenol, alcohols, chlorine, ethylene oxide)

Definition of Agents and Processes

            Sterilization--process of destroying all forms of life,

                        sterile objects are free of microbes (absolute)

Disinfectant--typically a chemical agent that kills the vegetative cells but

perhaps not the spores of pathogenic microbes (inanimate objects)

                        Antiseptic--substance that opposes infection or prevents the growth or

action of microbes by destroying them or inhibiting their growth and activity (body surface)

                        Sanitizer--agent that reduces the microbial population to levels

considered safe by public health requirements (99.9% effective)

                        Germicide (microbicide)--agent that kills the vegetative cells but not

necessarily the resistant spore forms (any application)

                        Bactericide--agent that kills vegetative forms of bacteria

                        Bacteriostasis--condition in which the growth of bacteria is inhibited

                        Antimicrobial agent--any agent which interferes with the growth and

metabolism of microbes (therapeutic agents)


Pattern and Rate of Bacterial Death--exponential/logarithmic


Conditions Influencing Antimicrobial Action

            Concentration or intensity of antimicrobial agent

            Number of microorganisms--more microbes require more time


1)      chemical agents damage microbes through chemical reactions

2)      rate of a chemical reaction is accelerated by increased temperature

Species of microorganism

            resistant spores

            susceptibility of vegetative cell to agents


Presence of organic material--inactivation/protection

1)      combining disinfectant with organic material to form a product which

is not microbicidal

2)      combining disinfectant with organic material to form a precipitate

3)      accumulation of organic matter on microbial cell surface, coating impairs disinfectant-cell contact

Acidity or alkalinity (pH)

            acidified bacteria are destroyed at lower temp. and shorter times


Mode of Action of Antimicrobial Agents

            Damage to cell wall (genesis or alteration)

            Alteration of cell permeability (inhibits growth)

            Alteration of protein and nucleic acid molecules

                        denaturing by temp. and concentration

            Inhibition of enzyme action (terminates or slows metabolism)

            Inhibition of nucleic acids and protein synthesis

Selection of Antimicrobial agents or techniques

            Each situation must be assessed in terms of result desired and the agent or method

that will best achieve this result




            High temperatures--humid and arid methods

                        Moist heat kills by coagulating proteins

                        Arid heat kills by oxidizing chemical constituents

                        Vegetative cells more sensitive to heat than spores

                                    moist heat 5-10 min. at 60-70 C

                                    spores--100 C extended times

                        Thermal death time--shortest period time required to kill a suspension of

 bacteria (or spores) at a given temp.  and under specific conditions

                        Decimal reduction time--time required to reduce the bacterial population

by 90%

            time in minutes for the thermal death time

            curve to pass through one logarithmic decrease


Application of high temp.  for killing microorganisms

            Moist heat--saturated steam under pressure--higher temp.

                        Advantages:  rapid heating, penetration, and abundant moisture to

 facilitate protein coagulation

                        Essential to remove air from autoclave  (raises temp.)

                        15 lbs / sq in at 121 C

                        Fractional sterilization--successive introductions to steam  (100 C) spores

 have time to germinate between treatments

                        Boiling water--vegetative cells destroyed in 10 min.

                                    spores can withstand treatment for hours

                                    disinfects rather than sterilizes

                        Pasteurization--temp. based on thermal death time of

 most resistant pathogen

            Mycobacterium and Rickettsial organisms

            62.8 C fro 30 min.

            Dry heat

                        Hot-air sterilization--2 hours at 160 C



Low temperatures--metabolism and growth cease--good preservation

            Refrigeration--viabile for months at 4-7 C

            Subzero temperatures

                        mechanical freezers:  -20 C

                        dry ice / Carbon dioxide:  -70 C

                        liquid nitrogen:  -195 C

                        mortality upon freezing and thawing


Desiccation--drying or microbe and its environment which greatly

reduces metabolic activity

                        Survival after desiccation depens upon:

1)      kind of microbe

2)      material in or on which the organism is dried

3)      completeness of the drying process

4)      conditions to which dried organism is exposed

Gram negative cocci are sensitive to deiccation--Neisseria

Gram positive cocci are resistant to desicction-Strepto-

Lyophilization--“freeze drying”

Osmotic pressure         

            hypertonic solutions cause plasmolysis (atrophy)

            hypotonic solutions cause plasmoptysis (hypertrophy)

            The mechanism of microbial inhibition is plasmolysis:  the cells are dehydrated

 and hence are unable to metabolize or grow


Radiation--lethal to microbes as well as other organisms


Ultraviolet light 265 nm (solar radiation 280-390 nm)

                        low ability to penetrate matter

                        most significantly absorbed by nucleic acids


X-rays--considerable energy and penetration ability


Gamma rays--more energetic than x-rays (Co isotopes) used in sterilization of

 thick and volumous materials


Cathode ray tube (electron beam radiation)--high intensity

                        lower power of penetration--brief exposures

                        used on surgical equipment and drugs

            Filtration--used on thermolabile substances

                        Bacteriological filters--evolved from asbestos-porcelain-diatomaceous

 earth-sintered glass to a polymeric

membrane filter (pores ranging from .01 to 10 um dia.)

                        Air filters--HEPA (high efficiency particulate air filters)

                                    used in combination with laminar airflow systems

                        Face masks--limited protection

            Physical cleaning