Physical and Chemical Control of

Microorganisms

Control of Microorganisms by Physical and Chemical Agents

Physical methods of microbial control

HeatFiltrationRefrigerationOsmotic PressureRadiationDesiccation

What is sterilization?

DefinitionsSterile: Free from any living organisms.

Sterilization: Process of killing or removing microorganisms from a product to insure that it is sterile.

Terminology and Methods of ControlSterilization : a process that destroys all

viable microbes, including viruses and endospores; microbicidal.

Disinfection : a process to destroy vegetative pathogens, not endospores; inanimate objects.

Antiseptic : disinfectants applied directly to exposed body surfaces.

Sanitization : any cleansing technique that mechanically removes microbes.

Degermation : reduces the number of microbes from a limited area.

Difference between sterilization and disinfection:

In sterilization there is complete destruction or removal of all bacteria & their spores, but in disinfection there is destruction of bacteria but not their spores

…protective clothing…hand washing…bench cleaning…loop flaming

Aseptic Techniques

Preparation for Sterilization

-All instruments must be double wrapped in special paper before sterilization. -The white stripes on the tape change to black when the appropriate conditions (temperature) have been met.

- Indicators should be on the inside and outside of equipment pack. - Expiration dates should be printed on all equipment packs.

Choice of a antimicrobial method depends on :

1 .Type of Microorganism :

Products contaminated with microorganisms more resistance to killing require a more heat or chemical treatment. Some of more resistant microbes include

–Bacterial endospores: Bacillus and Clostridium. They requireextreme heat or chemical treatment.

–Mycobacterium species: The waxy wall of these organisms makethem resistant to many chemical treatments so stronger and moretoxic disinfectants are required.

–Pseudomonas species: Resistant to common disinfectants and can sometime even grow in them

– viruses: viruses such as polio, are more resistant to disinfectants.

cont..Selection of an antimicrobial procedure

2 .Numbers of Microorganisms present:

– The time taken by a heating or chemical procedure to kill a population of microbes is depend on the number of organisms initially present.

–More time is required to kill a larger population than it does to kill a small population.

–Removing organisms by washing or scrubbing can minimize the time necessary to sterilize and disinfect a

product .

Sterilization

- Defined as a validated process used to render a product free from viable microorganisms.

-The presence of microorganisms on the individual items is expressed in terms of probability.

-While the probability may be reduced to a very low number, it can never be reduced to zero.

- The probability can be expressed as a Sterility Assurance Level (SAL), it means probability of a viable microorganism being present on the product unit after sterilization.

What it isTo ensure that validated sterilization processes provide the required Sterility Assurance Level (SAL) either during validation or for routine process monitoring, there is a wide range of biological indicator and product sterility testing capabilities.

Why you need it -In order to label a product as "sterile," specific requirements

for validation and routine monitoring of sterilization processes must be met .

-For most products, a theoretical SAL is set so the probability of a non-sterile (PNS) product making it through the validated sterilization process is one unit per million sterilized. This is

referred to as an SAL level of 10-6 .

Kinetics of microbial death (survivor curves)

Permanent loss of reproductive capability, even under optimum growth conditions.

This can be represented graphically with a survivor curve by plot log of survivors number against the exposure time or radiation dose.

A- continuous curve having a linear portionB- an initial shoulder curveC- Biphasic curve

Factors That Affect Death RateThe effectiveness of a particular agent is

governed by several factors:

Number and nature of microbes in the population

Temperature and time pH of environment Concentration or dosage of agent Mode of action of the agent Presence of solvents, organic matter, or

inhibitors

Thermal death point (TDP): Lowest temperature atwhich all cells in a culture are killed in 10 min. Timeis fixed, need to know temperature.

Thermal death time (TDT): Shortest time to kill all cells in a culture. Need to know time and temperature, related to population size.

Decimal reduction time (D-value): Minutes to kill 90% of a population at a given temperature.

High D value indicates greater heat resistance.

• Z- value: The amount of temperature increased to reduce D- value by 90%.

Expression of Resistance

Heat transfer: Direct: incineration or flaming as loop.

Indirect:

Through medium as air, water or metal1- radiation: using infra red sources2- conduction: usually through metals3- convection: through water, air or oil