Using Chemicals to Destroy Microorganisms and Viruses Chapter 5

Using Chemicals to DestroyMicroorganisms and Viruses

Chapter 5

Approaches to Control

• Control mechanisms either physical or chemical– May be a combination of both– Physical methods

• Heat• Irradiation• Filtration• Mechanical removal

– Chemical methods• Use a variety of antimicrobial chemicals• Chemical depends on circumstances and degree of control

required


• Principles of control– Sterilization

• Removal of ALL microorganisms– Sterile item is absolutely free of microbes, endospores and

viruses

• Can be achieved through filtration, heat, chemicals and irradiation

– Disinfection• Eliminates most pathogens

– Some viable microbes may exist

• Disinfectants = used on inanimate objects and surfaces• Antiseptics = used on living tissues


• Principles of control– Pasteurization

• Brief heat treatment used to reduce organisms that cause food spoilage

– Surfaces can also be pasteurized

– Decontamination• Treatment to reduce pathogens to level considered

safe to handle

– Degerming • Mechanism uses to decrease number of microbes

in an area– Particularly the skin (antiseptics)

• Principles of control– Sanitized

• Implies a substantially reduced microbial population

– This is not a specific level of control

– Preservation• Process used to delay spoilage of perishable items

– Often includes the addition of growth-inhibiting ingredients



• Situational considerations– Microbial control

methods are highly variable

• Depends on situation and degree of control required

– Daily life

– Hospital

– Microbiology laboratories

– Food and food production facilities

– Water treatment

• Daily life– Washing and scrubbing with soaps and

detergents achieves routing control• Hand washing single most important step to

achieving control

– Soap acts as wetting agent• Aids in mechanical removal of microorganisms

– Removes numerous organisms from outer layer of skin» Normal flora usually unaffected because it resides in

deeper layers


• Hospitals– Minimizing microbial population very important

• Due to danger of nosocomial infections– Patients are more susceptible to infection– Pathogens more likely found in hospital setting

» Numerous organisms develop antimicrobial resistance due to high concentrations of antibiotics

• Instruments must be sterilized to avoid introducing infection to deep tissues


• Microbiology laboratories– Use rigorous methods of control

• To eliminate microbial contamination to experimental samples and environment

– Aseptic technique and sterile media used for growth» Eliminates unwanted organisms

– Contaminated material treated for disposal» Eliminates contamination of environment


• Food and food production facilities– Retention of quality enhanced through

prevention of microbial growth and contamination

• Achieved through physical removal and chemical destroying organisms

• Heat treatment most common and most reliable mechanism

• Irradiation approved to treat certain foods• Chemicals prevent spoilage

– Risk of toxicity


• Water treatment facilities– Ensures drinking water is safe– Chlorine generally used to disinfect water

• Can react with naturally occurring chemicals– Form disinfection by-products (DBP)

» Some DBP linked to long-term health risks

• Some organisms resistant to chemical disinfectants


Selection of Antimicrobial Procedure

• Selection of effective procedure is complicated– Ideal method does not exist

• Each has drawbacks and procedural parameters

• Choice of procedure depends on numerous factors– Type of microbe– Extent of contamination

• Number of organisms

– Environment– Risk of infection– Composition of infected item

• Type of microorganism– Most critical consideration

• Is organism resistant or susceptible to generally accepted methods?

• Resistant microbes include– Bacterial endospores

» Resistant to heat, drying and numerous chemicals– Protozoan cysts and oocysts

» Generally excreted in feces and cause diarrheal disease– Mycobacterium species

» cell wall structure initiates resistance– Pseudomonas species

» Can grow in presence of many chemical disinfectants– Naked viruses

» Lack envelope and are more resistant to chemical killing


Bacterial endosporesClostridium botulinum – causes botulism, resists boiling, but autoclaving kills

ProtozoansGiardia lamblia and Cryptosporidium parvum

Cause digestive problems


• Number of organisms initially present– Time it takes to kill it directly

affected by population size• Large population = more time

– Commercial effectiveness is gauged by decimal reduction time

• a.k.a D value• Time required to kill 90% of

population under specific conditions

– Washing reduces time required to reach disinfection or sterilization

• Environmental conditions– Environmental conditions strongly influence

effectiveness• pH, temperature and presence of organic materials

can increase or decrease effectiveness– Most chemicals are more effective at higher

temperatures and lower pH– Effectiveness can be hampered by the presence of

organism molecules» Can interfere with penetration of antimicrobial agent


• Potential risk of infection– Medical items categorized according to

potential risk of disease transmission• Critical items = come in contact with body tissues

– Needles and scalpels

• Semicritical instruments = contact mucous membranes but do not penetrate body tissues

– Endoscope

• Non-critical instruments = contact unbroken skin only

– Show little risk of transmission– stethoscope


• Composition of the item– Some sterilization and disinfection methods

inappropriate for certain items• Heat inappropriate for plastics and other heat

sensitive items


Heat as Control

• Heat treatment most useful for microbial control– Relatively fast, reliable, safe and inexpensive

• Heat can be used to sterilize or disinfect

• Methods include– Moist heat– Dry heat

Heat as Control

• Moist heat– Destroys through irreversible coagulation of

proteins– Moist heat includes

• Boiling• Pasteurization• Pressurized steam

• Boiling (100° C)– Destroys most microorganisms and viruses– Not effective means of sterilization

• Does not destroy endospores

• Pasteurization– Pasteur developed to avoid spoilage of wine– Does not sterilize but significantly reduces organisms– Used to increase shelf life of food– Most protocols employ HTST method

• Heated to 72°C and held for 15 seconds– Other protocol UHT

• Heated to 140°C - 150°C, held for several seconds then rapidly cooled

Heat as Control

Heat as Control• Pressurized steam

– Autoclave used to sterilize using pressurized steam

• Heated water steam increased pressure

• Preferred method of sterilization

– Achieves sterilization at 121°C and 15psi in 15 minutes

• Effective against endospores• Flash autoclaving sterilizes at

135°C and 15psi in 3 minutes• Prions destroyed at 132°C

and 15psi for 4.5 hours

• Dry heat– Not as effective as moist heat

• Sterilization requires longer times and higher temperatures

– 200°C for 1.5 hours vs. 121°C for 15 minutes

– Incineration method of dry heat sterilization

• Oxidizes cell to ashes• Used to destroy medical waste and animal

carcasses• Flaming laboratory inoculation loop

incinerates organism– Results in sterile loop

Heat as Control

Other Physical Methods of Control

• Heat sensitive materials require other methods of microbial control– Filtration– Irradiation– High-pressure treatment


• Filtration– Membrane filtration

used to remove microbes from fluids and air

– Liquid filtration• Used for heat sensitive

fluids• Membrane filters allow

liquids to flow through– Traps microbes on

filter

• Depth filters trap microbes using electrical charge

– Filtration of air• High efficiency particulate

air (HEPA) filter removes nearly all microbes from air

– Filter has 0.3µm pores to trap organisms


• Radiation– Electromagnetic radiation

• Energy released from waves

– Based on wavelength and frequency

» Shorter wavelength, higher frequency = more energy

• Range of wavelength is electromagnetic spectrum

• Radiation can be ionizing or non-ionizing

• Ionizing radiation– Radiation able to strip electrons from atoms– Three sources

• Gamma radiation• X-rays• Electron accelerators

– Causes damage to DNA and potentially to plasma membrane

– Used to sterilize heat resistant materials• Medical equipment, surgical supplies, medications• Some endospores can be resistant


• Ultraviolet radiation– Non-ionizing radiation

• Only type to destroy microbes directly• Damages DNA

– Causes thymine dimers

– Used to destroy microbes in air, drinking water and surfaces

– Limitation• Poor penetrating power

– Thin films or coverings can limit effect


• High pressure processing– Used in pasteurization of commercial foods

• Does not use high temperatures• Employs high pressure

– Up to 130,000 psi

• Destroys microbes by denaturing proteins and altering cell membrane permeability


Chemicals as Control

• Chemicals can be used to disinfect and sterilize– Called germicidal

chemicals

• Reacts with vital cell sites– Proteins– DNA– Cell membrane


• Potency of chemicals– Formulations generally

contain more than one antimicrobial agent

– Regulated by• FDA

– Antiseptics

• EPA– Disinfectants

– Germicidal agents grouped according to potency

• Sterilants = – Destroy all microorganisms

• High-level disinfectants– Destroy viruses and vegetative

cells, – Not endospores

• Intermediate-level disinfectants– Kill vegetative cells fungi, most

viruses, – Not endospores

• Low-level disinfectants– Removes fungi, vegetative bacteria

and enveloped viruses,– Not mycobacteria, naked viruses or

endospores

• Selecting appropriate chemical– Points to consider

• Toxicity– Benefits must be weighed against risk of use

• Activity in presence of organic material– Many germicides inactivated in presence of organic matter

• Compatibility with material being treated– Liquids cannot be used on electrical equipment


• Selecting appropriate chemical– Points to consider

• Residue– Residues can be toxic or corrosive

• Cost and availability• Storage and stability

– Concentrated stock relieves some storage issues

• Environmental risk– Is germicidal agent harmful to environment


• Classes of chemicals– Germicides represent a number or chemical families

• Alcohols• Aldehydes• Biguanides• Ethylene oxide• Halogens• Metals• Ozone• Peroxides• Phenolics • Quaternary ammonium compounds


Preservation of Perishable Products

• Preservation extends shelf-life of many products– Chemicals are often added to prevent or slow

growth of microbes• Other methods include

– Low temperature storage– Freezing– Reducing available water

• Chemical preservatives– Numerous chemicals are used as preservatives

• Formaldehyde, Quats, and phenols– Weak organic acids often used as food preservatives

• Benzoic, ascorbic and propionic acids• Used in bread, cheese and juice• Mode of action

– Alter cell membrane function– Interfere with energy transformation

– Nitrates and nitrites used in processed meats• Inhibits germination of endospores and growth of vegetative

cells• Have been shown to be potent carcinogen


• Low temperature storage– Microbial growth is temperature dependent

• Low temperatures slow down or stop enzymatic reactions of mesophiles and thermophiles

– Some psychrophiles still able to grow

– Freezing as means of food preservation• Essentially stops microbial growth• Irreversibly damages cell

– Kills up to 50% of microbes» Remaining cells still pose potential threat


• Reducing water availability– Decreasing water availability accomplished by salting

or drying food.• Addition of salt increases environmental solutes

– Causes cellular plasmolysis

• Numerous bacteria can continue to grow in high salt environments

– Staphylococcus aureus can survive in high salt concentrations

• Desiccation or drying is often supplemented by other methods

– Salting

• Lyophilization (freeze drying)– Widely used to preserve foods like coffee, milk and meats


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Using Chemicals to Destroy Microorganisms and Viruses Chapter 5