Control of Microbial Growth
Chapter 7
Terminology• Sepsis refers to microbial contamination.• Asepsis is the absence of significant
contamination.
• Antisepsis: Removal of pathogens from living tissue
• Degerming: Removal of microbes from a limited area
• Sanitization: Lower microbial counts on eating utensils
Terminology cont’d
• Decontaminated
- item that has been treated to reduce # of disease causing organisms
• Preservation
- delaying spoilage of foods
• Sterilization, Disinfection, Antiseptic, Bacteriocidal, Bacteriostatic
Terms used:
Sterilization vs. Disinfection
• Sterilization: destroying all forms of life
• Disinfection: destroying pathogens or unwanted organisms
Disinfectant vs. Antiseptic Disinfectant: antimicrobial agent used on inanimate
objects Antiseptic : antimicrobial agent used on living tissue
cidal vs. static• Biocide/Germicide: Kills microbes• Bacteriostasis: Inhibiting, not killing, microbes• Examples:
- Bactericidal - kills bacteria- Bacteriostasis (Bacteriostatic)- inhibits bacterial growth- Fungicidal- Fungistatic- Algacidal- Algastatic
Methods of control
• Physical or chemical?– physical control includes heat, irradiation,
filtration and mechanical removal
– Chemical control involves the use of microbial chemicals
– Depends on the situation
– degree of control required
Methods of control cont’d
• Daily life
- Cooking
- refrigeration
- cleaning
- soap water
mechanical
Mechanical and chemical
• Hospitals– Important to minimize nosocomial infection
(hospital acquired infection) due to
- weakened patients’ condition
- breaching of intact skin
- high concentration of pathogens from patients and workers
– Sterile condition
Methods of control cont’d
• Microbiology lab utilizes– Sterile equipment
– Aseptic technique
– And possesses workers who takes care of the nature of of microbiologists (GLP)
Methods of control cont’d
• Foods/food production industry
- physical removal
- adding chemicals
- may result in toxicity
- clean surface/ machinery
Methods of control cont’d
Selection of Control Method
• Antimicrobial procedure used for control of microbial growth is based on – Types of microbe
– Extent of contamination
– Environmental conditions
– Potential risk
Selection of Control Method cont’d
• Types of microorganism
- some organisms are more resistant and require stronger measures for control
- endospores require chemical treatment for 10 hours
- Mycobacterium’s waxy cells are resistant to chemicals
Mycobacterial cell wall: 1-outer lipids, 2-mycolic acid, 3-polysaccharides (arabinogalactan), 4-peptidoglycan, 5-plasma membrane, 6-lipoarabinomannan (LAM), 7-phosphatidylinositol mannoside, 8-cell wall skeleton
• Extent of microbial population
- larger population take more time to destroy
- usually 90% of the population is destroyed in
a given period
e.g if in 1st 3 minutes 90% of the population is destroyed, then 90% of the remaining population gets destroyed in the next 3 minutes and so on
Selection of Control Method cont’d
• Environmental conditions- pH, temperature- presence of
- organics: blood- dirt- grease
- the potential risk of transmitting infectious agents- critical items- semicritical items- non-critical items
Selection of Control Method cont’d
Must be cleaned first, then controlled
• Critical items have
- indirect contact with body tissues
- needles, scalpels
• Semicritical items have contact with
- mucous membranes but it does not penetrate
endoscopes, endotrachial tubes
Selection of Control cont’d
Selection of Control cont’d
• Non-critical items have
- indirect contact with unbroken skin
- countertops, stethoscopes
Methods to Control Microbial Growth
Physical Microbial Controls: Heat
• Heat as a microbial control- fast, reliable, inexpensive- does not introduce potential toxic substances
• Types of heat control include- moist heat- pasteurization- pressurized steam- dry heat
• Moist heat
- causes irreversible coagulation of proteins found in microorganisms
- 10 minutes of boiling
- most microbes and viruses will be destroyed except endospores and few
others which can survives hours of boiling
Physical Microbial Controls: Heat cont’d
• Pasteurization
- reduces number of heat sensitive organisms
- widely used in milk and juices
increases shelf life and does not alter quality
- original pasteurization was 62ºC, 30 mins
- now: UHT-shorter time 72ºC, 15 secs
Physical Microbial Controls: Heat cont’d
• Pressurized steam
- pressure cooker or autoclave
- higher air pressure increases the temperature
at which steam forms
- 15 psi (lbs/square inch) at 121ºC for 15 mins
-effective to kill endospores
Physical Microbial Controls: Heat cont’d
The autoclave: Moist heat and pressure
• 15psi, 121ºC, 15 minutes
• Thermal death point (TDP): Lowest temperature at which all cells in a culture get killed in 10 mins
• Thermal death time (TDT): time to kill all cells in a culture
• Decimal reduction time (DRT): Minutes to kill 90% of a population at a given temperature
The autoclaving machine
Temperature of steam and Pressure at sea level
The autoclaving machine
• Dry heat- without moisture e.g. flaming- burns cell constituents- object is oxidized to ash- irreversibly denatures proteins- takes longer (200ºC, 1.5 hrs dry=121ºC, 15 min moist)- advantages are for powders, does not corrode metals and blunt sharps- e.g flasks, tubes, pipettes in microbiological laboratories.
Physical Microbial Controls: Heat cont’d
Physical Microbial Controls: Filtration
• Used for heat sensitive fluids
• air
• Fluids– solutions of antibiotics, vitamins,
tissue extracts, animal serum, etc.– Depth filters
–able to retain microorganisms while allowing fluids to pass through
–Membrane Filter- The use of graded pore size 0.2-0.4µm
Physical Microbial Controls: Filtration cont’d
• Air- HEPA (High Efficient Particulate Air) filter and laminar air flow are commonly used
- filter incoming air and outgoing air respectively
- HEPA filter prevents the income of 0.3µm and large size particles to enter.
Physical Microbial Controls: Filtration cont’d
• “Cold sterilization“ for disposable materials made up of plastics, wool, cotton, etc without altering the material.
• Radiation damages DNA• Ionizing radiation (X-rays, gamma rays, electron
beams) • Non-ionizing radiation
- UV- Microwaves kill by heat not especially
antimicrobial
Physical Microbial Controls: Radiation
• Gamma irradiation
- penetrate deeply
- for heat sensitive materials
- causes biological damage to microorganisms
- does not alter food flavor (meat)
Physical Microbial Controls: Radiation cont’d
• UV light– damages the structure and function of nucleic acids
– Penetrate poorly- cannot penetrate even into liquid.
– Used to disinfect surfaces
– Can cause damage to human cells
– Germicidal lamps -kill or reduce the number of viable microorganisms to sterilize microbiological laboratories hospital operating rooms, and specific filling rooms in various industries
Physical Microbial Controls: Radiation cont’d
• Microwave– Kills by heat
– Does not affect microorganisms directly
Physical Microbial Controls: Radiation cont’d
Physical Methods used to control Microbial growth
Chemical Microbial Control
• Grouped according to potency– Sterilants
– High-level
– Intermediate level
– Low level
Chemical Microbial Control cont’d
Chemical Control
sterilants
• destroy microorganisms, endospores and viruses
• used for critical equipment-scalpels
chemical control cont'd
• high level
- destroy viruses and vegetative microorganisms (no endospores)
- used for semicritical equipment: endoscopes
• intermediate level
- destroy vegetative microorganisms, some viruses
- used for non-critical equipment: sthetoscopes
• low level
-destroy fungi, vegetative microorganisms
- used for general purpose disinfectants
Selecting Germicidal Chemical
• Germicide: An agent capable of killing pathogens and non-pathogens but not necessarily endospores
• toxicity to human or environment?
- weigh the benefits vs the risks
• presence of organic material
- hypochlotrite is inactivated by the presence of organic matter
• compatibility
-electrical equipment with a liquid??
• residue
- some have to be rinsed with sterile water
• cost and availability
Selecting germicidal Chemical cont'd
• storage and stability
- may come in concentrated form for ease in storing
- those have to be mixed
• environmental risk
- is neutralization necessary before disposal?
Classes of Germicidal Chemicals
• alcohols alcohols
• aldehydes
• biguanides
• ethylene oxide
• halogens: oxidize proteins
Classes of Germicidal Chemicals
• alcohols
- coagulated enzymes and proteins
- damage lipid membranes
- on-toxic
- inexpensive
- no residue
Classes of Germicidal Chemicals cont’d
• aldehydes
- inactivate proteins and nucleic acids
- toxic to humans
• Peroxygens
- oxidizing agents
- hydrogen peroxide
- leaves no residue
Classes of Germicidal Chemicals cont’d
• biguanides
- extensive antiseptic use
- adheres and persists on skin, mucous membranes
- low toxicity
• Phenolic compounds– Hitorically important
– Irritant, unpleasant odor
– Destroy cytoplasmic membrane and denatures protein
Classes of Germicidal Chemicals cont’d
• ethylene oxide
- reacts with proteins
- gas: penetrable
- mutagenic
• Metal compounds– Interfere protein function
– Toxic
– pollutants
Classes of Germicidal Chemicals cont’d
• halogens: oxidize proteins
-chlorine
-irritating to skin
-organic compounds consume free chlorine
-iodine
-tincture
- Iodophore
Effect of germicidal activity on Microbes
Effect of germicidal activity on Microbes
Chemical methods of microbial control
• Evaluating a disinfectant
- Disk diffusion method
Preservation of Perishables
• Extends shelf life– Slow or halts microbial growth thus delaying
spoilage
• Chemical preservatives– Some chemical preservatives are used in non-food
items
– Food preservatives must be non-toxic to humans• Benzoic acid, propionic acids, nitrate are commonly
used
• Nitrate– Inhibits germination of C. botulinum endospores
Preservation of Perishables
Preservation of Perishables
• Low temperature storage– Temperature dependent• most microorganisms do not reproduce in ordinary
refrigerator (0-7ºC)
– Freezing• ice crystals can cause irreversible damage to many
microorganisms (kills up to 50% growth)
• Freezing stops all growth, but may start to reproduce again once food is thawed
• Reducing water– Salt/sugar
– Draw water out of cell
– Less available for microorganisms
• Drying– Desiccation• Removing water such as milk powder
Preservation of Perishables
Salt cured meat
• Lyophilization– Freeze drying• Freeze food first
• followed by putting in vacuum
Preservation of Perishables
Factors that influence effectiveness
• Number of microbes
• Environmental influences
• Time of exposure
• Microbial characteristics
Microbial death curve
Limitations: Microbial Characteristics
Actions of Microbial Control Agents
• Alteration of membrane permeability
• Damage to proteins
• Damage to nucleic acids
Questions????
Sand rich in salts n nitrates preserved mummy
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