Control of microorganisms Many bacteria cause disease and food spoilage Need exists to kill or inhibit the growth of these bacteria
Dec 27, 2015
Control of microorganisms
Many bacteria cause disease and food spoilage
Need exists to kill or inhibit the growth of these bacteria
Frequently used terms
Sterilization - removal or destruction of all living cells, viable spores, viruses and viriods
Disinfection - removal or destruction of pathogens (spores and some other microorganisms remain)
Sanitization - reduction of microbial population to safe levels
Antisepsis - prevention of infection (accomplished by antiseptics)
Frequently used terms
Bactericide - substance that kills bacteria
Bacteriostatic - substance that prevents growth of bacteria
Pattern of microbial death
Microorganisms usually die logarithmically (i.e. the population will be reduced by the same fraction at regular intervals)
Pattern of microbial death
Microorganisms usually die logarithmically (i.e. the population will be reduced by the same fraction at regular intervals)
Conditions influencing effectiveness of antimicrobial agents
Population size
Population composition
Concentration or intensity of agent
Duration of exposure to agent
Temperature
Local environment (e.g. pH, presence of organic material)
Physical methods of control
Heat
Low temperature
Filtration
Radiation
Measuring heat-killing efficiency
Thermal death point (TDP) - lowest temperature at which all microorganisms in a suspension are killed in 10 minutes
Decimal reduction time (D value) - time it takes to kill 90% of the cells or spores in a sample at a specific temperature
Decimal reduction time (D value)
Measuring heat-killing efficiency
Z value - the increase in temperature required to reduce D to 1/10 its value
F value - time in minutes at a specific temperature required to kill a population of spores or cells
Measuring heat-killing efficiency
Moist heat
Effective against all types of microorganisms
Degrades nucleic acids, denatures proteins, and disrupts membranes
Moist heat
Autoclaves are used to kill endospores
Uses steam under pressure to achieve temperatures above boiling
Pasteurization
Controlled heating at temperatures below boiling
Does not sterilize
Kills pathogens and reduces levels of spoilage microorganisms
First used to preserve wine, now used for milk, beer, juice and other beverages
Pasteurization
Traditional method: 63 ºC for 30 minutes
Flash pasteurization: 72 ºC for 15 seconds
Ultrahigh temperature (UHT) sterilization
Sometimes used by the milk industry
Milk heated at 140 to 150 ºC for 1 to 3 seconds
Products can be stored at room temperature for 1 to 2 months
Dry heat sterilization
Less effective, requiring higher temperatures and longer exposure times
Does not corrode metal and glass instruments
Oxidizes cell constituents and denatures proteins
Low temperature
Refrigeration- storage at 4 ºC slows microbial growth (only used for short-term storage)
Freezing- storage at - 20 ºC stops microbial growth (does not kill microorganisms)
Freezing at -30 to -70 ºC used to preserve microbial samples
Filtration
Can be used to sterilized or reduce the microbial population of heat-sensitive liquids
Removes microorganisms rather than destroying them
Solutions often forced through filters by pressure or a vacuum
Depth filters
Thick layers of fibrous or granular material with small diameter channels
Made of various materials (e.g. diatomaceous earth or asbestos)
Membrane filters
Porous membrane about 0.1 mm thick
Entrap microbes on their surface
Pore size of 0.2 um diameter removes most cells but not viruses
Air filtration
Surgical masks
Cotton plugs on culture vessels
Air filtration
Laminar flow biological safety cabinets
Employ high efficiency particulate air (HEPA) filters
Remove 99.97 % of particles larger than 0.3 um
Air forced through filter forms a curtain of sterile air across the opening of the cabinet
Laminar flow biological safety cabinets
Radiation
Ultraviolet (UV) radiation
Ionizing radiation
Ultraviolet (UV) radiation
Wavelength of about 260 nM
Lethal but does not penetrate glass
Used to sterilize air or exposed surfaces
Can sterilize water in a thin layer
Burns the skin
Ionizing radiation
Penetrates deep into objects
Not always effective against viruses
Gamma radiation from Cobalt 60 often used
Used to treat meat, fruits, vegetables and spices
Chemical agents
Are most commonly used agents for disinfection and antisepsis
Phenolics
Phenol first used by Lister
Phenol and derivatives used as disinfectants in hospitals and labs
Effective in the presence of organic material
Can cause skin irritation
Phenolics
Act by denaturing proteins and disrupting cell membranes
Alcohols
Not effective against spores or lipid-containing viruses
Ethanol and isopropanol most commonly used (at 70-80 %)
Act by denaturing proteins and possibly dissolving membrane lipids
Halogens
Include fluorine, chlorine, bromine, iodine and astatine
Iodine used as a skin disinfectant
Chlorine used to disinfect water
Both act by oxidizing cell material and iodinating or chlorinating molecules
Aldehydes
Formaldehyde and gutaraldehyde are the most commonly used
Are highly reactive molecules
Inactivate proteins and DNA by cross-linking alkylating molecules
Quaternary ammonium compounds (QUATS)
Are a class of detergents
Consist of a positively charged quaternary nitrogen with a long hydrophobic chain
Act by disrupting membranes and denaturing proteins
Sterilizing gases
Ethylene oxide (EtO) is used to sterilize heat-sensitive material
Kills by combining with proteins
Rapidly penetrates packing material
Sterilizing gases
Betapropiolactone (BPL) is occasionally used
Less difficult to eliminate and kills more quickly
Does not penetrate well and may be carcinogenic
Heavy metals
Ions of mercury, silver, arsenic, zinc and copper were used for many years as germicides (less toxic and more effective compounds have generally replaced them)
Silver nitrate can be used in the eyes of newborns (antibiotics more common now)
Silver sulfadiazine used on burns
Copper sulfate used as an algicide
Heavy metals
Act by combining with proteins and inactivating or precipitating them
Evaluation of antimicrobial agent effectiveness
The Environmental protection agency (EPA) regulates disinfectants
The Food and Drug Administration (FDA) regulates agents used on humans and animals
Phenol coefficient
Potency of disinfectant compared to phenol
Coefficient greater than 1 indicates agent is more potent than phenol
Not always indicative of potency during normal use
Other evaluation methods
Use dilution test
Measures rate at which selected bacteria are destroyed by various chemical agents
In use test
Testing done using conditions that approximate normal use of disinfectant