Control of Microbial Growth Flashcards
Microbicidal
kills the microorganism
Microbistatic
prevents growth of the microorganism as long as agent is present
Primary targets are microorganisms capable of causing infection or food spoilage
Vegetative bacterial cells and endospores Fungal hyphae and spores, yeast Protozoan trophozoites and cysts Viruses Prions
Decontamination
Physical, chemical, and mechanical methods to destroy or reduce undesirable microbes in a given area
Highest resistance of Microbes
bacterial endospores, prions
Moderate resistance
Pseudomonas sp. Mycobacterium- tuberculosis Staphylococcus aureus protozoan cysts Naked viruses
Least resistance
most bacterial vegetative cells
fungal spores and hyphae, yeast
enveloped viruses
protozoan trophozoites
Sterilization
a process that destroys all viable microbes, including viruses and endospores; microbicidal (may still contain toxins…)
Disinfection
a process to destroy vegetative pathogens, not endospores; from inanimate objects
Antiseptic
disinfectants applied directly to living tissues on exposed body surfaces
Sanitization
any cleansing technique that mechanically removes microbes from inanimate objects in public places (plates, utensils, toilets)
Degerming
reduces the number of microbes on skin (handwashing)
Antiseptic
disinfectants applied directly to living tissues on exposed body surfaces
Sepsis
microbial contamination or infection in the body
Growth of microbes in tissues
[septic shock is the body’s reaction to sepsis]
Asepsis
absence of contamination
Aseptic surgery: prevent microbial contamination during invasive technique
Medical asepsis: prevent microbial contamination through aseptic technique
Microbial Death
Permanent loss of reproductive capability Hard to detect Measured as: \+/- growth -plate counts on petri dishes
Cellular targets of physical and chemical agents: The cell wall
cell wall becomes fragile and cell lyses
some antimicrobial drugs, detergents, and alcohol
Cellular targets of physical and chemical agents: The cell membrane
loses integrity
detergent surfactants
Cellular targets of physical and chemical agents: Protein and nucleic acid synthesis
- prevention of replication, transcription, translation, peptide bond formation, protein synthesis
- chloramphenicol, ultraviolet radiation, formaldehyde
Cellular targets of physical and chemical agents: Proteins
- disrupt or denature proteins
- alcohols, phenols, acids, heat
The effectiveness of a particular agent is govern by several factors
- Number of microbes present
- Nature of microbes in the population
- Temperature and pH of environment
- Concentration or dosage of agent
- Mode of action of the agent
- Presence of solvents, organic matter, or inhibitors
Heat–Decimal Reduction Time (DRT)
-the time required to kill 90% of the organisms in a suspension at a specified temperature
-One log decrease is 90% of population killed
-Moving decimal place by 1 place
Ex: If a population of 1000 is reduced to 100, 900 were killed (900 of 1000 is 90%)
Selection of methods of control depends on circumstances
Does the application require sterilization?
Is the item to be reused?
Can the item withstand heat, pressure, radiation, or chemicals?
Is the method suitable?
Will the agent penetrate to the necessary extent?
Is the method cost- and labor- efficient, and is it safe?
Physical control:
Heat
- Heat-moist and dry
- Moisture: lower temperatures and shorter exposure times; coagulation and denaturation of proteins
- Dry: moderate to high temperatures; dehydration, alters protein structure; incineration
Thermal Death Measurements–Thermal death time (TDT)
shortest length of time required to kill all test microbes at a specified temperature
Thermal Death Measurements–Thermal death point (TDP)
lowest temperature required to kill all microbes in a sample in 10 minutes
Moist Heat– more penetrating than dry heat
-(Coagulates and denatures proteins–60 – 135ºC)
-Boiling: 100ºC-Boiling water for 30 minutes to destroy non-spore forming pathogens, and protozoal cysts-(disinfection)
(Not capable of killing endospores or killing all viruses)
(30 minutes will also kill botulism toxin)
-Steam: autoclaves or pressure cookers
Autoclave: sterilization
An autoclave is a high pressure device used to allow the application of moist heat above the normal atmosphere boiling point of water
-Important: Exposure to 121ºC at 15 psi for 15+ minutes is typically sufficient to sterilize–
Pasteurization
application of moist heat – gentle (less than boiling temperatures) process for liquids (milk, juice)disinfect and kill spoilage agents, but not affect flavor kills anything that is heat labile
Ex: 15 sec. at 70ºC
Tyndallization
- Cycles of moist heat, then incubation
- Non-pressurized steam
- Heat kills vegetative cells (disinfectant)
- Incubation periods allow endospores to germinate
- Some highly resistant spores might survive
Dry Heat
-Typically need higher temperatures than for moist heat (160C - 6500C)
-Direct Flaming
-Incineration (over 600C)
-Disposal of medical waste
-Only reliable method for prions
-Organic materials=charcoal
Hot air sterilization (an oven): 171ºC for at least one hour, 160ºC for at least two hours, 121ºC for at least 16 hours
Filtration
-Neither bacteriostatic nor bacteriocidal
-Used when materials are heat labile
Disadvantage: clogs easily
-Used in research labs
Filtered air (HEPA):
just reduces number of organisms
Ionizing Radiation
- Radiation that ionizes water
- Temporarily turns water into an oxidizing agent
- Damages DNA: bactericidal, sporicidal
- Gamma rays, X-rays, cathode rays
- Penetrating
- Cold (low temperature) sterilization
Ionizing Radiation: Applications
- Now used to treat food products
- Approved by the World Health Organization (WHO)
- Used to sterilize bandages and medical devices
- Requires specialized equipment, highly regulated
UV Radiation
-Non-ionizing (UV light)
-Causes thymine dimers
(disrupts DNA use-replication and transcription)
-Does not penetrate
-Good for surface disinfection & air (decontaminate hospital rooms but also damages human)
Microwaves
- Useful for heating food
- Reaches a high enough temperature to kill some bacteria by moist heat
Low Temperature-Refrigeration
(1-15C)–Will not sterilize (Only bacteriostatic)
But… for psychrophiles a viable environment
Low Temperature-Deep freezing
- Stops growth
- Freezing is employed to prevent food deterioration due to bacterial or fungal growth
- Long term storage of microorganisms (20-30 years)
Desiccation
-Removes water needed for bacterial growth (bacteria can wait for water to restart growth)
Prevents metabolism= prevents growth
Lyophilization
- Freeze drying
- For products that don’t dry easily (powdered milk, powdered eggs, coffee crystals)
- Microbial cultures can last centuries
Plasmolysis
- High concentrations of solute (sugar and salt)
- Water loss by osmosis
Disinfectants
used on inanimate objects
Antiseptics
used on living tissue
Chemical Agents in Microbial Control: Factors to consider
- Concentration of the chemical
- Nature of material
- Time of contact
- Temperature
- Cost
- Stable during storage
- Harmless to humans, animals and objects
High-level germicides
- Kill endospores; may be sterilants
- Devices that are not heat sterilizable and intended to be used in sterile environments (body tissue)
Intermediate-level
- Kill fungal spores (not endospores), tubercle bacillus, and viruses
- Used to disinfect devices that will come in contact with mucous membranes but are not invasive
Low-level
- Eliminates only vegetative bacteria, vegetative fungal cells, and some viruses
- Clean surfaces that touch skin but not mucous membranes
Halogens
- The seventh (VII) column of the periodic table of elements
- Two halogens are regularly employed as antimicrobials: iodine and chloride (also fluoride in toothpaste)
- 30% of marketed antimicrobials
- Disinfectant and antiseptics (can be sporicidial with long exposure)
Halogens: Chlorine
- Drinking water is commonly disinfected using hypochlorite
- Adding bleach, NaOCl, or bubbling chlorine gas through the water-(Oxidizing agent, households, food-processing)
Halogens: Iodine and Iodophors
- Iodine is often employed as a tincture or an iodophor
- Tincture: an alcohol solution of a substance, in this case iodine
- Iodophors: organic compounds added to iodine–slow the release of iodine
- Serve as surfactants
- Examples are betadine and Isodine (Skin and wound treatment)
Phenol:
Denatures bacterial/viral cell proteins and damages the bacterial cell membrane
Phenolics:
- Similar to phenol
- Turn cloudy when added to water
- Stable in presence of organic material
- Lysol, Pinesol (from pine oil), triclosan
Carbolic Acid - Phenolics
-Lister was the first to use this as a disinfectant (Listerine, an antiseptic, named for him)
Chlorhexidine
- Organic base with chlorine and two phenolic rings
- Bactericidal: both Gm positive and Gm negative organisms
- Antiseptics in hospitals: Hibiclens, Hibitane-(Choice disinfectant to control MRSA and Acinetobacter)
- Low toxicity, mild antiseptic, fast acting
Alcohols
- Work best as 70 to 99% mixtures with water (Disinfecting)
- Destroys membranes and proteins, dissolves lipids
- Either ethanol or isopropyl (rubbing) alcohol
- Advantage: can evaporate away (e.g., to disinfect skin prior to injection)
Surfactants: Surface-active agents
- Dissolve lipids but can mix with water
- Remove microbes from the surface
- Examples of surfactants are soaps and detergents-(Differ in strength of anionic group, Soap is not biocidal but is useful for cleansing)
Surfactants: Quats-Quaternary ammonium compounds (NH4+)
Cationic detergent disinfectants
- Bind to cell membranes
- Good for when small levels of microbes
- Used widely in medical field but they have some problems:
- inactivated by soaps
- inhibited by divalent cations (calcium and magnesium ions)
- inhibited by cotton and other porous organic substances
- unable to kill certain species of Pseudomonas)
Surfactant Uses - Soap
Degerming
Quarternary ammonium compounds(Cationic detergents)
Bactericidal, Denature proteins, disrupt plasma membrane, used in mouthwash, lab bench disinfectant
Surfactant Uses - Anionic detergents
Sanitizing
Heavy Metals: examples
- silver nitrate: eye drops, burn cream (silvadene)
- Silver treated wound pads
- Mercurochrome
- Silver lined urinary catheters
- Thimerosal
Heavy Metals:
- biocidal or antiseptic
- Oligodynamic action: active in small amounts
- Silver, Copper, Zinc, Mercury
Alkylating Agents: inactivate proteins
Formaldehyde: inactivates viruses and toxins for vaccines and toxoidvaccines
- Glutaraldehyde is capable of sterilizingequipment
- Requires many hours of exposure (cold sterilization)
Gas Chemosterilizers (alkylating)
Denature proteins
Sporicidal- Sterilizing
Used on paper and plastic (heat sensitive)
-Ethylene oxide gas: Slow, toxic to humans,Chamber required
Peroxygens: oxidizing agents
- Ozone, O3
- Hydrogen peroxide: H2O2
- Benzoyl peroxide (acne)
- Peracetic acid: cold sterilant
Peroxygens: oxidizing agents-Hydrogen peroxide(H2O2)
Produces reactive free radicals–dilute (3%) home use
- Antiseptic at low concentrations
- Concentrated (35-90%): medical use: sterilizing (Endoscopes)
- Chamber required)now have wipes
- Vaporized form can sterilize rooms(Bioquell)
Dyes as Antimicrobial Agents (NH2)
Aniline dyes are very active against gram-positive species of bacteria and various fungi
- Sometimes used for antisepsis and wound treatment
- Low level, narrow spectrum of activity
- Gentian violet- treat thrush
- Malachite green-(treat water molds in fish farms and aquariums, Carcinogen in the EU)
Chemical preservatives - Organic acids
Prevent spore germination and bacterial and fungal growth:
- Acetic acid inhibit bacterial growth
- Propionic acid retards molds
- Lactic acid prevents anaerobic bacterial growth
- Benzoic acid and sorbic acid inhibit yeast
Chemical preservatives - Nitrate and nitrite
Used in processed meats
- Inhibit endospores germination and vegetative cell growth–But shown to be carcinogenic
- Can form nitrosamines if heated… known carcinogen–don’t fry bologna…-“Nitrate free” increases risk of Listeria
