Microbiology Chapter 11
Surfactants
Polar molecules with hydrophilic and hydrophobic regions Physically bind to the lipid layer and penetrate the internal hydrophobic region of membranes This opens up leaky spots that allow injurious chemicals to seep into the cell and important ions to leak out
Bacteristatic agents:
Prevent the growth of bacteria on tissues or objects in the environment
Modes of action of germicides are to attack cellular targets:
Proteins Nucleic acids Cell wall Cell membrane
Ionizing radiation:
Radiation ejects orbital electrons from an atom, causing ions to form Causes the most damage to proteins
Ultraviolet (UV) radiation:
Ranges from 100 nm to 400 nm Most lethal from 240 nm to 280 nm (peak at 260 nm) Germicidal lamp: generates 254 nm Not as penetrating as ionizing radiation
Desirable qualities of a germicide:
Rapid action in low concentrations Solubility in water or alcohol and long-term stability Broad-spectrum microbicidal action without toxicity to human and animal tissues Penetration of inanimate surfaces to sustain a cumulative or persistent action Resistance to becoming inactivated by organic matter Noncorrosive or nonstaining properties Sanitizing and deodorizing properties Affordability and ready availability
Aqueous solutions:
Solutions containing pure water as the solvent
Disinfection:
destroys most microbial life, reducing contamination on inanimate surfaces.
Virucide:
inactivates viruses, especially on living tissue
Fungicide:
kills fungal spores, hyphae, and yeasts
Bactericide:
A chemical that destroys bacteria except for those in the endospore stage
Sterilization
A process that destroys or removes all viable microorganisms, including viruses. Any material that has been subjected to this process is said to be sterile Sterilized products are essential to human well-being. Surgical instruments Syringes Commercially packaged foods
Dry heat:
Air with a low moisture content that has been heated by a flame or electric heating coil Temperature ranges from 160°C to several thousand degrees Celsius Lack of water increases stability of some protein configurations, necessitating higher temperatures At high temperatures, dry heat oxidizes cells, burning them to ashes
Irradiation:
Bombardment of microbes with radiation
Hypochlorites:
Broadly used in industry and allied health Household bleach is a weak solution of sodium hypochlorite
How antimicrobial agent works
Cellular targets of physical and chemical agents: The cell wall The cell membrane Cellular synthetic processes (DNA, RNA) Proteins
Moist heat:
Hot water, boiling water, steam Temperature ranges from 60°C to 135°C Operates at lower temperatures and shorter exposure times than dry heat Most microbicidal effect is coagulation and denaturation of proteins to permanently halt microbial metabolism
Fungistatic chemicals:
Inhibit fungal growth
Effects of UV radiation:
Initially absorbed by DNA Form pyrimidine dimers Abnormal linkages between adjacent pyrimidines (thymine and cytosine) Interfere with normal DNA replication and transcription Lead to inhibition of growth and death
Lyophilization:
A combination of freezing and drying Common method of preserving microbes and other cells in a viable state Pure cultures are frozen instantaneously and exposed to a vacuum that removes water
Antisepsis:
Also called degermation The same as disinfection, but on a living surface
Decontamination:
Also called sanitization The mechanical removal of most microbes from an animate or inanimate surface
Filtration:
An effective method to remove microbes from air and liquids Fluid is strained through a filter with openings large enough for liquid to pass through, but too small for microbes to pass through Most filters are perforated by precise, uniform pores Coarse: 8 microns Ultrafine: 0.02 micron Allows selection of the minimum particle size to be trapped Even smaller pore diameters permit true sterilization by removing viruses and even large proteins
Tinctures:
Antimicrobial chemicals dissolved in pure alcohol or water-alcohol mixtures
Antispesis
Antisepsis: application of chemical agents (antiseptics) to exposed body surfaces and surgical incisions to destroy or inhibit vegetative pathogens
Heat resistance and thermal death
Bacterial endospores exhibit the greatest resistance to disinfection methods. Destruction of spores usually requires temperatures above boiling Resistance varies Vegetative cells vary in their sensitivity to heat.
Susceptibility of Microbes to Heat
Both temperature and length of exposure should be considered for adequate sterilization. Higher temperatures allow shorter exposure times Lower temperatures require longer exposure times
Microbistatic agents:
Chemicals used to control microorganisms in the body (antiseptics and drugs)
The Effects of Cold and Desiccation
Cold treatment merely retards the activities of most microbes. Some microbes are killed by cold temperatures Most are not adversely affected by gradual cooling, long-term refrigeration, or deep freezing
Semicritical medical devices:
Come into contact with mucosal membranes
Iodophors:
Complex of iodine and alcohol Have replaced free iodine solutions in medical antisepsis
Bacterial endospores:
Considered the most resistant microbial entities Destruction of endospores is the goal of sterilization Any process that will kill endospores will invariably kill less resistant microbial forms Other methods of control (disinfection, antisepsis) act on microbes that are less hardy than endospores
Effects of agents on the cell wall
Damage to the cell wall: Blocking cell wall synthesis Digesting the cell wall Breaking down the surface of the cell wall A cell with a damaged cell wall is fragile and becomes lysed easily. Detergents and alcohols disrupt the cell wall
What is microbial death?
Death of microscopic organisms: Harder to detect No conspicuous vital signs Lethal agents do not alter the overt appearance of microbial cells Loss of movement cannot be used to indicate death Special qualifications are needed to define and delineate microbial death
Desiccation:
Dehydration of vegetative cells directly exposed to normal room air Some delicate pathogens can be killed by desiccation. Some pathogens can be preserved upon desiccation. Desiccation can preserve foods.
Phenolics:
Destroy vegetative bacteria, fungi, and some viruses Able to act in the presence of organic matter Too toxic to use as antiseptics Phenolic derivatives combined with soap Bisphenols: Aerosol sprays and cleansing soaps Triclosan: Disinfectant and antiseptic chemical added to many products Chlorhexidine: Hand scrubbing, surgical prep, and other medical uses
Practical Concerns in Microbial Control
Does the item in question require sterilization, or is disinfection adequate? Is the item to be reused or permanently discarded? If it will be reused, can the item withstand heat, pressure, radiation, or chemicals? Is the control method suitable for a given application? Will the agent penetrate to the necessary extent? Is the method cost- and labor-efficient, and is it safe?
Low-level germicides:
Eliminate only: Vegetative bacteria Vegetative fungal cells Some viruses Used to clean noncritical materials: Electrodes Straps Furniture that touches skin but not mucous membranes
Radiation:
Energy emitted from atomic activities and dispersed at high velocity through matter or space Radiation suitable for microbial control: Gamma rays X rays Ultraviolet radiation
Nonionizing radiation:
Excites atoms, raising them to a higher energy state Leads to the formation of abnormal bonds within molecules such as DNA
Critical medical devices:
Expected to come into contact with sterile tissues
Phenol:
First used as the major antimicrobial chemical Toxic and irritating side effects
Halogens
Fluorine, bromine, chlorine, iodine Microbicidal and sporicidal Active ingredients in ⅓ of all antimicrobial chemicals
Iodine and its compounds
Free iodine (I2) Iodophors All classes of organisms are killed by iodine if proper concentrations and exposure times are used Not adversely affected by organic matter
Chlorine and Its Compounds
Gaseous chlorine (Cl2) Hypochlorites (ClO1) Chloramines (NH2Cl) Kill bacteria, endospores, fungi, viruses Less effective if exposed to light, alkaline pH, and excess organic matter
Methods of physical control
Heat is the most widely used method of microbial control. Other methods: Radiation Filtration Ultrasonic waves Cold
Alcohols as Antimicrobial Agents
Only ethyl and isopropyl alcohol are appropriate for microbial control. Greater efficacy at 70% Destroys vegetative microbial forms but not endospores. More effective against enveloped viruses than non-enveloped viruses
Antisepsis/degermation: (decontamination)
Reduction of the number of microbes on the skin Involves scrubbing the skin or immersing it in chemicals, or both Emulsifies oils on the outer cutaneous layer Mechanically removes potential pathogens on the outer layers of the skin
Historical microbial control methods:
Salting food Smoking food Pickling food Drying food Exposing food, clothing, and bedding to sunlight
Decontamination
Sanitization: Any cleansing technique that mechanically removes microbes and debris Reduces contamination to safe levels Sanitizer: a soap or detergent used to sanitize
Antimicrobial chemicals
Solid Liquid Gaseous For convenience, solid or gaseous antimicrobial chemicals are dissolved in water, alcohol, or a mixture of the two
Aseptic techniques:
Sterile methods that exclude all microbes
Four possible outcomes of microbial control outside the body:
Sterilization Disinfection Antisepsis Decontamination
How Agents Affect the Cell Membrane
The cell membrane and viral envelope are composed of lipids and proteins. Disruption of the cell membrane causes: Loss of selective permeability Loss of vital molecules Allows the entry of damaging chemicals
Thermal death point:
The lowest temperature required to kill all microbes in a sample in 10 minutes
Thermal death time:
The shortest length of time required to kill all test microbes at a specified temperature
Disinfection
The use of a physical process or a chemical agent (disinfectant) to destroy vegetative pathogens but not bacterial endospores. Disinfectants are normally only used on inanimate objects
Noncritical medical devices:
Those that do not touch the patient or are only expected to touch intact skin
Aqueous iodine:
Topical antiseptic, treatment for burned skin00
Chloramines:
Used as alternatives to pure chlorine in water treatment
Iodine tablets:
Used for disinfecting water
Applications of filtration
Used in liquids that cannot withstand heat. Alternative method for decontaminating milk and beer. Important step in water purification. Efficient means of removing airborne contaminants.
Iodine tincture:
Used in skin antisepsis
Sporicide:
capable of killing endospores
Germicide and microbicide:
chemical agents that kill microorganisms
Sterilization:
the destruction of all microbial life.
Factors Affecting the Microbicidal Activity of Chemicals
Nature of microorganisms being treated Nature of material being treated Degree of contamination Time of exposure Strength and chemical action of the germicide
Factors that effect death rates
Number of microorganisms Nature of the microbes in the population Type of microbial growth Temperature and pH of the environment Concentration of the agent Mode of action of the agent Presence of solvents, interfering organic matter, and inhibitors
Cold sterilization:
Ionizing radiation used as an effective alternative for sterilizing materials Used for materials sensitive to heat or chemicals
High-level germicides:
Kill endospores Sterilants if properly used Critical items that are not heat-sterilizable Catheters Heart-lung equipment
Intermediate-level germicides:
Kill fungal but not bacterial spores, resistant pathogens such as the tubercle bacillus, and viruses Used to disinfect semi-critical items: Respiratory equipment Thermometers
Agents that Affect Protein and Nucleic Acid Synthesis
Microbial cells depend on an orderly and continuous supply of proteins. - Substances that inhibit ribosomes will also inhibit protein synthesis Nucleic acids are necessary for the continued functioning of microbes - Agent that impedes the transcription of DNA replication or DNA transcription or changes the genetic code is antimicrobial
Primary targets of microbial control:
Microorganisms capable of causing infection or spoilage Constantly present in the external environment and on the human body Often contains mixtures of microbes