Microbiology Chapter 11: Physical and Chemical Control of Microbes PDF

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Chapter 11: Phy Physical sical and Chemical Con Contr tr trol ol of Micr Microbes obes 11.1 Controlling Microorganisms Joseph Lister started aseptic techniques with medical applications by using carbolic acid (phenol) soaked rags and instruments during and after surgery. Gengrene and other infections following surgery greatly disminished. There are four methods of microbial control: ● Sterilization: the destruction of all microbial life ● Disinfection: destroys most microbial life, reducing contamination on inanimate surfaces ● Antisepsis (degermation): the same as disinfection except a living surface is involved ● Decontamination (sanitization): the mechanical removal of most microbes from an animate or inanimate surface

Relative Resistance of Microbial Forms The main target of control are microorganisms capable of causing infection or spoilage. These are constantly present in the external environment and on the human body. ● Usually has a mixture of microbes: bacterial vegetative cells and endospores, fungal hyphae and spores, yeasts, protozoan trophozoites and cysts, worms, viruses, and prions

● Bacterial endospores are the most resistant microbial entities ○ Destroying them is the goal of sterilization because it will kill less-resistant microbial forms ■ Other methods act on endospores that are less hardy

● Sepsis: the growth of microorganisms in the blood and other tissues

● Asepsis: refers to any practice that prevents the entry of infections agents into sterile tissues and thus prevents infection

The Agents vs. the Processes The processes: ● sterilization ● disinfection

The agents used in the process: ● Bactericidal is an agent that kill bacteria; bacteriostatic prevents the growth of bacteria ● Fungicidal is an agent that kills fungi; fungistatic prevents the grwoth of fungi ● Virucidal is an agent that inactivates viruses

● Sporicidal is an agent that destroys bacterial endospores ● Germicidal and microbicidal are agents that kill microorganisms; microbiostatic prevent the growth of microorganisms

Practical Concerns in Microbial Control Considerations govern the selection of a workable methods of microbial control: ● Does the item require sterilization or is disinfection adequate? Are there spores to be destroyed or only vegetative pathogens? ● Is the item to be reused or discarded? If discarded, then the quickest and least expensive method should be chosen ● If reused, can the item withstand heat, pressure, radiation, or chemicals? ● Is the control method suitable for a given application? Or in the case of a chemical, will it leave an undesirable residue? ● Will the agent penetrate to the necessary extent? ● Is the method cost and labor-efficent, and is it safe? There are three categories for determining how devices that come in contact with patients should be handled: ● Critical: devices that are expected to come in contact with sterile tissues ○ Syringe needles, an artificial hip, surgical instruments, and biopsy forceps ● Semicritical: devices that come in contact with mucosal membranes ○ Endoscope, respiratory equipment, and vaginal speculum ● Noncritical: items that either don’t touch the patients or only come in contact with the skin ○ Stethoscopes and blood pressure cuffs

What is Microbial Death? Death is the permanent termination of an organism’s vital processes. Structural or metabolic damage that renders the microbe unable to reproduce any longer even under ideal conditions.

Factors That Affect Death Rate Death is not instantaneous but begins when a certain threshold of microbial agent (time and concentration) is met. Death continues in a logarthmic manner as time or concentration of the agent is increased. ● Since agents target the cell’s metabolic processes, active cells (younger, rapidly dividing) die more quickly than those less metabolically active (older, inactive). ● Survival of any cells is unlikely = point of sterilization

Length of time an agent is in contact with the microbes

How many microbes are present

Relative resistance of spores vs. vegetative forms

The difference between microbistatic and microbicidal agents

Additonal factors influence the action of antimicrobial agents: 1. The number of microorganisms. Higher amount requires more time to destroy 2. The type of the microorganisms 3. The type of microbial growth 4. The temperature and pH of the environment 5. The concentration (dosage, intensity) of the agent 6. The mode of action of the agent. How does it kill the microorganism? 7. The presence of solvents, interfering organic matter, and inhbiitors

How Antimicrobial Agents Work: Their Modes of Action An antimicrobial agent’s adverse effect on cells is known as its mode (or mechanism) of action. Agents affect one or more cellular targets, but more selective agents tend to target only a single cellular component and are more restricted to microbes. The targets of physical and chemical agents fall into four categories: 1. Cell wall 2. Cell or cytoplasmic membrane 3. Cellular synthetic processes (DNA, RNA)

4. Proteins

The Effects of Agents on the Cell Wall Chemical agents damage the cell wall by blocking its synthesis, digesting it, or breaking down its surface. ● The cell then becomes fragile and is lysed easily

The Effects of Agents on the Cell Membrane If the cell membrane is disrupted, a cell loses its selective permeability and cannot prevent the loss of vital molecules or stop the entry of damaging chemicals. ● Loss of these abilities leads to cell death. ● Detergents called surfactants work as microbicidal agents ○ Surfactants are polar molecules w/ hydrophilic and hydrophobic regions that can bind to the lipid layer and penetrate the hydrophobic region of membranes ■ This “opens up” the interface, leaving leaky spots that allow chemicals in and ions out

The Effects of Agents on Protein and Nucleic Acid Synthesis ● ● ● ●

Can inhibit the ribosomes Bind the DNA and inhibit transcription Mutagens can permanently inactivate DNA Chemicals that interfere with the function of DNA or RNA

The Effects of Agents on Protein Function ● Denaturation of proteins ● Blocking active sites

11.2 Methods of Physical Control There are two categories of controlling microorganisms: physical and chemical. Most prominent of the physical agents is heat. Other agents include radiation, filtration, radiosonic waves, and even cold.

Heat ● Higher temperatures (exceeding max growth temp) are microbicidal ● Lower temperatures (below minimum growth temp) are microbistatic Heat can be applied in either moist or dry forms. ● Moist heat occurs in the form of hot water, boiling water, or steam ○ 60°C to 135°C ● Dry heat refers to air w/ a low moisture content that has been heated by a flame or heating coil ○ 160°C to several thousand degrees

Mode of Action and Relative Effectiveness of Heat Moist and dry heat differ in their modes of action and their efficiency. ● Moist heat operates at lower temperatures and shorter exposure times ○ Coagulation and denaturation of proteins ● Dry heat dehydrates the cell and denatures proteins ○ It oxidizes cells (incineration)

Susceptibility of Microbes to Heat: Thermal Death Measurements Proper sterilization requires that temperature and length of exposure must be considered. ● A combo of these two constitutes the thermal death time (TDT): the shortest length of time requires to kill all test microbes at a specified termperature ● Another way to compare the two is the themal death point (TDP): the lowest temperature required to kill all microbes in a sample in 10 minutes

Common Methods of Moist Heat Control There are three ways that moist heat is employed to control microbes: ● Boiling water ○ Disinfection and NOT sterilization ● Pasteruization ○ Heat is applied to liquids to kill potential agents of infection and spoilage while retaining the liquid’s flavor and food value ○ NOT sterilization ● Steam under pressure ○ NOT sterilization ○ As pressure increases, the temperature increases and more steam is produced ○ Autoclave: a device used by health and commercial industries. Like a pressure cooker

Dry Heat: Hot Air and Incineration Dry heat is not as versatile or widely used as moist heat. There are two methods: ● Incineration ○ Most rigorous ○ Fast and effective but limited to metals and heat-resistant glass materials ○ 800°C to 6,500°C ● Hot-air oven ○ Uses gas and and has coild that radiate heat within an enclosed compartment ○ Heated circulated air transfers its heat to the materials in the oven ○ Sterilization requires exposure to 150°C to 180°C for 2-4 hours

The Effects of Cold and Desiccation Cold treatment slows down the growth of cultures and microbes in food during processing and storage. ● Some are killed by cold temperatures, by most are not affected enough Vegetative cells exposed to normal room air gradually become dehydrated, or desiccated. ● It can be a good way to preserve foods because it reduces the amount of water available to support microbial growth Lyophilization: a combo of freezing and drying. It is a common method of preserving microorganisms and other cells in a viable state for years. ● Cultures are frozen and exposed to a vacuum that removes the water ● Avoids the formation of ice crystals These methods are not good methods of disinfection or sterilization because their effects are uncertain and does not ensure all pathogens have been killed.

Radiation as a Microbial Control Agent Radiation: energy emitted from atomic activities and dispersed at high velocity through matter or space ● Only gamma rays, X rays, and UV radiation are suitable for microbial control

Modes of Action of Ionizing vs. Non-Ionizing Radiation Irradiation: bombardment of cells with radiation ● When a cell is bombarded, its molecules absorb some of the energy, leading to two consequences: ○ If the radiation ejects electrons from an atom, it causes ions to form. This is called ionizing radiation. ■ Causes catastrophic mutation in DNA and damages proteins that would usually fix it ■ Gamma rays and X rays ○ Non-ionizing radiation, UV, excites atoms by raising them to a higher energy state, but it does not ionize them ■ Leads to formation of abnormal bonds in molecules like DNA and thus a source of mutations ■ UV rays

Ionizing Radiation: Gamma Rays, X Rays, and Cathode Rays Ionizing radiation is an effective alternative for sterilizing materials that are sensitive to heat or chemicals (like plastics). It is a type of cold sterilization.

Devices like gamma-ray, X-ray, and Cathode-ray machines emit ionizing rays. ● Most to least penetrating

Non-ionizing Radiation: UV Rays UV rays ranges in wavelength from 100 nm to 400 nm. It is most lethal 240 nm to 280 nm. ● Does not penetrate as good as ionizing radiation so objects need to be directly exposed to it ○ Works against microbes in the air or on clean surfaces As UV passes through a cell, it is first absorbed by DNA. ● Specific damage occurs on the pyrimidine bases (thymine and cytosine) which form abnormal linkages with each other called pyrimidine dimers. ○ It interferes with normal DNA replication and transcription and results in inhibition of growth and cellular death

○ Also disrupts cells by generating toxic photchemical products called free radicals

Decontamination by Filtration: Techniques for Removing Microbes Filtration removes microbes from air and liquids. ● High Efficency Particulate Air (HEPA) filters remove airborne contaminants ○ It is used in operating rooms, for those with allergies ● In fluid filtration, solids are separated from liquids with fine ores ○ Mechanical force or vacuum suction help fluid through the filter ○ Does not sterilize unless pore size is small enough to trap everything

Osmotic Pressure Adding large amounts of salt or sugar to foods creates a hypertonic enivronment for bacteria in food, causing plasmolysis and not allowing bacteria to multiply ● Like cured meats and preserved jams

11.3 Chemical Agents in Microbial Control Selecting a Microbicidal Chemical Qualities we look for when selecting a chemical agent: 1. Rapid action even in low concentrations 2. Solubility in water or alcohol and long-term stability 3. Broad-spectrum microbicidal action without toxicity to human and animal tissues 4. Penetration of inanimate surfaces to sustain a cumulative or persistent action 5. Resistance to becoming inactivated by organic matter 6. Noncorrosive or nonstaining properties 7. Sanitizing and deodorizing properties 8. Affordability and ready availability No chemical can fulfill all of these requirements. There are three levels of chemical decontamination procedures. ● High: used on critical items that cannot tolerate heat ○ Kills endospores and if used properly, are sterilants ● Intermediate: used on semi-critical items ○ Kills fungal spores, resistant pathogens and viruses ● Low: used on non-critical items ○ Kills vegetative bacterial cells, vegetative fungal cells, and some viruses

Factors Affecting the Microbicidal Activity of Chemicals Factors that affect the usefulness of a germicide include: ● The nature of the microorganism being treated ● The nature of the material being treated ● The degree of contamination

● The time of exposure ● The strength and chemical action of the germicide

Germicidal Categories According to Chemical Group Several groups of chemical compounds are used for antimicrobial purposes in medicine and commerce. Includes: halogens, heavy metals, alcoholds, phenolic compounds, oxidizers, aldehydes, detergents, and gases.

Phenol and its Derivatives ● 5% Phenol is the standard against which chemical agents are tested and compared

● Each chemical is compares for the same length of time on the same organism under identical conditions ○ If the chemical being tested requires a greater concentration or a longer time than phenol, its efficiency is less than phenol ○ If the chemical being tested requires a lower concentration or a shorter time than phenol, its efficiency is greater than phenol ● Ration of tested chemical activity to phenol activity ○ Less than 1 means less efficient than phenol ○ More than 1 means more efficient then phenol...