ET-Ch08 - CONTROL OF MICROBIAL GROWTH WHY Major reasons...

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Unformatted text preview: CONTROL OF MICROBIAL GROWTH WHY? Major reasons include: Preventiorh’itreatinent of infection (general) Prevention of infection (aseptic surgery) Prevention of food spoilage Terminology ' Sterilization - the complete destruction of all microbes on an object . Disinfection - the reduction or inhibition of microbial growth on a non—living surface (i.e._., disinfectants) . Antisepsis - the reduction or inhibition of microbial growth on living tissue (i.e., antiseptics) - - -I flnlhmmic numb-Fr of microbial summon Bacterial death curves -Baeteria tend to die at a fixed exponential rate; when plotted logarithmically: this looks like a straight, downward sloping line (sort of like death phase from last lecture). 'i .flflflflflfl Line lug decrease — - 9|] “ti-I of population killed ”mm SDDDGD {J '3' 25am logarithm ol' I‘IurlibEr o! mlrroblal summon Lngnrithm of numb-Er of mirmhial survivors Time [mir‘Li Time {min}: How do various agents harm bacteria? Damage to the cell wall Alteration of membrane permeability (or membrane destruction) Damage to Proteins andfor Nucleic acids: This can cause enzymes to malfunction or inhibit the processes of DNA replication and/or protein synthesis Since bacteria have both physical and chemical needs, they can be controlled by either physical or chemical agents. Physical Agents * HEAT - Quiekly denatures preteins; alse may damage the membrane (makes it mere fluid) and denature DNA and RNA * Meist heat - very effective — Beiling - kills mest haeteria in 10 111inntes3, but endesperes and seme Viruses are resistant (30 minutes reeemmended fer drinking water) — Anteelaving - 111eistheat1mder high pressm'e allews the temperature te inerease te 1'21 degrees C in mest auteelaves (under 15 psi efpressm‘e [2 atm. tetal]). Very pepnlar in sterilizing enltm'e media and lab instrmnents. In essences a “pressm'e eeeker”. Enruust valve 5 Dpel'fllng “he [M mum ”ml-glummimm mrmfllflfiml Stun-Ito “M :med-mbm I chub-er “$333“ 2" inhuman-II: alert-1r value bhrmusufluly {annulledlnd chanson \ mudwllh purl Hum Iain-an: when ii! I! flhlllslld. Moist heat cont'd — Pasteurizatien - shert duratien/high temperature treatment ef feed (72 degrees C fer 15 seeends.) Dees net sterilize, but kill rnan}r undesirable baeteria and does net alter the flavor er texture ef the feed. Dry heat . Good example - flaming a loop used to transfer bacteria to a petri plate. Dry heat kills by oxidizing effects, can denature proteins and DNA. ' Methods - — incineration (combustion) — dry ovens COLD - COLD inhibits baeterial growth: but usually does not kill the microorganism! I I l! - Refrigeration and freezing are common uses of eold to inhibit bacterial growth. Desiccation . Desiccation - removal of water - often stops growth, but again doesn*t necessarily kill. - Many endospores are resistant to desiccation . Freeze-drying, or lyophilization - the simultaneous combination of freezing and desiccation. A great way of preserving samples of microbes or even food! Filtration . Passage of a liquid or gas through a filter with pores small enough to retain microorganisms. ' Membrane filters are available that oan eliminate most baoteria; air filters oan do the same. ' Cannot remove toxins that might be present. More Physical methods ' Osmotic pressure treatment (use of salts). Causes plasmolysis, and is often used in preservation of food. Salts and sugar solutions are used to “cure"meats and preserve fruits, respectively. . Radiation: kills bacteria in many ways depending on type and dosage: — Ionizing radiation (gamma rays. X-rays) - can cause large scale mutations in DNA. Often used in disinfecting food (somewhat controversial). — Non-ionizing radiation (UV. microwaves) - can cause abnormal base linkages in DNA (e. g. thymine dimers). and heat (microwaves). Chemical Agents - Disinfectants - Halogens (Group VII elements) — Chlorine (In gaseous form or in a compoimd) — When chlorine in various forms is added to water. hypochlorous acid fonus. which is a strong oxidizing agent and dis1upts enzymes — ICompressed chlorine gas is used in disinfection of public drmkmg water and pools (small quantities [1 ppm] are useful). — Hypochlolite is the chemical of choice in treatment of swimming pools. — Bleach (sodiiun hypochlorite) (Clorox) is a common household disnifectant. — Pros: Kills not only bacterial cells but most endospores, fiuigi. Viruses. and algae. — Cons: Not effective at basic pHs; relatively imstable, especially if exposed to light. including smflight. Chemicals - Disinfectants ' Halogens II — Iodine (acts similarly to chloriner by denaturing enzymes) — In tincture forms which is a solution in alcohol, it is (was) often used as an antiseptic. — lodine tablets can be used to disinfect drinking water (cg, while camping); the water doesn't taste so great thereafter! . Fluorine and Bromine are potentially dangerous to handle, and not generally used for these purposes, although some Bromine-based pool cleaning systems exist Chemicals - Disinfectants ALCOHOLS — Work by dissolving membrane lipids and denaturing proteins — Ethanol and Isopropyl aloohol (rubbing aloohol) are most popular. — They oan be used as antiseptiosr or for wiping down surfaces: they then quiokly evaporate. Chemicals - Disinfectants Surface acting agents (surfactants) — Detergents and scaps — Chemical structure: lcng chain hydrccarbcn with a charged “head“; they are amphipathicr like phcsphclipids. — Detergents act by disrupting cell membranea and are alsc gccd cleansing agents and emulsifiers. — Scap (an anicnic detergent) isn't much cf a disinfectant but thrcugh emulsificaticn cf cils and scrubbing acticnfl its use remcves man}r germs frcm skin surfaces. Chemicals - Disinfectants Surface acting agents 11 — Quaternary ammonium salts (QUATS) — They are cationic detergents (ammonium ion bound to four other groups) and work by disrupting plasma membranes. — Very popular in household disinfectants, like LysoL bathroom cleaners, even mouthwashes, in which case we must call them antiseptics — Popular example: n—Alkyl dimethyl benzyl ammonium chloride (where the alliyl groups are a mixture of long chain hydrocarbons [12-1 8Cs]). Common household cleaners may use QUATS with alcohol. Chemicals - Disinfectants Phenol and phenolics — Disrupt plasma membranes and denature proteins — First chemieal used by Lister to disinfect wounds — Toxic: so not used often as an antiseptic: exeept in threat lozenges and sprays (at low concentration!) — Phenolies are related compounds (usually an aromatic ring with a different functional group or added groups) that are less toxic and or smelly - some of these are used in some formulations of Lysol and other household disinfectants. Chemicals - Disinfectants - Aldehydes (organic compounds with a terminal CHO group) — Formaldehyde is the most famous and very effective. They work by inactivating proteins and nucleic acids. Formaldehyde is very toxic. which limits its general usefulness. — Formalin is the commonly used liquid preservative made by dissolving formaldehyde gas in water. — Glutaraldehyde (with two terminal CHO groups) is less toxic and very effective and commonly used (c. g. for sterilization of medical instruments). Heavy metals . The}! work by inactivating proteins by interacting with functional groups and include mercury, silver, copper, and zinc. ' An antiseptic example would be ”mercurochrome”, a solution of mercury often used similarly to tincture of iodine, which is no longer considered a good antiseptic due to the toxicity of mercury, Chemical food preservatives - Many are weak organic acids. Most popular are benzoic acid and sorbic acid, usuallv encountered as sodium salts, (eg, sodium benzoate.) . Nitrite salts prevent germination of C. bomlfmim endospores in meat and so often are used as presert.-'atix.-'es in bacon, etc. Who's more susceptible? . Gram-positive bacteria are generally more suseeptible than gram-negatix-e baeteria . Endespores are very resistant ' Enveloped Viruses are often NIORE SUSCEPTIBLE to chemicals than non- enveleped ones. ...
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