Physical and Chemical Control of Microorganisms

Transcription

1 1 Physical and Chemical Control of Microorganisms I. Terms II. Factors which determine the effectiveness of control methods III. Methods of physical control IV. Chemical agents Terms 1) Control -- Limiting exposure to agents Microorganisms are part of our environment Infection can be controlled by limiting exposure to infectious agents 2) -- Removal or destruction of all microorganisms. sterile 3) Disinfection -- Removal or destruction of forms of pathogens. disinfectant 4) Antisepsis -- Removal or destruction of vegetative forms from. antiseptic Note: Antiseptics and disinfectants do the same thing, but in different environments

2 2 Terms (continued) 5) Sanitization -- any cleansing technique that mechanically removes microorganisms. Usually soaps More terms 6) -cide refers to. Bacteriocide kills bacteria, fungicide kills fungi, etc. 7) -static means to stand still. Refers to an agent which prevents but doesn t kill. Bacteriostatic or fungistatic or virustatic agent

3 3 Factors which determine the effectiveness of a control method 1. The number of microorganisms # of cells Death curve Time (min.) Note log scale on y axis (each increment is 10X reduction in numbers) Time (min.) Decimal reduction time (DRT) -- Time for a reduction in population size. Here DRT = If the goal is to reduce numbers to 10 2 then the size of the initial population will determine how long it will take, in this case, minutes.

4 Factors which determine the effectiveness of a control method (cont.) 4 Spore formers vs. non-spore formers # of viable cells Vegetative cells Time (min.) Factors which determine effectiveness of a control agent (cont.) Temperature and of the environment of the agent Mode of action of the agent Microbistatic vs. microbicidal Cellular target Presence of compounds or inhibitors of the control agent

5 5 Methods of Physical Control: Heat heat Hot water, boiling water, steam C Denatures proteins, nucleic acids heat Ovens >1000 C Denatures, oxidizes At lower temperatures, moist heat is effective than dry heat Moist heat methods I. Boiling water -- min. boiling kills all vegetative cells II. Pasteurization -- Heating to temp. below boiling to kill specific pathogens and increase shelf-time Batch method for 30 min. method for 15 sec. Results in killing of most viruses and 97-99% reduction in veg. stages of bacteria and fungi UHT (Ultra High Temp) Milk can last up to 3 months unrefrigerated Primary pathogen targets of pasteurization:, Listeria, Mycobacterium, Campylobacter, Brucella

6 6 Moist heat methods (cont.) III. Steam under pressure Highest temp. of moist heat at sea level -- C If increased, higher temps. can be reached Autoclaving Commonly: C at 15 PSI min. at this temp. is very effective Effects on cellular proteins Text, Fig. 11.4

7 7 Physical methods: radiation Physical methods: radiation -- knocks electrons off atoms --> ions e.g. gamma rays, x-rays Damages DNA and proteins by breaking bonds -- Fig Exposure hazard to humans Penetrates and liquids Used in fruits & vegetables, now meats Non-ionizing -- excites atoms but doesn t ionize e.g., sunlight Damages DNA by creating T-T dimers -- Fig. 11.8, Relatively safe to use Doesn t penetrate solids or liquids well sterilization, water treatment

8 8 Text, Fig Text, Fig

9 9 Other physical control methods Sonication -- Disruption using waves Filtration Text, Fig Chemical control agents I. Many kinds but most function by: Disrupting Altering and/or nucleic acid structure II. Effectiveness is determined by: Concentration Contact time

10 10 Major groups Phenol and its derivatives Alcohols Hydrogen peroxide Detergents Heavy Halogens Mainly, also iodine and fluorine Iodine iodophors most common iodine compound used in hospitals: Betadine, Povidone, etc. Less than free iodine due to slow release of free iodine. 3 Chlorine forms: Cl 2 (chlorine gas) OCl (hypochlorite -- common bleach ) NH 2 Cl (chloramines) In solution, these compounds combine with water to form hypochlorous acid (HOCl). This is the active form which destroys by Effective against: bacteria, fungi, spores, viruses Limitations: ineffective at ph : light and O 2 cause breakdown

11 11 Phenols phenol OH p-cresol (lysol) OH Cl CH 3 Hexachlorophene (phisohex) OH OH CH 2 Cl Phenolics affect protein function and/or disrupt membranes Cl Cl Cl Cl Alcohols Ethanol -- 70% to 95% Isopropanol -- more microbicidal but also more Destroy cell membranes, can coagulate proteins

12 12 Hydrogen peroxide (H 2 O 2 ) Forms free radicals (e.g. superoxides, hydroxyl radicals: see last lecture) which are toxic to all cells Also breaks down to H 2 O and O 2, therefore is strongly effective against. Detergents All solubilize membranes and disrupt proteins Text, Fig. 11.3

13 13 Detergents (cont.) Two types: (charged) and non-ionic Non-ionic and anionic detergents (like soaps) are not very microbicidal, although they may be very effective in sanitization. detergents (like benzalkonium chloride) are the most effective ionic detergents. Heavy metals Mainly only mercury and silver preparations are used now. Merthiolate, mercurochrome, silver nitrate, etc. Form ions which complex with cell components, stopping growth Disadvantages: Microbes can develop