in this lecture we're going to discuss methods to control microbial growth when you hear the word sepsis this generally refers to microbial contamination so asepsis is the absence of significant contamination in the lab you employ aseptic technique every class to prevent contaminating both yourself and the cultures that you're working with in medicine aseptic surgery techniques are used to prevent microbial contamination of wounds in addition control of microbes also prevents food spoilage and foodborn illness for control of microbial growth there are varying levels in which microbial growth can be inhibited in some cases all microbial growth can be removed and in other cases the number of microbes is reduced but does not remove all of the organisms sterilization is the process of destroying all microbial life on an object including endospores typically this can be achieved by heat like for an autoclave which you've learned about in lab the rest of the methods that we will discuss will reduce the number of organisms but won't destroy them all the process of reducing or inhibiting microbes on non-living or inanimate surfaces is referred to as disinfection and the process of reducing or inhibiting microbes on living tissue is referred to as antisepsis and disinfection and antisepsis kill only vegetative pathogens but do not destroy endospores for example also often the chemicals that are used for disinfection are too harsh to also be used as an antiseptic for example you wouldn't spray Lysol directly onto a wound that you have but you would use it to disinfect a surface in addition to disinfectants and antiseptics there's also something called der germing which is the removal of microbes from a limited area Deering which mostly results in mechanical removal rather than the killing of most microbes in The Limited area includes things such as wiping an injection site with alcohol before giving a patient a vaccine for example washing your hands is another example of der germing you're not able to remove all of the organisms but you are trying to uh reduce the microbial number sanitation is the lowering of microbial counts on eating surfaces and utensils sanitation is usually accomplished by high temperature washing or in the case of glass wear in a bar washing in a sink followed by a dip in a chemical disinfectant for salmonella less than 1,000 cells is considered safe whereas staff needs 1 million cells to be considered infectious names of treatments that cause outright death of microbes have the suffix side which means to kill for example bacteriocide kills bacteria fungicide can you guess what fungicide kills fungicide kills fungi and virde inactivates viruses we don't necessarily say it kills viruses because remember that viruses are not considered to be living other treatments only inhibit the growth and multiplication of the bacteria and but they don't kill them and their names have the suffix Stat or stasis meaning to stop or steady their purpose is just to stop the microb from growing and so we're going to look at now the rate of microbial death when bacterial populations are heated or treated with antimicrobial chemicals they usually die at a constant rate for example suppose a population of 1 million microbes has been treated for 1 minute and 90% of the population is died and so if you look here so here we start with our initial population which is 1 million and after 1 minute uh 90% of the population has died so notice that 900,000 have died and we're now left with a population of 100,000 and for each consecutive minute that passes 90% more die off and so notice that in 100,000 990,000 of those would die we'd be left with 10,000 and again if we reduce it by 90% we would go from 10,000 down to 1,000 and the process would keep cons would keep continuing if we were to plot that growth curve if we plot it on an arithmetic scale notice that you're going to notice that quickly the number of microbes drops very fast as you're going from a larger number down to a smaller one and eventually it's going to kind of level off when you get to these lower numbers and so this type of growth curve is not an accurate way to actually describe um the way that the microbes grow in this case it's actually better to plot it on a log scale in which now you can see that the microbes do grow or I'm sorry do die at a constant rate and for the microbes the kill time is proportional to the number of microbes meaning that the more more the number of microbes that are present to begin it's going to take longer amounts of time to get rid of those microbes and we're going to talk in a minute that different species and stages meaning vegetative or endospores have different susceptibilities to both physical and chemical control agents and so this slide is just showing you that the kill time is proportion to the number of microbes and the more microbes there are to begin with the longer it's going to take to eliminate the entire population and so what you're looking at is this solid line here is a high population load so notice that you start with a higher initial population the dotted line represents a lower starting population and so notice that because you're starting with a lower number of microbes it takes less time to kill the entire population than if you start with a larger population and again this has to do with the fact that these microbes are going to kill be killed off in a proportional manner now there are several factors that will influence the effectiveness of the control agents um the first one is going to be the number of microbes present and again like I mentioned before the greater the size of the microbial population the longer it's going to take to eliminate that entire population next we have the concentration and the age of the agent often times the concentration of the agent matters for example 60% ethanol which is approximately the concentration found in most alcohol-based hand sanitizers is more effective to kill microbes compared to a 50% ethanol solution and so for different types of um chemicals they would have different concentrations that they would work ideally at in addition the age of the agent also matters because the chemical itself can change over time next we have temperature and pH affecting um these control agents and antimicrobial uh drugs often work better under War warmer temperatures because their activity is due to temperature dependent chemical reactions heat is also measurably more effective under acidic conditions and so in this way both temperature and pH play a role on these control agents next we have the presence of organic matter and presence of organic matter usually inhibits the action of chemical antimicrobial agents and the presence of organic matter in blood vomit or feces influence the selection of the type of disinfectants that you use microbes and surface biofilms when they're encased in The mucoid Matrix are difficult for biocides to reach effectively next we have evaporation and diffusability so evaporation meaning how quickly does that particular agent evaporate if you think of alcohol for example alcohol evaporates very quickly and so when you guys did your disinfectant and antiseptic test you might have noticed that when you tested um alcohol-based disinfectants that um in our test they didn't necessarily work very well and that remember could be due to the fact that the alcohol could have evaporated while it was in the incubator diffusibility also matters because the chemical would need to be able to diffuse and to get to the microbes in order to have an effect next we have time of exposure and chemical antimicrobials often required extended exposures to affect more resistant microbes or endospores meaning that you you'll need to treat the microbes for a longer period of time for it to be effective the age of the cells matter as well as the species and so we're going to talk about some of the characteristics that microbes have that make them more resistant to these control agents and it's important to note that very few chemicals achieve sterility meaning that they're not used used to sterilize but instead are only used to reduce microbial counts so for this slide we're looking at the microbial characteristics and what makes a organism to be more resistant or least resistant to these antimicrobial uh chemicals and so you'll notice that at the top of the list are the prons and prons are infectious proteins and these infectious proteins are the cause of neurological diseases like mad cow disease prons are extremely resistant to antimicrobial agents because they're not alive and they're heat resistant proteins and so to destroy prons infected animals like cows for example have to be incinerated in order to destroy these prons in addition surgical instruments are treated with enzymes called proteases and proteases are these uh protein enzymes and those enzymes are used to degrade or destroy uh protein and so when they treat the surgical instruments with the proteases they're trying to destroy those pron proteins and so again the reason that prons are so resistant is because they're not alive and they're heat resistant proteins next on the list we have endospores and we've been talking a lot about endospores in class and remember that we said that endospores the function of endospores is that bacteria produce them in order to survive harsh conditions so if food becomes limiting if they're exposed to UV um if they're um lacking nutrients right some bacteria such as billus clostridium they're able to pack up their DNA and remember that they're going to put the DNA in a tough keratin shell and it's this tough keratin shell that makes endospores to be very resistant to antimicrobial uh chemicals next on the list we have Micco bacteria and remember that an example of this that we've talked about is micob bacterium tuberculosis the cause of TB and for mobac terium remember that what makes them resistant is that they have a waxy lipid Rich cell wall that's made of 60% molic acid and this waxy cell wall is what allows them to be resistant to different types of chemicals as well as to environmental influences like drying out right mob bacteria can live on Surface for long periods of time and again it has to do with the fact that mobac terium has this waxy cell wall next on the list we have the cyst of protozoans and in Labs we've been looking a lot at protozoans and what you probably can recall is that most of the time it's the cyst that are what are the infective stage of the protozoans and that's because the cyst is able to survive in the stomach acid for example and so the cyst the reason that they are resistant is because they have a thick wall on the outside made of kiten and if you remember back to our macro molecules kiten remember is a polysaccharide it's a polymer of sugar and it's many glucose molecules linked together and remember that kiten is a structural polysaccharide meaning that it gives structure and strength and so this wall that Cy have allows them to be very resistant to chemicals and if you remember when we talked about Giardia or in the uh protone video that um Giardia for example is resistant to chlorination often and so again the cysts are going to be the more resistant part of the protozoan life cycle next we have the vegetative protozoans and the vegetative protozoans um are resistant because they have a pelic which is like a cell wall and so they're not as resistant as the cyst but they're still more resistant than other types of organisms the next organism on our list are the gr negative bacteria and the reason that gr negative BAC Bia are resistant is because remember that if we look at the cell wall of gr negative bacteria they have an outer membrane and in that outer membrane remember are these porins and porin are highly selective and they can regulate which molecules can penetrate the bacterial cell um an example of a gram negative bacteria that's extremely resistant is pseudomonas and remember that in our um antibiotic experiment that pomonis was resistant to almost every drug that we tested it against except the noroxin pomonis is a gram negative bacteria that's resistant to bacterioides as well and in fact pomonis can even grow in quary ammonium ions like the disinfectants that we looked at in lab and so gr negative organisms tend to be uh more resistant to microbial uh drugs than Grand positive bacteria would be next on the list we have fungi including most fungal spores and again these can be um fairly resistant due to the fact that they have a cell wall uh with kiten in them when we get to our lecture on virology we'll talk about that viruses generally fall into two categories those that have a virus or have an envelope and those that do not and the envelope that viruses have if you look down here notice that it says a virus with a lipid envelope and so this envelope has a lipid structure on the outside and viruses with lipid envelopes are not not resistant to antimicrobial control agents and that's because often times um antimicrobial uh agents that are lipid soluble will be able to dissolve those lipid envelopes and so this would have an effect on viruses like um for lipid enveloped ones these are uh vericella vaccine which causes chickenpox lenta virus which causes HIV and fluza virus which causes the flu so ones that have a lipid envelope are not very resistant those without the envelope and they just have a protein coat the non- envelope viruses are more resistant because they can't they're not dissolved by lipid soluble antimicrobial agents and so these type of non-enveloped viruses would be like rhinovirus adov virus um which caus the cold uh papiloma virus which causes the warts and again we'll talk a lot more about viruses when we get to our lecture on virology now gr positive bacteria and viruses with the lipid envelope both of these two categories are not resistant to antimicrobial control agents and so it's not that one is more than the other um they're both just not not very resistant to microbial uh control agents so antimicrobial agents typically destroy microbes by one of four mechanisms and the first is going to be the disruption of the cell membrane and damage to the lipids or the proteins of the cell membrane by antimicrobial agents causes cellular contents to leak into the surrounding medium and it interferes also with the growth of additional cells another way that antimicrobial agents inhibit microbes is by denaturation of proteins and or interference in enzymatic functions and remember that most enzymes are proteins and that proteins only function if they're in their correct confirmation if a protein becomes denatured then the enzymes won't function properly and the cell can cannot survive another way that um antimicrobial agents work um is through damage to nucleic acids nucleic acids carry the cell's genetic information and damage to nucleic acids makes it so that the cell can no longer replicate nor can it carry out normal metabolic functions such as the synthesis of enzymes because remember nucleic acids code for proteins and and lastly some antimicrobial uh agents work by destruction by free radicals and they act as oxidizing agents and the way that these types of um agents work is that they interfere with enzymatic functions and so these are the four most common ways um that antimicrobial control agents work and so we're going to look at now some physical methods of microbial control and the first one that we're going to look at is moist heat and moist heat functions to denature proteins and it's quick it's faster than dry heat um and boiling water gets it to be to about 100° C but some endospores can withstand 20 hours of boiling and so to get water above 100° C you need pressure and so one would use an autoclave which is steam under pressure and we talked about the autoclave in lab and that the way that the autoclave works is that by putting pressurized steam in there it allows the temperature to get above 100° C and in fact that the effective temperature or the standard operating procedure to run the autoclave is to set the autoclave at 121° C for 15 minutes at 15 psi and so we can write this on here so uh 121° C for 15 minutes at 15 psi or pounds per square inch and at that amount of time the autoclave is very efficient at sterilizing even endospores and so um the autoclave is used for heat stable items like metal glass cloth um it's also used to sterilize culture media like to make our broths and our plates that we use um in lab um it's used to sterilize instruments dressings intervenous equipments um applicators sponges Etc in order for the autoclave to be effective in sterilizing items the steam must contact the surface and you can see that in the picture on the right here so here we're looking at um a uh autoclave strip and this indicator strip was wrapped in foil and put in the autoclave the one on the bottom was not wrapped in foil and what you can see is that the one that was not wrapped in foil says that it's been sterilized and the one that is wrapped in the foil meaning that the steam didn't contact the strip directly was not sterilized and so in order for the autoclave to sterilize things it needs to be able that whatever you're trying to sterilize needs to be in direct contact to the steam in order for it to be sterilized if you didn't have access to an autoclave you could also use a pressure cooker that was set to similar conditions that I listed above and so now we're going to come to pasteurization and remember that this was named after Louis pasture and pasteurization is moist heat also but the temperature is lower than it would be for an autoclave and pasteurization is used routinely for milk juice ice cream yogurt beer Etc and the purpose of pasturization is to reduce spoilage organisms and pathogens but but it does not sterilize them so again its job is to reduce microbial numbers but not to eliminate all of them and the meth the reason we would use this method for food or beverages is because heating food too much like canning for example can change the taste of the food and so pasteurization is a good alternative to reduce spoilage microorganism but to not sterilize um the food and so there's several treatments that can be used for pasteurization um one treatment is to use uh 63° C for 30 minutes an equivalent treatment would be to go to a higher temperature but a short time and so notice that for this one so our high temperature we go up to 72° C and now we only have to use a 15-second exposure and this is the one this is what the dairy industry typically uses so dairy industry for milk for example typically uses this high temperature shorttime exposure um another type of um pasteurization that occurs is what's called ultra high temperature and so this is using 140° C for 4 seconds and and again this is used for uh milk sterilization and this can allow the milk to be stored for several months without refrigeration and this is most commonly used in Europe and also in the creamers that you see on the shelves that are not refrigerated at the grocery store now again remember pasteurization does not sterilize the items it simply reduces the microbial number and so because of this what are called thermoduric or heat sensitive organisms that are not likely to cause disease or spoilage those thermoduric organisms survive but again they don't cause disease and they don't cause spoilage in contrast dry heat typically kills by oxidation and this includes techniques like flaming items for example when we're talking about flaming your Loop in the lab and remember that when you flame your Loop you need to get it to be red hot in order to sterilize the loop before you use it for um your samples there's also a technique referred to as incineration and incineration is an effective way to sterilize and dispose of contaminated paper cups bags and dressings and then lastly we have hot air sterilization so simply putting the items into an oven for an extended period of time and notice that when we look at dry Heat versus moist heat that dry heat sterilization like in an oven for example takes longer to sterilize the items than an autoclave would so for an oven you would have to put the sample in at 170° cus for 2 hours the equivalent treatment for that is in the autoclave for 121 de C for 15 minutes at 15 psi and so again moist heat sterilization occurs faster than hot air sterilization another method to use is to use use low temperature to inhibit growth and this makes low temperature bacterio static meaning that it inhibits microbial growth but it doesn't sterilize it and when we talk about uh freezing things slow freezing is typically more harmful to bacteria because ice crystals can form and grow and disrupt cellular and molecular structures of the bacteria but it might might also have an effect on let's say the food that you're trying to freeze and so slow freezing might be bacterio cdal but under most conditions low temperature is bacteriostatic it just simply stops or inhibits the bacteria from growing and some examples of low temperature include refrigeration and if you look at a refrigerator it's typically set between a temperature of of 0 to 7° C usually around 3 to 4° C at this temperature this greatly reduces the metabolic rate of most microbes so that they can't reproduce or produce toxins another method is to use Flash freezing and this quickly freezes the food and makes the bacteria that could be present dormant but it doesn't kill them and then lastly we're going to see uh lyophilization which is a type of freeze drying and again all of these low temperature conditions are typically bacteriostatic high pressure is another way that microbial agents um can work and high pressure Alters the molecular structure of proteins and carbohydrates and this results in the rapid inactivation of vegetative bacterial cells and therefore is bacterio cyal because if the proteins are denatured that's going to cause the bacteria to die however endospores are resistant to high pressure so this doesn't work on all cells but it does help to reduce microbial number and so an example of a use for high pressure is that often times fruit juices can be preserved by high pressure because it has the advantage of preserving the flavors the colors and the nutritional value of that fruit juice next we have desiccation and in desiccation that refers to the removal of water and this typically prevents metabolism and is bacteriostatic so again they dry out it doesn't kill them but it does inhibit their growth uh tuberculosis bacteria can survive desiccation for months and viruses are fairly resistant to this as well um a use for desiccation is used to make dry fruit for example and again this is going to help to inhibit microbial growth next we have the use of high concentrations of salts and sugars to preserve foods and this is based on the effect of osmo osmotic pressure and remember that if you have a high concentration of sugar or salt we would say that that environment is hypertonic remember hyper means more so a hypertonic solution is one that has high salt or high sugar and if you put a cell in a hypertonic solution there's going to be more water inside the cell relative to outside and the water is going to go from in the cell out meaning it's going to leave the cell and that's going to cause something called plasmolysis remember that in bacteria which have a cell wall that means that as the water goes out the cell membrane collapses in now again this method is not bacterio cidal it's simply bacterio static meaning that as the high sugar salt concentrations are there it's dehydrating the bacteria but it's not killing them and so um for osmotic pressure um typically these would be used um so concentrated Salt Solutions are used to cure meats and thick sugar Solutions often are used to preserve fruits however molds and yeast are typically more resistant to osmotic pressure also soaps and detergents act as surface tension depressants and these function to loosen contamination from surfaces so again it's not going to sterilize it's simply going to try and loosen the contamination from a surface filtration is the passage of liquid or gas through a screen like material in this method there are different size pores available and in the process the bacteria gets stuck on the filter and the sterile filtrate is going to pass through and so in this method we're going to be able to filter this liquid and have the bacteria get stuck on the filter and the solution that comes through is what's going to be sterile and remember that we did a version of this in the lab where we used membrane filtration to test for the presence of coliforms in water samples in our experiment we weren't trying to get the sterile filtrate like most other experiments would be doing but instead we were trying to trap the bacteria on the filter and then see if there were any coliforms present in the water sample that we were testing when we look at the types of filters there's several types of filters that can be used and the first type is something called a HEPA filter and you guys have probably all heard of HEPA filters before HEPA stands for high efficiency particulate air and this typically removes microbes that are greater in size than3 micrometers and so again it's going to remove most of the microbes however if you think of micro uh micoplasma micoplasma remember is the smallest bacteria and it's 22 microns and so this would not remove microplasma there are also filters that remove microbes for example that are um greater than 22 micrometers and in some cases if you were trying to um make sure that the uh cell wall deficient micoplasma doesn't get through um we would typically use a filter that's .1 micrometers and a filter that's 0.1 micrometers would typically retain uh micoplasma even some viruses and even some large proteins and so one use for filtration is to uh filter the Municipal Water Supply so to filter the water supplies that um go to your house for example because again remember when we talked about Gardia that Gardia is resistant to um some types of chlorination and so in addition to try and help prevent pathogens from being in the water uh Municipal Water Supplies undergo a filtration process and so this is a class paper to get you think so the question says in what situation is filtration the only practical way to eliminate undesirable microbes so under what conditions would you think that using filtration might be an advantage and so I want you to pause the video think about your answer and then when you're ready hit play and you will get the answer okay so go ahead pause it and so the answer is that we would use filtration for heat sensitive liquids liquids that we would not be able to autoclave for example because it would it would uh change the chemical composition of those liquids and so this would be used for example um for some culture mediums so for example if I wanted to grow um human cells the media that we would use for human cells to grow cannot withstand the proteins that are in there cannot withstand uh the autoclave and so instead to filter that culture media we would use filtration instead it's also used to um sterilize um enzymes for example if for trying to purify enzymes vaccines medications again any type of heat sensitive liquids could be uh could use filtration to try and eliminate undesirable microbes so the next physical control that we're going to come to is looking at high energy or ionizing radiation and this includes xrays and gamma rays and so if we look on the Spectrum here so here is visible light meaning the light that we can see here we have ultraviolet light and then notice over here we have infrared we have microwaves we have radio waves and on this side of the spectrum we have UV X-ray and gamma rays and what you'll notice about this electromagnetic spectrum is is that these numbers here so notice that on this side it says 400 nanm and on this side it says 750 nanm and what that is referring to is the distance between the Peaks meaning that for these 750 nmet Peaks there's a greater distance between the two peaks for the 450 nanometer Peaks they're going to be closer together and the distance between the Peaks is going to be smaller now when we look at these uh wavelengths or the distance between the Peaks what's interesting to notice is that wavelength and energy are inversely related meaning that as the wavelengths get larger the energy gets lower as the wavelengths get smaller they're higher energy and so notice that ionizing radiation like gamma and x-rays these are very very high energy and these high energy uh radiations are used to ionize water which creates free radicals and the free radicals that get created damage DNA and again we've talked about that DNA if it's damaged is going to be bacteriocidal so it's going to sterilize uh whatever it is you're trying to sterilize it's going to kill the bacteria and we typically use um high energy or ionizing radiation to sterilize Pharmaceuticals disposable dental and medical supplies such as gloves plastic syringes suturing materials catheters Etc the food industry is also expanding the use of radiation for food uh preservation like for spices for example certain types of meats and produce are actually starting to use um ionizing radiation to sterilize um the food to allow it to um prevent spoilage for longer periods of time and then we have our non ionizing radiation and for the non-ionizing radiation these have a wavelength that is longer than ionizing radiation and so if you look back to the previous slide an example of this would be ultraviolet light so looking at wavelengths of around 260 NM now you probably know that going out in the sun for long periods of time is not healthy for you right you've probably all been told at some point in your life that don't go out in the sun because it can lead to skin cancer but I've bet you never really thought about why that is and what you need to know is that the reason that UV light can lead to skin cancer is because UV light is high energy and when UV light hits DNA it actually damages the DNA and so if we look at this DNA backbone here and we're looking at the nucleotide and so here is thyine here is thyine and when high energy UV light hits DNA and it damages it it causes something called a thyine thyine dier meaning that it's going to actually link those two thines together now the problem with that is that when the DNA goes to replicate like when the cell goes to divide normally an enzyme comes along and it would go okay here's a t I'm going to put an A and here's a t I'm going to put an A but when that enzyme which is called DNA polymerase when DNA polymerase comes along it gets to this thyine thyine dier and it doesn't recognize it it doesn't look like a thiamine anymore it doesn't look like anything it recognizes and so the enzyme then puts in some other nucleotide and then that change in the sequence is going to be what we call a mutation and mutations are what lead to cancer so damages to DNA is what causes cancer and bacteria are also sensitive to UV light remember in our experiment in lab right that the vegetative cells were more susceptible to U light compared to endospores endospores were much more resistant than uh the vegetative cells but again at some point notice that the UV did Kill the endospores as well and so this is going to sterilize um it uh it's bacterio cyal so again it's going to kill the DNA or kill the bacteria it's used to control micro in the air to disinfect vaccines and for other Medical Products as well and UV light is used also in a surgical Ward to help kill off endospores now one major disadvantage to using UV light is that it disinfects surfaces only and is not very penetrating meaning that again if you forgot in our UV experiment to take the lid off your plate remember that the UV light would not penetrate the lid and it wouldn't sterilize uh the samples and so UV light the drawback for it is that it's not very penetrating it only disinfects surfaces um only for microwaves microwaves typically Kill by heat now there was a study that showed that microwaving a wet sponge for 1 minute killed 99.999% of bacteria and this is in contrast to other studies that have found that microwaves are not very effective to control microbial growth and that in fact in some cases they actually still found uh microbes growing um inside the microwave and so the debate is still out as to whether or not uh microwaves work well um and one of the one of the reasons that we say this is that sometimes people will microwave their sponge in order to try and disinfect the sponge and so again there's kind of these um contrasting data some people say it does help some people say it doesn't and so the jury's still kind of out as to whether or not this is actually effective and so I just put this slide in here this is just to um summarize the different types of uh physical control methods that I just discussed with you um I'm not going to go through this again but you can look at it it kind of sums up everything that we've talked about so there's another one here again telling you what these different control methods are used for and so I have a question for you which of the following should not be relied upon for sterilization so notice the keyword sterilization so red ionizing radiation yellow dry heat green autoclaving or blue pasteurization so take a minute go ahead and pause your video and think of your answer and then when you're ready go ahead and push play again to hear the answer okay so hopefully now you've thought about your answer and if you said blue pasteurization you are correct and that's because remember that pasteurization is going to be a lower temperature for a shorter period of time and this low temperature is not enough to kill all organisms pasteurization is just there to reduce and to um control the number of microbes that are present in things like milk and Juice Etc and so pasteurization is not going to sterilize remember that sterilize means to remove all organisms pasteurization doesn't do that thermoduric organisms can survive pasturization ionizing radiation can be used for sterilization dry heat can be used for sterilization autoclaves these are all methods that can be used for sterilization with the exception of pasteurization so pasteurization is the one on the list that we would not use for sterilization now there's slides you'll notice at the end of your presentation um that talks about con uh chemical methods of microbial growth and um for chemical methods of uh microbial control we're not going to cover these in class um you already got to hear about some of these in lab for our disinfectant antiseptic um lecture and so for my class you won't be responsible I won't test you on those chemical methods of microbial controls but I will ask you to know the ones from the physical methods and so use your study guide as a tool to kind of figure out the level of detail that you would be required uh to know and so hope you enjoyed this lecture have a good night