2Learning intentionTo describe the main factors that are required to culture microbes
3Factors affecting microbial growth Slide to brainstorm with class
4Factors affecting microbial growth ContaminationGrowth mediaAvailability of oxygenTemperaturepHList of topics to be covered in the presentation
5Preventing contamination Agents that destroy or kill bacteria are called bactericidal agents.However, killing bacteria is not always possible so agents that inhibit bacterial growth are often used. These are called bacteriostatic agents.
6Why control microbial growth? This is done in the food industry to preserve food and prevent spoilage.It can prevent the spread of infectious disease, given the rapid growth rate of microbes.
7How to prevent contamination Heat sterilisationPasteurisationRadiation sterilisationFilter sterilisationUsing chemical agentsUsing disinfectants and antiseptics
8Heat sterilisation This involves using an autoclave. This sealed device allows the entrance of steam under pressure.The temperature used is 121ºC for 10–15 minutes.It is not the pressure in the autoclave that causes the death of the microorganisms but the high temperature achieved when the steam is placed under pressure.This procedure also ensures that bacterial spores are eliminated.Students will be asked to find out about spores in a task.
9PasteurisationThe process used to reduce the microbial population in milk and other heat-sensitive products.Named after Louis Pasteur, who first used heat for controlling the spoilage of wine.Pasteurisation involves passing milk through a heat exchanger. The temperature of the milk is raised to 71°C for 15 seconds. The milk is then rapidly cooled.
10Radiation sterilisation The number of microbes can be significantly reduced by using electromagnetic radiation.UV waves, for example, break down the DNA molecules in the microbe, thus causing death.Microwaves use thermal effects to kill bacteria.
11Filter sterilisationMembrane and depth filters can be used. This method of sterilisation is used for media that are sensitive to heat.
12Chemical agentsChlorine is used in air-conditioning systems to prevent the growth of bacteria such as Legionella.
13Disinfectants and antiseptics Disinfectants are chemicals that kill microorganisms.They should be used on inanimate objects.Antiseptics are chemical agents that kill or inhibit the growth of microorganisms but are non-toxic enough to be applied to living tissue.
14Nutrients Microbes are grown on culture media. This can be a solid or liquid preparation made to ensure the microbe has all the nutrients it needs for growth.The media is sterilised before use.
15Types of mediaComplex media contain one or more crude sources of nutrients. Often their exact chemical composition and components are unknown.Defined media are also called synthetic media. The components of these media are known and are relatively pure.
16Type of medium Example Extra constituents Bacteria grown Complex Nutrient agarMeat extracts, yeast extractMany bacteria will grow on thisDefinedM9Escherichia coliEnrichedBlood agarBloodStreptococcus pyogenesSelectiveMacConkey agarBile salts and crystal violet dyeGram –ve bacteria
17OxygenNot all microbes need oxygen, in fact for some microbes exposure to oxygen can be fatal!A microbe that must have oxygen present in order to extract energy from foodstuffs is called an obligate aerobe. It needs oxygen for respiration.An organism that would die in the presence of oxygenis an obligate anaerobe. This is because these organisms lack the enzymes needed to break down the dangerous chemicals (peroxides) made in aerobic reactions.
18Some microbes (especially yeast) can grow in the presence or absence of oxygen. These are called facultative anaerobes. They grow better in the presence of oxygen.
19Getting oxygenIn fermenters the actual process of delivering oxygen can be quite tricky.Piping may need to be used.Stirrers may be placed in the fermenter to ensure that the oxygen is available to all the microbes.
21TemperatureAll microbes have a range of temperatures within which they are able to grow.However, within this range there will be a temperature at which the organism grows best: its optimum temperature.
23Growth occurs due to chemical reactions and since enzymes are often involved, temperature will have to be tightly controlled.Some species of microbes live in extremes of temperature and can survive and grow despite the temperature.
24Psychrophiles Found in oceans, at the north or south poles. Adapted to live in very cold temperatures.
25MesophilesMicrobes that live at ‘normal’ temperatures, eg room temperatureThey live on the human body.
26Thermophiles Microbes that can live in very hot environments. Examples: volcanic vents, compost heaps, hot springs.
27pH As with temperature, microbes have an optimum pH. Fungi tend to prefer a more acidic environment (pH 4–6).Bacteria like a neutral pH.Vinegar, citric acid and other chemicals can be used to preserve food because they stop microbes from growing.Buffers can be used to keep the pH relatively constant.