Session 2 Factors affecting growth Session overview • pH • Temperature • Water activity • Redox potential • Combination of factors pH • pH is an important factor in affecting growth • All organisms have a pH range in which they can grow • Bacteria tend to have a smaller pH range for growth than fungi • Some microbes can grow at the extremes of the pH scale e.g. Acinetobacter can grow in pH as low as 2.8 Fusarium can grow between pH 1.8 and 11.1 Temperature • Like pH, all micro-organisms have a minimum, maximum and optimum temperature for growth • This is called the cardinal temperature range • Some micro-organisms can grow at extremes of temperature, but most grow within a narrow range • (Obligate) Psychrophiles Min –10, Optimum 10-15, Max 20oC These are microbes that can only grow at low temperatures e.g. Flavobacterium • Psychrotrophs Min –10, Optimum 25-30, Max 42oC These organisms are able to grow at low temperatures, but the cardinal ranges tend to be mesophilic e.g. Listeria monocytogenes • Mesophiles Min 5, Optimum 28-43, Max 52oC Microbes that grow at intermediate temperatures e.g. E.coli • Thermophiles Min 30, Optimum 50-65, Max 70oC Microbes found in locations with high temperatures e.g. Bacillus stearothermophilus Water activity • Water is essential for metabolic processes and reproduction • Water activity is the measure of water that is available for microbial growth-it is not simply a measure of the amount of water present e.g. ice will not support microbial growth because the water is not available for growth • Halophiles These microbes have adapted to grow in high concentrations of sodium chloride moderate halophiles can grow in 1-10% NaCl extreme halophiles can grow in up to 40% • Halotolerant These microbes can tolerate high NaCl concentrations e.g. Staphylococcus aureus • Osmophiles These microbes can grow in high osmotic pressures- e.g. environments with high sugar • Osmotolerant These organisms can withstand high osmotic pressures Both these categories to apply to yeasts only Redox potential This term refers to the state of oxidation of an environment and is connected to the availability of molecular oxygen Redox potential is given as either a positive or negative value • positive values are considered aerobic • negative values are considered anaerobic • Obligate aerobe Requires oxygen to grow. Most bacteria and fungi are in this group • Microaerophiles Require oxygen to grow, however will not grow in air. They can only withstand low levels of oxygen. e.g. Campylobacter will only grow in oxygen levels of 1-10% • Obligate anaerobes Requires the complete absence of oxygen. Oxygen is toxic to the organism • Aerotolerant anaerobe Normally require anaerobic conditions to grow, but can survive with low amounts of oxygen present • Facultative anaerobes Can grow with or without oxygen. Grow better with presence of oxygen • Oxygen independent Grow equally well with or without oxygen Name Redox range Ps fluorescens +500 to +100 Staph aureus +180 to –80 P vulgaris +600 to –150 Cl botulinum -30 to –550 Cl perfringens +16 to -230 Combination of factors • All the above factors should be considered in combination • Can be used in combination to prevent microbial growth-hurdle effect-defined as the combined effect of pH, redox etc in extending shelf-life e.g. The other factors tend to have an increasingly inhibitory effect, as water activity approaches the limit of growth Session summary • pH, temperature, water activity and redox potential all affect the growth of micro-organisms • Most micro-organisms grow best at neutral pH, high water activity, moderate temperature and positive redox potential • These factors interact to affect growth-the hurdle effect.