Population growth patterns Learning objectives: To be able to describe a typical growth curve of a bacterial population and reasons for lag phase, exponential phase, stationary phase and death phase.To be able to describe a typical growth curve of a bacterial population and reasons for lag phase, exponential phase, stationary phase and death phase. To be able to describe the effect of temperature, pH, nutrient availability and oxygen availability on growth.To be able to describe the effect of temperature, pH, nutrient availability and oxygen availability on growth.

4 phases of bacterial growth lag phase when the cells are active but there is little increase in number as the organisms acclimatise to their environment Individual bacteria may be increasing in size. Enzymes may be being synthesised to utilise the nutrient medium

Log phase (or exponential) nutrients in plentiful supply, ample space, little waste. State of balanced growth

Stationary or constant phase nutrients become depleted, fall in pH as CO2 and other metabolites build up. Death rate = birth rate

Death phase more cells die than are produced so the number of living cells declines. Causes of death include starvation, lack of oxygen and toxicity due to waste products

Factors Affecting Cell Growth Rate TemperatureTemperature –In general growth rate increases with temperature until the microbes' enzymes denature. –Bacteria are found in an astonishing variety of habitats, sometimes with extreme temperatures, where no other organisms can survive. –Different bacteria can grow in the range -7°C to 118°C. –These cells are adapted so that their enzymes have unusual optimum temperatures. –Bacteria can be classified according to their optimum temperature:

Optimumtemperature Name of group examples 20-45°C>45°C<20°CMesophilesThermophilesPsychrophiles Mammalian symbiotic bacteria. These are the commonest bacteria.Mammalian symbiotic bacteria. These are the commonest bacteria. Rotting vegetation, hot springs, volcanic vents. These are useful in biotechnology, since there enzymes do not denature easily (e.g. in PCR or biological detergents).Rotting vegetation, hot springs, volcanic vents. These are useful in biotechnology, since there enzymes do not denature easily (e.g. in PCR or biological detergents). Soil bacteria or polar ocean dwellers. These bacteria can cause food spoilage in refrigerators.Soil bacteria or polar ocean dwellers. These bacteria can cause food spoilage in refrigerators.

In school labs bacteria must not be grown above 30°C by law.In school labs bacteria must not be grown above 30°C by law. Why?Why? This prevents the growth of human pathogenic bacteria (which are mostly mesophiles).This prevents the growth of human pathogenic bacteria (which are mostly mesophiles).

pH Most microbes grow best in neutral pH and die in extreme acid or alkali conditionsas their enzymes slow down. The optimum pH for different microbes variesfrom 5 to 7.5. A few acid-tolerant species can grow at pH 2, but almost all microbes are killed below pH 4, which is why human stomach acid is so effective. The pH of a growth medium can change over time due to production of waste acids or alkalis, so it is important to use a pH buffer when growing microbes.

Nutrients All microbes need to be provided with the major elements CHONSP in an appropriate form.All microbes need to be provided with the major elements CHONSP in an appropriate form. Organotrophs require simple carbohydrates such as glucose or sucrose for respiration (and some may be able to digest starch), together with minerals, especially a source of nitrogen such as ammonia or amino acids. Why?Organotrophs require simple carbohydrates such as glucose or sucrose for respiration (and some may be able to digest starch), together with minerals, especially a source of nitrogen such as ammonia or amino acids. Why? to make proteins.to make proteins. Lithotrophs require mush simpler inorganic nutrients, such as methane, carbon dioxide, water and minerals.Lithotrophs require mush simpler inorganic nutrients, such as methane, carbon dioxide, water and minerals.

Oxygen Many microbes need molecular oxygen for respiration, just as animals do, and are termed obligate aerobes.Many microbes need molecular oxygen for respiration, just as animals do, and are termed obligate aerobes. In a solid agar medium these microbes will only grow on the surface, and a liquid medium must be well aerated.In a solid agar medium these microbes will only grow on the surface, and a liquid medium must be well aerated.

Some microbes are facultative anaerobes, which means they will use aerobic respiration if oxygen is available, but can switch to anaerobic respiration if oxygen is absent.Some microbes are facultative anaerobes, which means they will use aerobic respiration if oxygen is available, but can switch to anaerobic respiration if oxygen is absent. These include yeasts and lactic acid bacteria, and their metabolic products can be controlled by varying the amount of oxygen.These include yeasts and lactic acid bacteria, and their metabolic products can be controlled by varying the amount of oxygen. For example in brewingFor example in brewing

A few microbes are obligate anaerobes, which means they die in the presence of oxygen.A few microbes are obligate anaerobes, which means they die in the presence of oxygen. Anaerobes are useful industrially as they will grow throughout a medium, whereas aerobes will only grow on surfaces, or in well-stirred liquid media.Anaerobes are useful industrially as they will grow throughout a medium, whereas aerobes will only grow on surfaces, or in well-stirred liquid media.

Any of these factors can be controlled to select particular microbes, or particular metabolic products, or to control the growth rate.Any of these factors can be controlled to select particular microbes, or particular metabolic products, or to control the growth rate. You don't always want the fastest growth of cells, but the fastest production of a metabolite, which may occur under different conditions.You don't always want the fastest growth of cells, but the fastest production of a metabolite, which may occur under different conditions.

Water potential Very salty or sugary solutions can draw water out of organisms by osmosisVery salty or sugary solutions can draw water out of organisms by osmosis Organisms adapted to very salty conditions are called halophilesOrganisms adapted to very salty conditions are called halophiles

Industrial Microbiology Microbes produce many useful products, and humans have made use of this for thousands of years.Microbes produce many useful products, and humans have made use of this for thousands of years. Today there is a wide range of products made by microbial biotechnology, most of which are too complex to be synthesised by purely chemical techniques.Today there is a wide range of products made by microbial biotechnology, most of which are too complex to be synthesised by purely chemical techniques.

These include: food (bread, cheese, yoghurt, single cell protein (SCP));food (bread, cheese, yoghurt, single cell protein (SCP)); drink (beer, wine, vinegar);drink (beer, wine, vinegar); fuels (ethanol, methane);fuels (ethanol, methane); enzymes;enzymes; hormones;hormones; antibiotics;antibiotics; chemicals (citric acid, amino acids, steroids);chemicals (citric acid, amino acids, steroids); plastics; etc.plastics; etc.

Microbes are particularly useful for industrial purposes because: They have fast growth rates.They have fast growth rates. They have simple nutritional requirements and can often be fed on cheap or even waste substrates such as molasses, whey, wood pulp, etc.They have simple nutritional requirements and can often be fed on cheap or even waste substrates such as molasses, whey, wood pulp, etc. They can be grown indoors and their growth does not depend on seasons, climate, latitude, etc.They can be grown indoors and their growth does not depend on seasons, climate, latitude, etc. They are often tolerant of a wide range of temperatures and pH.They are often tolerant of a wide range of temperatures and pH. There are fewer ethical problems, when compared to animals.There are fewer ethical problems, when compared to animals. Prokaryotes can be more easily genetically modified than eukaryotic cells, since they don't have a nucleus.Prokaryotes can be more easily genetically modified than eukaryotic cells, since they don't have a nucleus. –Genes from other species can easily be inserted into the bacterial DNA to produce a range of gene products by fermentation, or –the microbe can be altered to produce far more of the product than normal.

Questions

Similarities Lag phase.Lag phase. –Little increase in cell number; –Individual bacteria may be increasing in size; –Enzymes may be being synthesised to utilise the nutrient medium;max.2 Log phase.Log phase. –The population shows exponential phase; –Nutrient supply is not a limiting factor; Stationary phase.Stationary phase. –The population growth slows down; –The number of new cells formed is balanced by the number of cells dying; –Limiting factors, such a nutrient supply and metabolic wastes have started to influence further increase in population size; max 2

Differences Lag phase for lactose alone longer; nutrient less readily available;Lag phase for lactose alone longer; nutrient less readily available; Growth of population less than with glucose; fewer cells and longer to increase;max.2Growth of population less than with glucose; fewer cells and longer to increase;max.2 Growth with lactose and glucose gives a greater population;Growth with lactose and glucose gives a greater population; Has a second lag; around minute and then increases again;Has a second lag; around minute and then increases again; Uses glucose first as growth curve similar to glucose alone; and then uses lactose;max 3Uses glucose first as growth curve similar to glucose alone; and then uses lactose;max 3 Death phase is occurring for glucose alone as all the energysource/glucose has run out.1Death phase is occurring for glucose alone as all the energysource/glucose has run out.1

CAMS p20Q4,5 p21 Q6-8 p22 Q9 p23 Q10 Qs p27