Microbial Growth

Growth When we speak of bacterial growth, we are talking about an increase in population size not an increase in the size of a single bacterium Animation: Overview of Bacterial Growth Binary Fission Bacterial cells replicate by a form of asexual reproduction called binary fission Animation: Binary Fission

Generation Time Generation Time – time required to complete fission cycle from parent cell to 2 daughter cells. (Doubling time). In terms of a population it is the amount of time needed to double the population. The length of the generation time is a measure of the Growth Rate of the microbe.  It varies depending on environmental conditions. Different microbes have different generation times. Escherichia coli - 20 minutes Mycobacterium tuberculosis – 18 hours Mycobacterium leprae - 14 days

Phases of Microbial Growth Cycle Lag phase The bacteria adapt themselves to growth conditions. It is the period where the individual bacteria are maturing and not yet able to divide. Log phase or Exponential phase (Doubling period) Cells in this phase are typically in the healthiest state. Growth is at maximal rate. Stationary phase Growth rate of population is zero. Growth is equal to death Either an essential nutrient is used up or waste product of the organism accumulates in the medium Death phase Lack of nutrients and increasing accumulation of wastes lead to number of cell deaths so the number of new divisions decreases.

The Growth Curve

Batch culture Batch culture: a closed-system microbial culture of fixed volume Typical growth curve for population of cells grown in a closed system is characterized by four phases Lag phase Exponential phase Stationary phase Death phase Animation: Bacterial Growth Curve

Continuous Culture: Continuous culture is an open process in which microbial cultures also grow continuously in log phase. One of the nutrients of culture medium is kept limited. Hence, at log phase the cell growth stops as the nutrients of limited quantity is exhausted. In continuous culture, fermented medium is continuously removed from the fermentor. Chemostat: It controls both population density (i.e. cell density) as well as growth rate of the culture. The chemostat is controlled by two elements, the dilution rate and the concentration of limiting nutrient e.g. carbon or nitrogen sources.

Chemostat

Turbidostat It is another type of continuous culture system. It consists of a photocell which measures the turbidity of the culture. The flow rate of medium through vessel is automatically regulated to maintain turbidity.

Effects of Temperature on Microbial Growth Microorganisms can be classified into groups by their growth temperature optima Psychrophile: low temperature Psychrotolerant Mesophile: midrange temperature Thermophile: high temperature Hyperthermophile (extreme thermophile): very high temperature

Temperature and Growth Relations in Different Classes

Effects of Temperature on Microbial Growth Mesophiles: organisms that have midrange temperature optima; found in Warm-blooded animals Terrestrial and aquatic environments Temperate and tropical latitudes

Microbial Growth at Cold Temperatures Extremophiles Organisms that have evolved to grow optimally under very hot or very cold conditions Psychrophiles Organisms with cold temperature optima; the most extreme representatives inhabit permanently cold environments Psychrotolerant Organisms that can grow at 0ºC but have optima of 20ºC to 40ºC; more widely distributed in nature than psychrophiles

Microbial Growth at HotTemperatures Thermophiles Hyperthermophiles produce enzymes widely used in industrial microbiology (E.g., Taq polymerase; used to automate the repetitive steps in the polymerase chain reaction (PCR) technique)

pH

pH Most bacteria and protozoa grow best in a narrow range around neutral pH (6.5-7.5) – these organisms are called neutrophiles Other bacteria and fungi are acidophiles – grow best in acidic habitats Acidic waste products can help preserve foods by preventing further microbial growth Alkalinophiles live in alkaline soils and water up to pH 11.5

Physical Effects of Water Microbes require water to dissolve enzymes and nutrients required in metabolism Water is important reactant in many metabolic reactions Most cells die in absence of water Some have cell walls that retain water Endospores and cysts cease most metabolic activity in a dry environment for years

Oxygen and Microbial Growth Aerobes: require oxygen to live Anaerobes: do not require oxygen and may even be killed by exposure Facultative organisms: can live with or without oxygen Aerotolerant anaerobes: can tolerate oxygen and grow in its presence even though they cannot use it Microaerophiles: can use oxygen only when it is present at levels reduced from that in air