Microbial Growth Notes

Microbial Growth

Requirements for Growth

• Temperature
• Each species has different temperature requirements
• Minimum growth temperature:
• Lowest temperature at which growth will occur
• Most microorganisms go dormant below the minimum temperature
• Maximum growth temperature:
• Highest temperature at which growth occurs
• Above this temperature, enzymes can denature above the maximum temperature
• Optimum growth temperature:
• Temperature at which most rapid growth occurs

Classification by Temperature Requirements

• Different microbes can live at different temperature conditions
• Psychrophiles (cryophiles)
• Cold loving organisms
• Optimum growth temperature ranges below 25 degrees Celsius
• Psychrotroph
• Growth temp around 0-30 degrees Celsius
• Mesophiles
• Optimum temperature between 25-40 degrees Celsius
• Thermophiles
• Heat loving organisms
• Optimum temperature is above 40 degrees Celsius
• Hyperthermophiles
• Optimum is above 90 degrees Celsius

• Oxygen
• Microbes can be classified by how they use oxygen
• Obligate Aerobes (strictly aerobic)
• Require oxygen to grow
• Microaerophiles
• Grow best at low oxygen levels
• Facultative Anaerobes
• Use oxygen if it’s present, but can also grow without the presence of oxygen
• Aerotolerant Anaerobes
• They never use oxygen, but they can tolerate it
• Obligate Anaerobes
• They can only grow in the absence of oxygen

• pH
• Every organism has its minimum, optimum and maximum growth pH
• Microorganisms  usually create acidic environment

• Binary Fission: this is a process by which one bacterium splits into two
• Different in gram positive and gram negative
• Gram positive: after cell division, each wall is half old and half new
• Gram negative: after cell division, each cell wall has alternating bits of old and new cell wall

• When we talk about microbial growth, we are really referring to the number of bacteria, not the size of the cells
• The time required for a cell to divide (and its population to double) is called the generation time

Phase of Growth

• Lag phase
• In this period there is little or no cell division
• Log phase
• Cells in this phase begin to divide and enter a period of exponential growth
• Microbes are particularly sensitive to adverse conditions During this phase
• Stationary phase
• The number of microbial deaths balances the number of new cells
• Death phase
• The number of death exceeds the number of new cells formed
• Surviving phase
• In some species, there are cells that are able to survive for a long period of time

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