All life need basic physical and chemical requirements to be met to survive. For most microbes, survival is limited to certain environmental niches where these physical and chemical requirements may be met.
Physical requirements (environmental)
Temperature
pH
Osmotic pressure
Chemical requirements
Carbon
Nitrogen, sulfur, and phosphorous
Trace elements
Oxygen
Organic growth factors
Minimum growth temperature
The lowest temperature at which a species will grow
Optimum growth temperature
The temperature at which it grows best
Maximum growth temperature
The highest temperature at which growth is possible
Classification by optimal growth temp
Psychrophiles (~-10 to 20°C)
Mesophiles (~10 to 45°C)
Thermophiles (~40 to 70 °C)
Hyperthermophiles (~65 to 110 °C)
Hyperthermophiles
Thermophiles
Mesophiles
Most bacteria are mesophiles and thrive between 20 and 40°C. Exposure of foods to extended durations around these temperatures is likely to result in rapid spoilage and potential expansion of bacteria harboring toxins.
Some bacteria are capable of growth and activity extending into lower temperatures and are often responsible for the slow but steady decay of food. These bacteria are referred to as psychrotrophs due to their ability to eat and grow at lower temperatures, even though they may prefer and optimally grow at warmer temperatures.
Classification by Optimal pH
Acidophiles: pH 1 – 5.5
Neutrophile: pH 5.5 – 8.5
Alkaliphile: pH 7.5 – 11+
Acidic (pH ~ 0) Hot Spring at Yellowstone Nat'l Park
Alkaline (pH 9.4) Lake Turkana in Kenya
Organisms that require certain conditions
Halophiles: organisms which thrive in high salt concentrations [high osmolarity]
Osmophiles: organisms which thrive in solute concentrations such as high sugar concentrations [high osmolarity]
Xerophiles: organisms which tolerate desiccation or dry conditions [low osmolarity]
Piezophiles (Barophiles): organisms that tolerate intense atmospheric pressure
The elements required for all life are carbon, hydrogen, oxygen, nitrogen, phosphorus, and sulfur. Elements needed in small amounts are called trace elements (e.g. iron, copper, zinc).
Organisms classified based on their ability to use or tolerate oxygen
Obligate aerobes: organisms that require oxygen to live
Facultative anaerobes: organisms that can tolerate the absence of oxygen, but prefer to use it (generate more energy in its presence – recall ETC)
Obligate anaerobes: organisms that require the absence of oxygen for growth
Aerotolerant anaerobes: organisms that cannot use oxygen, but can tolerate it
Microaerophiles: aerobic organisms that require only low doses of oxygen
Chemotrophs
Organisms that acquire energy through chemical metabolism
Phototrophs
Organisms that acquire energy through light reactions
Chemotrophs further classified based on carbon source
Chemoheterotrophs: use organic compounds
Chemoautotrophs: use CO2
Phototrophs further classified based on carbon source
Photoheterotrophs: use organic compounds
Photoautotrophs: use CO2
Binary fission
A form of asexual reproduction whereby a bacteria divides producing two genetically identical daughter cells
Events of Binary Fission
1. The circular DNA chromosome of the bacterium is replicated from the origin of replication (Ori)
2. The replicated DNA chromosomes migrate to the poles of the cell
3. The cell elongates
4. The cell wall constricts forming a septum with a ring of proteins (FtsZ and possibly MreB) leading to a new cell wall and membrane between daughter cells
Generation Time
The time it takes a cell to divide (or the population to double)
Bacterial growth is logarithmic (grows exponentially rather than linearly).
Phases of a bacterial growth curve
Lag Phase
Log Phase
Stationary Phase
Death Phase
Lag Phase
Very little or no cell division, the bacteria are generating enzymes and adjusting to their environment in preparation of growth
Log Phase
Bacteria divide rapidly (logarithmically) at the generation time rate, continues as long as resources are available and the environment is stable
Stationary Phase
Bacterial growth is equal to bacterial death, the population has reached its top capacity based on the resources available
Death Phase
Bacterial death exceeds bacterial growth, as resources are consumed, the population capacity continues to diminish. Dying cells may provide some nutrients resulting in potentially a long and persistent surviving population.
Direct Methods for quantifying prokaryotic culture sizes
Direct Plate Count
Most Probable Number (MPN) Method
Filtration
Indirect Methods for quantifying prokaryotic culture sizes
Turbidity
Metabolic Activity
Dry Weight
Turbidity
Growth of bacteria in liquid culture results in the solution becoming cloudy (turbidity). Measurement of light transmission (optical density or OD) may be used as an indirect measure of the extent of microbial growth