Growth is an increase in cellular constituents that may result in an increase in cell size, an increase in cell number, or both
Because observing growth of single cells is difficult, microbiologists usually study growth of a population of microbes
The Growth Curve
Population growth is usually analyzed in a closed system called a batch culture; it is usually plotted as the logarithm of cell number versus the incubation time
Lag phase-the period of apparent inactivity in which the cells are adapting to a new environment and preparing for reproductive growth, usually by synthesizing new cell components; it varies considerably in length depending upon the condition of the microorganisms and the nature of the medium
Exponential (log) phase-the period in which the organisms are growing at the maximal rate possible given their genetic potential, the nature of the medium, and the conditions under which they are growing; the population is most uniform in terms of chemical and physical properties during this period
Stationary phase-the period in which the number of viable microorganisms remains constant either because metabolically active cells stop reproducing or because the reproductive rate is balanced by the rate of cell death
Microbial populations enter stationary phase for several reasons including nutrient limitation, toxic waste accumulation, and possibly cell density
Responses to starvation conditions are of practical importance for medical and industrial microbiology; these responses include morphological changes and changes in gene expression and physiology
Death phase-the period in which the cells are dying at an exponential rate
The mathematics of growth-microbial growth can be described by certain mathematical terms:
Mean growth rate constant is the number of generations per unit time, often expressed as generations per hour
Mean generation (doubling) time is the time required for the population to double
Generation times vary markedly with the species of microorganism and environmental conditions; they can range from 10 minutes for some bacteria to several days for some eucaryotic microorganisms
Measurement of Microbial Growth
Measurement of cell numbers
Direct count methods do not distinguish between living and dead cells, and may be accomplished by direct microscopic observation on specially etched slides (such as Petroff-Hausser chambers or hemacytometers) or by using electronic counters (such as Coulter Counters, which count microorganisms as they flow through a small hole or orifice)
Viable cell counts involve plating diluted samples (using a pour plate or spread plate) onto suitable growth media and monitoring colony formation; this type of method counts only those cells that are reproductively active; because it is not possible to be certain that each colony arose from a single cell, results are usually expressed as colony forming units (CFU); analysis of aquatic samples is frequently done by counting colonies growing on membrane filters having pores small enough to trap bacteria
Measurement of cell mass-may be used to approximate the number of microorganisms if a suitable parameter proportional to the number of microorganisms present is used (suitable parameters may be dry weight, light scattering in liquid solutions, or biochemical determinations of specific cellular constituents such as protein, DNA, or ATP)
The Continuous Culture of Microorganisms
Used to maintain cells in the exponential growth phase at a constant biomass concentration for extended periods of time (these conditions are met by continual provision of nutrients and removal of wastes)
A chemostat-a continuous culture device that maintains a constant growth rate by supplying a medium containing a limited amount of an essential nutrient at a fixed rate and by removing medium that contains microorganisms at the same rate
A turbidostat-a continuous culture device that regulates the flow rate of media through the vessel in order to maintain a predetermined turbidity or cell density; there is no limiting nutrient
The Influence of Environmental Factors on Growth
Microorganisms grow in a variety of environmental conditions; certain microorganisms, referred to as extremophiles, grow under harsh conditions that would kill most other organisms
Solutes and water activity
If a microorganism is placed in a hypotonic solution (one with a lower solute concentration), water will enter the cell and cause it to burst, unless the microorganism has a protective mechanism to reduce the osmotic concentration of the cytoplasm
If a microorganism is placed in a hypertonic solution (one with a higher solute concentration), water will leave the cell causing dehydration, unless the microorganism has a protective mechanism to increase the osmotic concentration of the cytoplasm
Water activity (aw) is the amount of water available to microorganism; it is reduced by the interaction of water with solute molecules; osmotolerant organisms can grow in solutions of both high and low water activity; halophiles require environments of low water activity (high osmotic pressure due to saline conditions) in order to grow
pH
pH is the negative logarithm of the hydrogen ion concentration
Each species has a pH growth range and pH growth optimum a. Acidophiles grow best between pH 0 and 5.5 b. Neutrophiles grow best between pH 5.5 and 8.0 c. Alkalophiles grow best between pH 8.5 and 11.5 d. Extreme alkalophiles grow best at pH 10.0 or higher
Microorganisms can usually adjust to changes in environmental pH by maintaining an internal pH that is near neutrality; some bacteria also synthesize protective proteins (acid shock proteins) in response to pH
Temperature
Temperature has a profound effect on microorganisms; as the temperature rises, there is an increase in the growth rate due to increasing the rates of enzyme reactions; eventually a temperature becomes too high and microorganisms are damaged by enzyme denaturation, membrane disruption, and other phenomena
Psychrophiles can grow well at 0°C, have optimal growth at 15°C or lower, and usually will not grow above 20°C
Psychrotrophs (facultative psychrophiles) can also grow at 0°C, but have growth optima between 20°C and 30°C, and growth maxima at about 35°C
Mesophiles have growth optima of 20 to 45°C, minima of 15 to 20°C, and maxima of about 45°C or lower
Thermophiles have growth optima of 55 to 65°C, and minima around 45°C
Hyperthermophiles have growth optima of 80 to 110°C and minima around 55°C
Oxygen concentration
An organism able to grow in the presence of O2 is an aerobe; one that cannot is an anaerobe
Obligate aerobes are completely dependent on atmospheric O2 for growth
Facultative anaerobes do not require O2 for growth, but do grow better in its presence
Aerotolerant anaerobes ignore O2 and grow equally well whether it is present or not
Obligate (strict) anaerobes do not tolerate O2 and die in its presence
Microaerophiles require lower levels (2 to 10%) for growth because normal atmospheric levels of O2 (20%) are damaging to the cell
The different relationships with O2 are due to several factors including inactivation of proteins and the effect of toxic O2 derivatives (superoxide radical, hydrogen peroxide, and hydroxyl radical), which oxidize and destroy cellular constituents; many microorganisms possess enzymes that protect against toxic O2 derivatives (superoxide dismutase and catalase)
Pressure
Barotolerant organisms are adversely affected by increased pressure, but not as severely as are nontolerant organisms
Barophilic organisms require, or grow more rapidly in the presence of, increased pressure
Radiation
There are many types of electromagnetic radiation, including visible light, ultraviolet light (UV), infrared rays, radio waves, and ionizing radiation; some of these can be harmful to organisms
Ionizing radiation such as X rays or gamma rays is very harmful to microorganisms; low levels produce mutations and may indirectly result in death, whereas high levels are directly lethal by direct damage to cellular macromolecules or through the production of oxygen free radicals
Ultraviolet radiation damages cells by causing the formation of thymine dimers in DNA; this damage can be repaired by photoreactivation (repairs thymine dimers by direct splitting when the cells are exposed to blue light) or by dark reactivation (repairs thymine dimers by excision and replacement in the absence of light)
Many microorganisms that are airborne or live on exposed surfaces use carotenoid pigments for protection against photooxidation
Microbial Growth in Natural Environments
Growth limitation by environmental factors
Microbial environments are complex, constantly changing, and may expose a microorganism to overlapping gradients of nutrients and environmental factors
Liebigís law of the minimum states that the total biomass of an organism will be determined by the nutrient present in the lowest concentration relative to the organismís requirements.
Shelfordís law of tolerance states that there are limits to environmental factors below and above which microorganisms cannot survive and grow, regardless of the nutrient supply
In response to low nutrient level (oligotrophic environments) and intense competition, many microorganisms change their morphology or physiology or both
Counting viable but nonculturable vegetative procaryotes
When microorganisms are stressed they can remain viable but lose the ability to grow on media normally used for their cultivation (viable but nonculturable cells)
Numerous microscopic and isotopic procedures to identify and count viable but nonculturable cells have been developed
Quorum sensing and microbial populations
Quorum sensing (autoinduction) is a process by which bacteria can communicate and behave cooperatively
Chemical signals are secreted by bacteria and used to communicate with each other; gram-negative bacteria use acyl homoserine lactones (HSLs) as signals; gram-positive bacteria often use oligopeptide signals
