Growth Factors

The growth environment can greatly affect the life of a cell. In unfavorable growth conditions the cell will make chemical and metabolic adjustments, however certain conditions can inhibit the growth and activity of the cell.

Solutes

The solute concentration of the surrounding environment can affect cell growth. In a hypotonic environment, where the solute concentration inside is greater than that outside of the cell, osmotic pressure will cause water to enter the cell and eventually cause it to burst. Cells may try to adjust to this situation by producing or taking in more solutes. In a hypertonic environment, where the solute concentration is greater outside than inside of the cell, water leaves the cell shrinking the plasma membrane and dehydrating the cell. In this case the cell will become inactive and stop growing.

Moisture Requirement and Water Activity

Water functions as the matrix through which the cellular chemistry takes place. Metabolic activity and reproduction of cells require water. The transport of material such as nutrients, wastes, toxic materials, and other compounds in and out of the cell requires water. Water exists in two different forms. Bound water has a physical bond to other compounds in the growth environment and is not available for use in cellular functions. Available water is free and available for microorganisms to use. The degree of water availability for chemical activity and growth is called the water activity (a_w). The water activity is equivalent to the ratio of the vapor pressure of the growth medium to the vapor pressure of pure water. The values range between zero and one. The activity of pure water is equal to one.

Osmotic pressure, relative humidity, freezing point, and boiling point affect the water activity. Water activity is inversely proportional to osmotic pressure and directly proportional to relative humidity. Increasing the solute concentration and lowering the relative humidity would result in a reduction in water activity. Microorganisms differ in their abilities to grow at various water activities (Table 1).

Table 1. Minimum Water Activity of Microbial Groups

Adapted from Langlois and Newman, Food Microbiology Lecture Material, University of Kentucky, 1999.

Hydrogen Ion Concentration (pH)

The measure of hydrogen ion activity is denoted by pH and is equivalent to

(1)

Microorganisms have different maximum, minimum, and optimum pH levels for growth. They can grow in very acidic environments (pH 1-2) to very alkaline environments (pH 10-11). Acidophiles are capable of growing between a pH of 0 and 5.5. Neutrophiles and most bacteria grow in a pH range of 5.5 to 8.0. Fungi grow well at a pH ranging from 4 to 6. Alkalophiles proliferate in pH levels of 8.5 to 11.5 while extreme alkalophiles can grow in environments with a pH of 10 or higher. Despite the habitat external to the cell, the internal pH is usually close to neutral (7). Adjustments are often made by the organism to survive in an environment with dynamic pH levels. Some cells alter their own surroundings through the production of metabolic waste products. For example, the fermentation of Clostridium thermocellum yields lactate, acetate, ethanol, and other products. Yet when the concentration of ethanol gets too high the organism inhibits its own growth. The addition of buffers to the growth medium can be used to prevent growth inhibition.

Temperature

Temperature greatly affects the growth and function of microorganisms. Cell temperature is related to its surroundings. Temperatures above and below the tolerance of the organism can prevent enzyme catalyzed reactions that are important to cell function from taking place.

Psychrophiles can grow at temperatures between 0 and 20° C with an optimum temperature of 15° C or lower. Many are found in arctic and antarctic habitats. Cellular functions of these organisms are adapted to withstand cold environments. For instance, the amount of unsaturated fatty acids in their cellular membranes is high so the cell can be semi-fluid in the cold. Psychrotrophs can grow at 0° C but their optimum growth temperatures range from 20 to 30° C. The maximum temperature they can grow at is 35° C. These organisms are a factor in the spoilage of refrigerated foods. Most microorganisms are mesophiles. They grow optimally between 20 and 45° C. The minimum temperature for growth is 15° C and the maximum temperature is 45° C. Thermophiles can grow at 55° C or higher and have optimum temperatures between 55and 65° C. Thermophiles are mostly bacteria but do include some algae and fungi. Typical habitats are compost, soils, and hot springs. The enzymes of these organisms are also heat stable. Like the psychrophiles, the membranes are adapted to the environment and contain more saturated fatty acids with higher melting points so the cells remain intact. Hyperthermophiles, on the other hand, grow poorly at 55° C and have optimum growth temperatures ranging from 80 to 100° C.

Oxygen Concentration

The gaseous environment surrounding microorganisms can have a significant impact on their growth and metabolic system. Aerobes grow in the presence of oxygen whereas anaerobes grow in the absence of oxygen. Most higher organisms are obligate aerobes and are completely dependent on atmospheric oxygen for respiration. Facultative anaerobes do not require oxygen to grow but do better in an abundant oxygen environment. Aerotolerant anaerobes do not utilize oxygen and show no difference in their growth capabilities with or without oxygen. Organisms that cannot tolerate the presence of oxygen and die upon exposure are referred to as obligate anaerobes. These organisms do not undergo respiration and utilize anaerobic metabolic pathways. Microaerophiles require oxygen but can be damaged when present in too high of levels, thus they require oxygen concentrations between 2 and 10%.

Radiation

Electromagnetic rays with short wavelengths and high energy such as X-rays and gamma rays can be harmful for many microorganisms. UV radiation is deadly for all microorganisms. Radiation is commonly used to sterilize instruments and work areas from microbial populations. It is also a means of inducing mutations in the genetic makeup of various organisms.

 The optimum growth conditions vary for different microorganisms. Interactions of the various growth factors will alter the growth responses of the working organisms. Understanding the growth requirements for specific microbes will allow individuals to optimally produce them and their metabolic byproducts or destroy them and inhibit their growth as in the case of food production.

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