Effect of various environmental factors on microbial growth- Effect of Temperature on the growth of microorganisms

The growth of microorganisms also is greatly affected by the chemical and physical nature of their surroundings. Microorganisms being mostly unicellular and poikilothermic (coldblooded), any change in the environment is easily reflected on them.  An understanding of environmental influences aids in the control of microbial growth and the study of the ecological distribution of microorganisms. 

Procaryotes are present anywhere life can exist. Many habitats in which procaryotes thrive would kill most other organisms. Some microorganisms can adapt to extreme and inhospitable environments. Procaryotes such as Bacillus infernus are even able to live over 1.5 miles below the Earth’s surface, without oxygen and at temperatures above 60°C. Microorganisms that grow in such harsh conditions are often called extremophiles.

Effect of Temperature on the growth of microorganisms 

Environmental temperature has a great effect on microorganisms since their temperature varies with that of the external environment.

Temperature has a major effect on microbial growth due to the temperature sensitivity of enzyme-catalysed reactions. Each enzyme has an optimum temperature at which it functions optimally. At temperatures below the optimum, it ceases to be catalytic. As the temperature rises from this low temperature, the rate of catalysis increases, till the optimal temperature is reached. The velocity of the reaction will roughly double for every 10°C rise in temperature. As the rate of each reaction increases, metabolism is more active, and the microorganism grows faster. However, beyond a certain point, further increases actually slow growth, and sufficiently high temperatures are lethal.

Microbial growth thus has distinct cardinal temperatures—minimum, optimum, and maximum growth temperatures. 

Based on their temperature ranges for growth, microorganisms  can be placed in one of five classes.

1. Psychrophiles grow well at 0°C and have an optimum growth temperature of 15°C or lower; the maximum is around 20°C. They are readily isolated from Arctic and Antarctic habitats; because 90% of the ocean is 5°C or colder, it is an enormous habitat for psychrophiles. Chlamydomonas nivalis is a psychrophilic algae seen in snowfield or glacier turning it pink with its bright red spores. Pseudomonas, Vibrio, Alcaligenes, Bacillus, Arthrobacter, Moritella, Photobacterium, and Shewanella are common psychrophiles  among bacteria. A psychrophilic archaeon, Methanogenium, has been isolated from Ace Lake in Antarctica.

Psychrophilic microorganisms have adapted to their environment in several ways. Their enzymes, transport systems, and protein synthetic mechanisms function well at low temperatures. The cell membranes of psychrophilic microorganisms have high levels of unsaturated fatty acids and remain semifluid when cold. Indeed, many psychrophiles begin to leak cellular constituents at temperatures higher than 20°C because of cell membrane disruption.

2. Many species can grow at 0 to 7°C even though they have optima between 20 and 30°C, and maxima at about 35°C. These are called psychrotrophs or facultative psychrophiles. Psychrotrophic bacteria and fungi are major factors in the spoilage of refrigerated foods.

3. Mesophiles are microorganisms with growth optima around 20 to 45°C; they often have a temperature minimum of 15 to 20°C. Their maximum is about 45°C or lower. Most microorganisms probably fall within this category. Almost all human pathogens are mesophiles, because their environment is a fairly constant 37°C.

4. Some microorganisms are thermophiles; they can grow at temperatures of 55°C or higher. Their growth minimum is usually around 45°C and they often have optima between 55 and 65°C. The vast majority are procaryotes although a few photosynthetic protists and fungi are thermophilic. These organisms flourish in many habitats including composts, self-heating hay stacks, hot water lines, and hot springs.

5. Hyperthermophiles are Thermophiles which can grow at 90°C or above and with maxima above 100°C. They usually do not grow well below 55°C.  Their growth optima can be between 80°C and about 113°C are called hyperthermophiles. Pyrococcus abyssi and Pyrodictium occultum are examples of marine hyperthermophiles found in hot areas of the seafloor.

High temperatures damage microorganisms by denaturing enzymes, transport carriers, and other proteins. Temperature also has a significant effect on microbial membranes. At high temperatures, the lipid bilayer melts and disintegrates. Thus, when organisms are above their optimum temperature, both function and cell structure are affected. At very low temperatures, membranes solidify.  If temperatures are  low, function is affected but not necessarily cell chemical composition and structure.

Thermophiles differ from mesophiles in many ways.

• They have more heat-stable enzymes and protein synthesis systems, which function well at high temperatures.
• Heat-stable proteins have highly organized, hydrophobic interiors; more hydrogen bonds and other noncovalent bonds strengthen the structure. 
• Larger quantities of amino acids such as proline also make the polypeptide chain less flexible.
• The proteins are stabilized and aided in folding by special chaperone proteins.
• In thermophilic bacteria, DNA is stabilized by special histone like proteins.
• Their membrane lipids are more saturated, more branched, and of higher molecular weight. This increases the melting points of membrane lipids thus making them temperature stable.
• Archaeal thermophiles have membrane lipids with ether linkages, which protect the lipids from hydrolysis at high temperatures. Sometimes archaeal lipids actually span the membrane to form a rigid, stable monolayer.

 

The cardinal temperatures for a particular species are not rigidly fixed but often depend to some extent on other environmental factors such as pH and the available nutrients. For example, Crithidia fasciculate, a flagellated protist living in the gut of mosquitoes, will grow in a simple medium at 22 to 27°C. However, to grow at 33 to 34°C extra metals, amino acids, vitamins, and lipids are required.

The cardinal temperatures vary greatly between microorganisms. The temperature optimum is always closer to the maximum than to the minimum. The growth temperature range for a particular microorganism usually spans about 30 degrees. Some species (e.g., Neisseria gonorrhoeae) have a small range (stenothermal); others, like Enterococcus faecalis, will grow over a wide range of temperatures(eurythermal).

The major microbial groups differ from one another regarding their maximum growth temperatures. The upper limit for protists (protozoa/algae) is around 50°C. Some fungi can grow at temperatures as high as 55 to 60°C. Procaryotes can grow at much higher temperatures than eucaryotes.

Optima usually range from 0°C to 75°C, whereas microbial growth occurs at temperatures extending from less than ^-20°C to over 120°C. Some archaea can even grow at 121°C (250°F), the temperature normally used in autoclaves.