Temperature and Microbial Growth

Growth Rate and Temperature

Bacteria may grow across a wide range of temperatures, from very cold to very hot.

Growth Rate and Temperature

Bacterial growth is the division of one bacterium into two daughter cells in a process called binary fission. Providing no mutational event occurs the resulting daughter cells are genetically identical to the original cell. Hence, local doubling of the bacterial population occurs. Both daughter cells from the division do not necessarily survive. However, if the number surviving exceeds unity on average, the bacterial population undergoes exponential growth. The measurement of an exponential bacterial growth curve in batch culture was traditionally a part of the training of all microbiologists. The basic means requires bacterial enumeration (cell counting) by direct and individual (microscopic, flow cytometry), direct and bulk (biomass), indirect and individual (colony counting), or indirect and bulk (most probable number, turbidity, nutrient uptake) methods. Models reconcile theory with the measurements.

Bacteria may grow across a wide range of temperatures, from very cold to very hot. A mesophile is an organism that grows best in moderate temperature, neither too hot nor too cold. All human pathogens are mesophiles. Organisms that prefer extreme environments are known as extremophiles: those that prefer cold environments are termed psychrophilic, those preferring warmer temperatures are termed thermophilic or thermotrophs and those thriving in extremely hot environments are hyperthermophilic.

For example, in molecular biology, the cold-shock domain (CSD) is a protein domain of about 70 amino acids which has been found in prokaryotic and eukaryotic DNA-binding proteins. Part of this domain is highly similar to the RNP-1 RNA-binding motif. When Escherichia coli is exposed to a temperature drop from 37 to 10 degrees Celsius, a four to five hour lag phase occurs and then growth is resumed at a reduced rate. During the lag phase, the expression of around 13 proteins, which contain cold shock domains is increased two- to ten-fold. These so-called cold shock proteins are thought to help the cell survive in temperatures lower than optimum growth temperature, by contrast with heat shock proteins, which help the cell survive in temperatures greater than the optimum, possibly by condensation of the chromosome and organization of the prokaryotic nucleoid.

Classification of Microorganisms by Growth Temperature

Bacteria can be classified on the basis of cell structure, metabolism or on differences in cell components.

Classification seeks to describe the diversity of bacterial species by naming and grouping organisms based on similarities. Bacteria can be classified on the basis of cell structure, cellular metabolism, or on differences in cell components such as DNA, fatty acids, pigments, antigens and quinones.

Bacteria can be classified by their optimal growth temperature. The following are the five classifications:

• Hyperthermophile (60 degrees C and upwards)
• Thermophile (optimal growth between 45 and 122 degrees)
• Mesophile (20 and 45 degrees C)
• Psychrotrophs (will survive at 0 degrees C, but prefer mesophilic temperature
• Psychrophiles (-15 and 10 degrees C or lower)

A mesophile is an organism that grows best in moderate temperature, neither too hot nor too cold, typically between 20 and 45 °C (68 and 113 °F). The term is mainly applied to microorganisms.The habitats of these organisms include especially cheese, yogurt, and mesophile organisms are often included in the process of beer and wine making. Organisms that prefer cold environments are termed psychrophilic, those preferring warmer temperatures are termed thermophilic and those thriving in extremely hot environments are hyperthermophilic. All bacteria have their own optimum environmental surroundings and temperatures in which they thrive the most. A thermophile is an organism — a type of extremophile — that thrives at relatively high temperatures, between 45 and 122 °C (113 and 252 °F). Thermophilic eubacteria are suggested to have been among the earliest bacteria. Thermophiles are found in various geothermally heated regions of the Earth, such as hot springs like those in Yellowstone National Park. and deep sea hydrothermal vents, as well as decaying plant matter, such as peat bogs and compost.As a prerequisite for their survival, thermophiles contain enzymes that can function at high temperatures. Some of these enzymes are used in molecular biology (for example, heat-stable DNA polymerases for PCR), and in washing agents.

The Heat-Shock Response

Heat shock response is a cell's response to intense heat, including up-regulation of heat shock proteins.

The bacterial stress response enables bacteria to survive adverse and fluctuating conditions in their immediate surroundings. Various bacterial mechanisms recognize different environmental changes and mount an appropriate response. A bacterial cell can react simultaneously to a wide variety of stresses, and the various stress response systems interact with each other by a complex of global regulatory networks.

In biochemistry, heat shock is the "effect of subjecting a cell to a higher temperature than that of the ideal body temperature of the organism from which the cell line was derived. "

Heat shock response is the cellular response to heat shock includes the transcriptional up-regulation of genes encoding heat shock proteins (HSPs) as part of the cell's internal repair mechanism. HSPs are also called 'stress-proteins' and respond to heat, cold and oxygen deprivation by activating several cascade pathways. HSPs are also present in cells under perfectly normal conditions. Some HSPs, called 'chaperones', ensure that the cell's proteins are in the right shape and in the right place at the right time. For example, HSPs help new or misfolded proteins to fold into their correct three-dimensional conformations, which is essential for their function. They also shuttle proteins from one compartment to another inside the cell and target old or terminally misfolded proteins to proteases for degradation. Additionally, heat shock proteins are believed to play a role in the presentation of pieces of proteins (or peptides) on the cell surface to help the immune system recognize diseased cells. The up-regulation of HSPs during heat shock is generally controlled by a single transcription factor; in eukaryotes this regulation is performed by heat shock factor (HSF), while σ32 is the heat shock sigma factor in Escherichia coli.