Bacterial Growth and Different Environmental Factors Affect Bacterial Growth

• Bacterial growth involves the proliferation of bacterium into two daughter cells, in a process known as binary fission.
• Both the daughter cells generated from the binary fission are genetically identical to the original cell.  
• The term growth is used to refer to an increase in size; eg, transforming from a tiny newborn baby to a large adult.
• Bacteria can increase their size before cell division, bacterial growth means an increase in the number of organisms rather than an increase in their size.
• It also can be defined as an orderly increase of all the chemical components of the cell.
• Cell duplication is an outcome of growth that leads to an expansion in the number of bacteria making up a population or culture.
• Bacterial growth can be equated to cell number: one bacterium divides into two, these two produce four, and then eight, and so on.

Environmental Factors Affect Bacterial Growth

In addition to knowing the proper nutrients for the cultivation of bacteria, it is also necessary to know the physical environment in which the organisms will grow best. Just as bacteria vary greatly in their nutritional requirements, so do they exhibit diverse responses to physical conditions such as temperature, gaseous conditions, and pH. A variety of factors affect the growth of bacteria. These are discussed below:

• Effect of Temperature on Bacterial Growth
• Effect of Solutes concentration and Water Activity on Bacterial Growth
• Effect of pH on Bacterial Growth
• Effect of Oxygen Concentration on Bacterial Growth
• Effect of Pressure on Bacterial Growth
• Effect of Radiation on Bacterial Growth
• Effect of Carbon dioxide on bacterial Growth

Effect of Temperature on Bacterial Growth

• Since all processes of growth are dependent on chemical reactions and since the rates of these reactions are influenced by temperature, the pattern of bacterial growth can be profoundly influenced by the Temperature. 
• An important factor influencing the effect of temperature on growth is the temperature sensitivity of enzyme-catalyzed reactions. Each enzyme has a temperature at which it functions optimally.
• At some temperature below the optimum, it ceases to be catalytic, as the temperature rises from this low point, the rate of catalysis increases to that observed for the optimal temperature.
• The velocity of the reaction roughly doubles for every 10°C rise in temperature. 
• When all enzymes in a microbe are considered together, as the rate of each reaction increases, metabolism as a whole becomes more active and the microorganism grows faster.
• However, beyond a certain point, further temperature increases actually slow growth, and sufficiently high temperatures are lethal. High temperatures denature enzymes, as well as transport and other proteins.
• Temperature also has a significant effect on microbial membranes. At very low temperatures, membranes solidify. At high temperatures, the lipid bilayer simply melts and disintegrates. 
• Thus when organisms are above or below their optimum temperature, both cell function and structure are affected

On the basis of their temperature relationships, bacteria are divided into three main groups;

1. Psychrophiles 

• Microbes that grow in cold environments are either known as psychrotolerants or psychrophiles. Psychrotolerants (sometimes called psychrotrophs) grow at 0°C or higher and typically have maxima at about 35°C. 
• Psychrophiles (sometimes called cryophiles) grow well at 0°C and have an optimum growth temperature of 15°C; the maximum is around 20°C.
• They are readily isolated from Arctic and Antarctic habitats.
• Oceans constitute an enormous habitat for psychrophiles because 90% of ocean water is 5°C or colder.
• An example of psychrophilic protist is Chlamydomonas nivalis which can actually turn a snowfield or glacier pink with its bright red spores (a phenomenon called “watermelon snow”). 
• Psychrophiles are widespread among bacterial taxa and are found in such genera as Pseudomonas, Vibrio, Alcaligenes, Bacillus, Photobacterium, and Shewanella.
• At very low temperatures their enzymes, transport systems, and protein synthetic machinery function well.
• The membranes of psychrophilic microorganisms contain high levels of unsaturated fatty acids and remain semifluid when cold. Indeed, at temperatures higher than 20°C many psychrophiles begin to leak cellular constituents because of membrane disruption. 
• Many psychrophiles accumulate compatible solutes rather than protecting against osmotic stress, in this case the compatible solutes decrease the freezing point of the cytosol. 
• Other psychrophiles use antifreeze proteins to decrease the freezing point of the cytosol.
• Psychrophilic bacteria and fungi are major causes of refrigerated food spoilage.

2. Mesophiles

• Those microorganisms that grow at moderate temperatures are known as the Mesophiles.
• They have growth optima around 20° to 45°C and often have a temperature minimum of 15° to 20°C and a maximum of about 45°C. 
• Almost all human pathogens are mesophiles, as might be expected because the human body is a fairly constant 37°C.

3. Thermophiles and Hyperthermophiles

• Microbes that grow best at high temperatures are thermophiles and hyperthermophiles.
• Thermophiles grow at temperatures between 45° and 85°C, and they often have optima between 55° and 65°C.
• The vast majority are members of Bacteria or Archaea, although a few photosynthetic protists and fungi are thermophilic. 
• Thermophiles flourish in many habitats including composts, self-heating hay stacks, hot water lines, and hot springs. 
• Hyperthermophiles have growth optima between 85° and 113°C. 
• Hyperthermophiles usually do not grow below 55°C. 
• Pyrococcus abyssi and Pyrodictium occultum are examples of marine hyperthermophiles found in hot areas of the seafloor.
• Thermophiles and hyperthermophiles contain heat-stable enzymes and protein synthesis systems that function at high temperatures. 
• Heat-stable proteins have highly organized hydrophobic interiors and many hydrogen and other noncovalent bonds to stabilize their structure. 
• Large quantities of amino acids such as proline make polypeptide chains less flexible and more heat stable.
• In addition, the proteins are stabilized and aided in folding by proteins called chaperones. 
• Nucleoid-associated proteins appear to stabilize the DNA of thermophilic bacteria.
• Hyperthermophiles have an enzyme called reverse DNA gyrase that changes the topology of their DNA and enhances its stability. 
• The membrane lipids of thermophiles and hyperthermophiles tend to be more saturated, more branched, and of higher molecular weight. This increases the melting points of membrane lipids. 
• archaea have membrane lipids with ether linkages. Such lipids are resistant to hydrolysis at high temperatures. 
• Furthermore, the diglycerol tetraethers observed in the membranes of some archaeal thermophiles span the membrane to form a rigid, stable monolayer.

Note: The temperature that allows for the most rapid growth during a short period of time (12 to 24 h) is known as the optimum growth temperature.

Descriptive Term	Definition	Representative Genera and Species
Psychrophile	Grows at O°C and has an optimum growth temperature of 15°C or lower	Bacillus psychrophilus, Chlamydomonas nivells
Psychrotolerant	Can grow at 0-7°C; has an optimum between 20 and 30’C and a maximum around 35°C	Listeria monocytogenes, Pseudomonas fluorescens
Mesophile	Has growth optimum between 20 and 45°C	Escherichia coll, Trichomonas vaginalls
Thermophile	Can grow at 55°C or higher; optimum often between 55 and 65°C	Geobacillus stearothermophilus, Thermus aquaticus, Cyanidium caldarium, Chaetomium thermophilum
Hyperthermophile	Has an optimum between 85 and about 113°C	Sulfolobus, Pyrococcus, Pyrodictum

Effect of Solutes concentration and Water Activity on Bacterial Growth

• Water is important to the survival of all organisms, but water can also be destructive.
• Solutes in an aqueous solution alter the behavior of water. One way this occurs is the phenomenon of osmosis, which is observed when two solutions are separated by a semipermeable membrane that allows the movement of water but not solutes. 
• If the solute concentration of one solution is higher than the other, water moves to equalize the concentrations. It moves from solutions with lower solute concentrations to those with higher solute concentrations due to a selectively permeable plasma membrane separates a cell’s cytoplasm from its environment, microbes can be affected by changes in the solute concentration of their surroundings. 
• If a microorganism is placed in a hypotonic solution (one with a lower solute concentration; solute concentration is also referred to as osmotic concentration or osmolarity), water will enter the cell and cause it to burst unless something prevents the influx of water or inhibits plasma membrane expansion. 
• Conversely, if the microbe is placed in a hypertonic solution (one with a higher osmotic concentration), water will flow out of the cell. In microbes that have cell walls, the membrane shrinks away from the cell wall.
• Dehydration of the cell in hypertonic environments may damage the plasma membrane and cause the cell to become metabolically inactive.
• Microbes in hypotonic environments are protected in part by their cell wall, which prevents overexpansion of the plasma membrane.
• Wall-less microbes can be protected by reducing the osmotic concentration of their cytoplasm; this protective measure is also used by many walled microbes to provide protection in addition to their cell walls. 
• Microbes use several mechanisms to lower the solute concentration of their cytoplasm. For example, some bacteria have mechanosensitive (MS) channels in their plasma membrane.
• Some microbes are adapted to extreme hypertonic environments, and can be called osmophiles.

Based on tolerant to osmotic pressure microbes are classified into different classes such as;

1. Halophiles

• Halophiles are required the presence of NaCl at a concentration above about 0.2 M.
• Archaeal halophiles can be isolated from the Dead Sea (a salt lake between Israel and Jordan), the Great Salt Lake in Utah, and other aquatic habitats with salt concentrations approaching saturation.
• Halophiles generally are able to live in their high-salt habitats because they synthesize or obtain from their environment molecules called compatible solutes.
• Compatible solutes can be kept at high intracellular concentrations without interfering with metabolism and growth. 
• Some compatible solutes are inorganic molecules such as potassium chloride (KCl). Others are organic molecules such as choline, betaines (neutral molecules having both negatively charged and positively charged functional groups), and amino acids such as proline and glutamic acid.
• Some of the best-studied halophiles belong to the archaeal order Halobacteriales. These archaea accumulate potassium and chloride ions to remain hypertonic to their environment; the internal potassium concentration may reach 4 to 7 M.

2. Extreme halophiles

• Extreme halophiles have adapted so completely to hypertonic, saline conditions that they require NaCl concentrations between about 3 M and saturation (about 6.2 M).
• Examples of extreme halophiles are Salinibacter ruber and organisms in the Halobacteria class. 
• Extreme halophiles live in the Dead Sea in the Middle East and in man-made solar salterns (lakes used for making sea salt).

3. Osmotolerant

• These osmotolerant microorganisms grow over wide ranges of water activity but optimally at higher levels. 
• Osmotolerant organisms can be found in all domains of life. For example, Staphylococcus aureus is halotolerant, can be cultured in media containing up to about 3 M sodium chloride, and is well adapted for growth on the skin. 
• The yeast Zygosaccharomyces rouxii grows in sugar solutions with aw values as low as about 0.65. 
• The photosynthetic protist Dunaliella viridis tolerates sodium chloride concentrations from 1.7 M to a saturated solution.

4. Xerotolerant

• Xerotolerant microbes can withstand either high solute concentrations or the effects of desiccation. 
• These microbes exist in desert regions, household dust, and in preserved foods.
• However, most microorganisms only grow well at water activities around 0.98 (the approximate aw for seawater) or higher. 
• This is why drying food or adding large quantities of salt and sugar effectively prevents food spoilage.
• This organism can grow best at low water activity, typically with optima at 0.85 or below.

Descriptive Term	Definition	Representative Genera and Species
Halophile	Requires high levels of sodium chloride, usually above about 0.2 M, to grow	Holobacterium, Dunaliella, Ecrothiorodospiro
Osmotolerant	Able to grow over wide ranges of water activity or osmotic concentration	Staphylococcus aureus, Zygosaccharomyces rouxii
Xerophile	Organisms that grow best at low water activity, typically with optima at 0.85 or below	Xeromyces bisporus

Effect of pH on Bacterial Growth

• pH is a measure of the relative acidity of a solution and is defined as the negative logarithm of the hydrogen ion concentration (expressed in terms of molarity).
• Most pathogenic bacteria grow between pH 7.2 and 7.6. Very few bacteria, such as lactobacilli, can grow at acidic pH below 4.0.
• If the external pH is low, the concentration of H+ is much greater outside than inside, and H+ will move into the cytoplasm and lower the cytoplasmic pH.
• Drastic variations in cytoplasmic pH can harm microorganisms by disrupting the plasma membrane or inhibiting the activity of enzymes and membrane transport proteins. 
• Most microbes die if the internal pH drops much below 5.0 to 5.5. Changes in the external pH also can alter the ionization of nutrient molecules and thus reduce their availability to the organism.
• When the pH drops below about 5.5, Salmonella enterica serovar Typhimurium and E. coli synthesize an array of new proteins as part of what has been called their acid tolerance response. 
• An ATPase enzyme contributes to this protective response by pumping protons out of the cell, at the expense of ATP. 
• If the external pH decreases to 4.5 or lower, acid shock proteins and heat shock proteins are synthesized. These prevent the denaturation of proteins and aid in refolding denatured proteins in acidic conditions.

Each species has a definite pH growth range and pH growth optimum. Based on the pH tolerance microorganisms are classified into the following groups;

1. Acidophiles

• They have their growth optimum between pH 0 and 5.5.
• Most fungi prefer more acidic surroundings, about pH 4 to 6; photosynthetic protists also seem to favor slight acidity. 
• Many archaea are acidophiles. For example, the archaeon Sulfolobus acidocaldarius is a common inhabitant of acidic hot springs; it grows well from pH  1 to 3 and at high temperatures.
• The archaea Ferroplasma acidarmanus and Picrophilus oshimae can actually grow very close to pH 0. 

2. Neutrophiles 

• Neutrophiles have their growth optimum between pH 5.5 and 8.0.
• Most known bacteria and protists are neutrophiles. 

3. Alkaliphiles

• Alkaliphiles have their optimum growth between pH 8.0 and 11.5.
• Alkaliphiles are distributed among all three domains of life. They include bacteria belonging to the genera Bacillus, Micrococcus, Pseudomonas, and Streptomyces; yeasts and filamentous fungi; and numerous archaea. Because seawater has a pH of about 8.3, marine microorganisms are alkaliphilic.

Descriptive Term	Definition	Representative Genera and Species
Acidophile	Growth optimum between pH 0 and 5.5	Sulfolobus, Picrophilus, Ferroplosma, Acontium
Neutrophile	Growth optimum between pH 5.5 and 8.0	Escherichia, Euglena, Paramecium
Alkaliphile	Growth optimum between pH 8.0 and 11.5	Bacillus alcalophilus, Natronobacterium

Effect of Oxygen Concentration on Bacterial Growth

• The importance of oxygen to the growth of an organism correlates with the processes of the electron transport chain (ETC), where involves the movement of electrons through a series of membrane-associated electron carriers. 
• Most microorganisms use oxygen as the terminal electron acceptor.

On the basis of oxygen requirements, bacteria can be divided into following different categories :

1. Aerobic bacteria

• Grow in ambient air, which contains 21% oxygen and small amount of (0,03%) of carbondioxide (Bacillus cereus).
• Examples of aerobic bacteria are Nocardia sp., Psuedomonas aeruginosa, Mycobacterium tuberculosis, and Bacillus sp., etc.

2. Anaerobic bacteria

• They do not use oxygen to obtain energy, moreover, oxygen is toxic for them and they cannot grow when incubated in an air atmosphere. 
• Some can tolerate low levels of oxygen constringent or tolerant anaerobes ), but others stringent or strict anacrobes ) cannot tolerate even low levels and may die upon brief exposure to air. 
• Examples of Anaerobic Bacteria are Bacteroides, Fusobacterium, Porphyromonas, Prevotella, Actinomyces, Clostridia etc.

3. Facultatively anaerobic bacteria 

• They do not require oxygen for growth, although they may use it for energy production if it is available. 
• They are not inhibited by oxygen and usually grow as well under an air atmosphere as they do in the absence of oxygen.
• Examples of the facultative anaerobes are bacteria are Escherichia coli, Pseudomonas aeruginosa, Staphylococcus spp., Listeria spp., Salmonella, Shewanella oneidensis, and Yersinia pestis.

4. Microaerophilic bacteria 

• They require low levels of oxygen for growth but cannot tolerate the level of oxygen present in an air atmosphere.
• Examples of microaerophiles are Borrelia burgdorferi, Helicobacter pylori, etc.

5. Aerotolarent Anaerobe

• Grow equally in the presence and absence of Oxygen.
• An example of Aerotolarent Anaerobe is Streptococcus pyogenes.

6. Obligate aerobes:

• They have absolute requirement for oxygen in order to grow.
• Example of Obligate aerobes are Psuedomonas aeruginosa, Mycobacterium tuberculosis.

7. Capnophiles

• Capnophilic bacteria require increased concentration of carbondioxide (5% to 10%) and approximately 15% oxygen.
• This condition can be achieved by a candle jar (3% carbondioxide) or carbondioxide incubator, jar or bags.
• Example of Capnophiles are Haemophilus influenzae, Neisseria gonorrhoeae.

Descriptive Term	Definition	Representative Genera and Species
Obligate aerobe	Completely dependent on atmospheric O2 for growth	Micrococcus luteus, most protists and fungi
Facultative anaerobe	Does not require O, for growth but grows better in its presence	Escherichia, Enterococcus, Saccharomyces cerevisiae
Aerotolerant anaerobe	Grows equally well in presence or absence of O2	Streptococcus pyogenes
Obligate anaerobe	Does not tolerate o, and dies in its presence	Clostridium, Bocteroides, Methonothermobacter
Microaerophile	Requires O levels between 2 and 10% for growth and is damaged by atmospheric O, levels (20%)	Campylobacter, Spirillum volutons, Treponema pallidum

Effect of Carbon dioxide on bacterial Growth

• The organisms that require higher amounts of carbon dioxide (CO2) for their growth are called capnophilic bacteria.
• They grow well in the presence of 5–10% CO2 and 15% O2 . In candle jar, 3% CO2 can be achieved.
• Examples of such bacteria include H. influenzae, Brucella abortus, etc.

Effect of Pressure on Bacterial Growth

• Organisms that spend their lives on land or the surface of the water are always subjected to a pressure of 1 atmosphere (atm; 1 atm is ~0.1 megapascal, or MPa for short) and are never affected significantly by pressure. 
• Other organisms, including many bacteria and archaea, live in the deep sea (ocean depths of 1,000m or more), where the hydrostatic pressure can reach 600 to 1,100 atm and the temperature is about 2° to 3°C. These high hydrostatic pressures affect membrane fluidity and membrane associated function.
• Many microbes found at great ocean depths are barotolerant—that is, increased pressure adversely affects them but not as much as it does nontolerant microbes. S
• Some are truly piezophilic (barophilic). A piezophile is defined as an organism that has a maximal growth rate at pressures greater than 1 atm.
• An important adaptation observed in piezophiles is that they change their membrane lipids in response to increasing pressure. For instance, bacterial piezophiles increase the amount of unsaturated fatty acids in their membrane lipids as pressure increases.
• They may also shorten the length of their fatty acids. 
• Piezophiles are thought to play important roles in nutrient cycling in the deep sea. Thus far, they have been found among several bacterial genera (e.g., Photobacterium, Shewanella, Colwellia).

Descriptive Term	Definition	Representative Genera and Species
Piezophile (barophile)	Growth more rapid at high hydrostatic pressures	Photobacterium profundum, Shewanella benthica

Effect of Radiation on Bacterial Growth

• Sunlight is the major source of radiation on Earth, it includes visible light, ultraviolet (UV) radiation, infrared rays, and radio waves.
• Only about 3% of the light reaching Earth’s surface is UV radiation, and this includes only certain types of UV light. There are three major types of UV radiation: UVA, UVB, and UVC, which range from longest (UVA) to shortest (UVC) wavelengths.
• Many forms of electromagnetic radiation are very harmful to microorganisms. One of the most damaging is ionizing radiation, radiation of very short wavelength and high energy,  which causes atoms to lose electrons (ionize). 
• Two major forms of ionizing radiation are X rays, which are artificially produced, and gamma rays, which are emitted during natural radioisotope decay. 
• Low levels of ionizing radiation may produce mutations that indirectly result in death, whereas higher levels are directly lethal. 
• Ionizing radiation causes a variety of changes in cells, It breaks hydrogen bonds, destroys ring structures, and polymerizes some molecules but the most severe effect is protein oxidation.
• Ionizing radiation can be used to sterilize items. However, bacterial endospores and bacteria such as Deinococcus radiodurans are extremely resistant to large doses of ionizing radiation
• Ultraviolet (UV) radiation is another very damaging form of radiation. It can kill microorganisms due to its short wavelength (approximately from 10 to 400 nm) and high energy. 
• The most lethal UV radiation has a wavelength of 260  nm, the wavelength most effectively absorbed by and damaging to DNA.
• Excessive exposure to UV light outstrips the organism’s ability to repair the damage and death results. 
• Longer wavelengths of UV light (near-UV radiation; 325 to 400 nm) can also harm microorganisms because they induce the breakdown of the amino acid tryptophan to toxic photoproducts. These toxic photoproducts plus the near-UV radiation itself produce breaks in DNA strands.

Descriptive Term	Definition	Representative Genera and Species
Radiation resistant	Survives high doses of gamma radiation	Deinococcus radiodurans, Thermococcus gammatolerans