Bacterial growth and nutrition
Nutrition requirement of bacteria: Bacteria were first carefully studied in association with infectious disease. It was early recognized that these MOs will not grow on simple substances but required a complex diet, frequently consisting of mammalian body fluids, such nutrients are needed to supply a source of energy and provide the necessary components for cell growth. All disease producing bacteria , all fungi, protozoa, and animals cells required organic chemical compounds as a source of carbon and energy such cells are called heterophs.
Certain bacteria , not of direct medical importance use CO2 as a carbon source i.e not use energy rich organic compounds for nutrition , such a microbe is called outotrophs.
The most common chemical elements needed by bacterial cells are; carbon, hydrogen, oxygen, Nitrogen, sulfur, phosphorus. Potassium, Magnesium, Calcium, iron, and sodium. In addition, elements like Zinc, Molybdium, Copper, and Manganese are needed in a small amounts ,these act as a co factor for essential enzymatic reaction in the cell and called trace elements.
Heterophic microbes obtain their carbon from organic compounds, such as sugar, proteins, and lipids. Hydrogen is usually obtain from water, and oxygen is obtained from the atmosphere or from water, where it is found in dissolved state.
Nitrogen Sulfur, and Phosphorus can be obtained from either organic or inorganic sources. Most of the other needed elements are obtained from soluble inorganic compounds , some bacteria, especially of several disease –producing species require special growth factors ( such as; (1) amino acids required for protein syntheses
(2) purins and pyrimidines requires for nucleic acid syntheses. (3) vitamins needed as a coenzymes and functional groups of certain enzymes)., which explains their need for blood, or other animal body fluids for their growth.
Culture media: The growth , or culture of a given bacterium requires a culture medium that provides all the essential nutrition, the proper concentration of salts and ions, and the proper PH (relative acidity or alkalinity) for optimum bacterial growth to occur. Moisture is always essential for bacterial growth because the various nutrition must be in a soluble from or in a form that can be solubilized to facilitate diffusion into the cell. So the environmental growth condition for bacterial growth are:
1-Oxygen requirements
a-Obligate aerobes requires oxygen for growth
b-Anaerobes do not require oxygen for growth.
1- Obligate anaerobes which are damaged by oxygen , include normal colonizers of the gastrointestinal tract, respiratory tract and/or skin.
2-Aerotolerant anaerobes can survive in the presence of small amounts of oxygen but grow best in it’s absence.
c- Facultative anaerobes can grow under aerobic or anaerobic conditions.
2-Water is needed for the (1) growth and reaction of metabolism like glycolysis and protein synthesis (2) various nutrient must be in a soluble form to facilitate diffusion into the cell, in absence of water (dehydration) some bacteria will form a spore for continue it1s survival.
3-Nutrient requirement (energy source): a-Undemanding eaters can be cultured on simple media (e.g. E. coli, Salmonella, and other gram-negative enteric bacteria)
b- Demanding eaters require complex media congaing numerous growth factors (e.g. Hemophilus and Neisseria).
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5- Temperature requirements most pathogenic bacteria of medical importance grow optimally at 35°C to 37°C near normal body temperatures.
Microbial growth
Cell division
Bacteria divide by binary fission , A simple process than mitotic division of eukaryotic cells.
1- Chromosomal duplication is initiated at a specific sequence in the DNA.
2- Synthesis of new membrane and cell wall in the center of the cell forms a septum that eventually divides the cytoplasm into two daughter cells, each contain a complete chromosome.
The growth of microorganisms can be measured by :
(1) increase in it s size but this a poor criterion of growth
(2) increase in the number of MOs by either countering the number of living cells(viable count) or all cells(total count).
(3) measurement of some component of cell structures such as protein or DNA as an indication of microbial increase (growth) or decrease(death).
The generation time
of a microbial cell is the time required for one complete cell division.
Some microbes are able to divide as rapidly as once every 12 to15 minutes, other requires up to several hours, and a few very slow growing bacteria may require more than 24hours per cell division when proper nutrients are available and environment growth conditions are favorable, the growth of MOs can be a dynamic event with profound effects on the surrounding environment . if a bacterial cell continue to divide once every 30minutes. For instance, there would be 64 cells in 3hours, 17million in 17hours, and 280 trillion cells in 24hours, if growth could continue for 48hours at this rate, the mass of cells produce would continue for 48hours at this rate the mass of cells produced would weight several thousands times the weight of the earth. Obviously, such rapid growth can not continue for very long periods. Yet under certain conditions it may occur for a short time.
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Growth curve: when microbial cells are placed in fresh nutrient broth under favorable growth the conditions, but with limited supply of available nutrition, multiplication follows the typical growth pattern shows in the growth curve figure; The first phase is the Lag phase that will encompass several hours. During this time the MOs grow in size, accumulate organic matter, and store large quantities of chemical energy such as ATP for biosynthesis. Second phase Logarithmic (Log phase)in which the microorganisms undergo rapid cell division and fulfill their generation time. The population doubles during each generation time and the population increase in size at a logarithmic or exponential rate and in which all the measurable components of the cell such as; protein, RNA,DNA, or biomass increase at the same rate. The exponential growth phase is usually fairly briefly lasting only 4-10 hours for most rapidly growing bacteria. The third is Stationary phase of the growth curve: logarithmic growth rapidly depleted, the available nutrients, and toxic waste products quickly accumulate. As nutrient concentration decrease, or the culture environment becomes more toxic, growth of the cells becomes unbalanced and various cellular components are synthesized at different rates. These factors cause a decrease in, and ultimately cessation of cellular division. The accumulated cells may then remain for a period of time in a static condition-that is not increasing in the numbers of viable cells. This is called Stationary phase of the growth curve in which the cells are smaller than in the log phase and their components are not uniform and they are most resistant to environment changes such as ;heat, dryness, radiation, and antibiotics.
The stationary phase is followed by a period in which the cells gradually die off, called Decline or death phase in which cellular death is also an exponential function.
Although the characteristic growth curve shown above probably only occurs under selected conditions, modification do happen in nature and in some clinical circumstances.
Products like bottled milk, for instance, if not properly refrigerated, could support the growth of microorganisms in the logarithmic phase, thus causing rapid souring. While the change in the number of bacteria follows the normal growth curve. In clinical conditions, such as a wound, where an abscess is forming a niche may exist that is filled with dead tissue and body fluids that could support the rapid growth of bacteria for a time. In most abscesses the bacteria have reach the stationary phase of the growth curve; in this condition they do not take in many nutrients or other substances from the surrounding environment. Thus antibiotics if given to the patient to cure the infection do not effectively penetrated into the abscess and may not be taken up by the bacteria if they do consequently such therapy may fail to improve the infection. In order to resolve this problem, it is nearly always necessary to drain abscesses in order to remove the waste products that are inhibiting the growth of the bacteria and preventing penetration of antimicrobial agents. Fresh nutrients then diffuse into the area and the remains bacteria begin to multiply if antibiotic is then given, it will be taken up by the growing bacteria, inhibit their growth, and help to cure the infection.
Growth of bacteria in an open environment, such as soil, water, or even the intestine generally dose not follow the curve shown above ,in these circumstances bacterial growth is most often continuous, so that the number of viable microorganisms remains fairly constant over long periods of time.
Laboratory (In vitro) studies of bacterial growth in continuous culture have shown that the microorganisms grow exponentially in a condition of balanced growth, and that the generation time is determined by the rate at which fresh nutrients are supplied to the culture. continuous culture is usually used by genetic engineering for synthesis of some wanted things like ;insulin, vitamins, toxins, enzyme….etc.
The human normal flora that live on the skin, mouth, intestine, vagina and upper respiratory tract are grow continuously while balanced microbial growth may occur to some extent in chronic disease conditions.
Endospore, and spore formation
Members of several bacterial genera are capable of forming endospores. The most common are gram positive rods; genus Bacillus and genus clostridium. Endospores formed when theses bacteria exposed to unfavorable conditions, like nutritional depletion, one spore develops per cell and forms a dormant resistant stage for the cell. Bacterial spores are the most stable forms of life and are the most resistant forms of life known to science, they resist boiling under temp, dryness, disinfectant, antibiotics , and alote of chemicals, but it can be destroyed by sterilization in autoclave (steam under pressure). This great stability of the bacterial endospore is related to two major factors: (1) The dehydrated state of the spore and (2) the presence of diplocolinic acid, this type of acid is not found in any other living organism.
The endospore is formed by a sequence of changes called sporulation. This process is very complex and involves the activities of a large number of proteins and enzymes. Following initiation, a chromosome near the terminal end of the cell is surrounded by in folding of the cell membrane, in addition to DNA , this membrane bounded structure includes ;ribosome and enzymes that will be needed when the spore germinates to a vegetative cell again. This spore core is then surrounded successively by a spore wall, that contains peptidoglycan (which upon germination will be the nidus of a new cell wall, a relatively thick cortex, and relatively impermeable proteineous layer known as a spore coat. Finally, water is removed to dehydrate and mature the spore. When the environment become favorable; as presence of energy, nitrogen….etc. germination process will stored, out growth process appear with external damage to the spore coat by heat or mechanical means.
Spores are very important in medical microbiology because (1) they are etiological agents of a dangerous and fatal diseases like; tetanus, botulism, gas gangrene, and anthrax., such a spore can be transmitted and stay for a long time, even million of years after which can cause a disease .(2) resist sterilization other than autoclave.
المادة المعروضة اعلاه هي مدخل الى المحاضرة المرفوعة بواسطة استاذ(ة) المادة . وقد تبدو لك غير متكاملة . حيث يضع استاذ المادة في بعض الاحيان فقط الجزء الاول من المحاضرة من اجل الاطلاع على ما ستقوم بتحميله لاحقا . في نظام التعليم الالكتروني نوفر هذه الخدمة لكي نبقيك على اطلاع حول محتوى الملف الذي ستقوم بتحميله .