Bacterial Growth and Requirements - 2024-2025 PDF

Bacterial growth and requirements Dr. Mazen ZAYLAA Beirut Arab University 2024-2025 Bacterial Growth Requirements Physical  Temperature  pH  Osmotic pressure  Moisture & desiccation Chemical  Oxygen  Carbon source, nitrogen, hydrogen, phosphorus and sulfur  Sodium...

Bacterial growth and requirements Dr. Mazen ZAYLAA Beirut Arab University 2024-2025 Bacterial Growth Requirements Physical  Temperature  pH  Osmotic pressure  Moisture & desiccation Chemical  Oxygen  Carbon source, nitrogen, hydrogen, phosphorus and sulfur  Sodium, potassium, chlorine, magnesium, calcium, and trace elements such as iron Bacterial growth  When microbes are provided with nutrients and the required environmental factors, they become metabolically active and grow.  Growth takes place on two levels: 1. A cell synthesizes new cell components and increases its size; 2. The number of cells in the population increases.  The division of a bacterial cell occurs mainly through an asexual process known as binary fission; binary means that one cell becomes two  During binary fission, the parent cell enlarges, duplicates its chromosome, and forms a central transverse septum that divides the cell into two daughter cells.  The time required for a complete fission cycle—from parent cell to two new daughter cells—is called the generation, or doubling, time. Bacterial Growth  This term refers to an increase in the number of organisms rather than an increase in their size  It is the proliferation or multiplication of bacteria  Bacterial division occurs by binary fission into two daughter cells Bacterial Growth  Generation Time : The time it takes one cell to become two cells by binary fission – Generation time varies between species – Some bacteria are rapid growers, others are slow growers Fastidious microbes : are difficult to grow in the lab and require special media and conditions – E.g. Mycobacterium tuberculosis, Mycobacterium leprae, and Treponema pallidum Growth curve  Typical growth curve is observed when microorganisms are cultivated in batch culture.  It is usually plotted as logarithm of cell number versus time.  Four principal phases in the growth curve are recognized, each phase is characterized by a certain rate of population change.  These phases are: 1. The lag phase  It is the time necessary for the cells of inoculum to adapt themselves to the new environment. The duration of the lag phase can vary.  The growth rate is almost zero.  Bacteria absorb nutrients, synthesize enzymes, and prepare for the cell division  The bacteria does NOT increase in number, only preparing for replication 2. The exponential growth phase (log phase)  In the log growth phase, also called the exponential growth phase, bacterial cell division begins and proceeds as a geometric progression.  One cell divides to form two, each of the two cells divides, the number of cells increases as an exponential function of time (doubling in number at regular intervals).  Bacteria multiplies quickly that the number of organisms increases very rapidly 3. The stationary phase  A bacterial population may reach stationary growth when a required nutrient is exhausted, when inhibitory end products accumulate, or when physical conditions change.  As all the nutrients and liquid media are used up and the concentration of toxic waste products builds up led to inhibition of growth and slowing of the rate of cell division  During this phase, there is no further net increase in bacterial cell numbers. The growth rate is equal zero but many cell functions continue (energy metabolism, some biosynthetic processes) 4. The death phase (decline phase)  As the concentration of waste keeps on increasing, the nutrient supply continues decreasing  The viable population declines. The cell mass may remain constant, but a large fraction of the population becomes nonviable.  The death phase is also exponential, however the rate of cell death is slower than that of exponential growth  The culture may die completely, or spores are produced if spore-forming bacteria are cultured Bacterial Growth Requirements Physical  Temperature  pH  Moisture & desiccation  Osmotic pressure Chemical  Oxygen  Carbon source, nitrogen, hydrogen, phosphorus and sulfur  Sodium, potassium, chlorine, magnesium, calcium, and trace elements such as iron Temperature  The range of temperatures for microbial growth can be expressed as three cardinal temperatures.  The minimum temperature is the lowest temperature that permits a microbe’s growth and metabolism, below this temperature its activities are inhibited.  The maximum temperature is the highest temperature at which growth and metabolism can proceed. If the temperature rises slightly above maximum, growth will stop, but if it continues to rise beyond that point, the enzymes and nucleic acids become permanently inactivated and the cell will die.  The optimum temperature covers a small range, intermediate between the minimum and maximum. It promotes the fastest rate of growth and metabolism during a short period.  Depending on their natural habitats, some microbes have a narrow cardinal range, others have a broad one Temperature  Temperature is the most important environmental factor affecting growth of microorganisms  As temperature increases within a given range, growth and metabolic function increase up to a point where protein denaturation occurs  Cardinal temperatures vary widely between organisms  Extremophiles : organisms that grow in extremely hot or cold habitats Temperature  Thermophiles: Organisms that grow best in high temperatures – Have temperature optima above 45°C  Hyperthermophiles have temperature optima above 80°C (these live in extreme hot environments in nature which are those associated with volcanic phenomenon, mainly hot springs)  Mesophiles: Microbes that grow best at moderate temperatures, like most species that grow on animals and plants (most pathogens) Temperature  Psychrophile : microbes that prefer cold temperature – Has optimal growth temp at 15°C or lower, a max temp below 20°C, and minimum at 0°C or lower – Psychrophiles grow in constantly cold regions (polar areas and cold oceans) – E.g. The green algae Chlamydomonas nivalis  Psychrotolerants : organisms that grow at 0°C but have optima of 20-40°C – Psychrotolerant microorganisms are more widely distributed than psychrophiles (soil and water in temperate climates, meat, milk, vegetables and fruit stored at 4°C) – Include various bacteria, fungi, algae and protozoa pH  pH refers to the hydrogen ion (H+) concentration of a solution ----˃ the acidity/alkalinity of a certain solution  Most microbes prefer a neutral or slightly alkaline growth medium (pH 7.2-7.6) – Very few bacteria grow below pH 4 – Lactobacilli grow in acidic pH – Vibrio cholera grow in alkaline pH pH  Most microorganisms have pH optima 5-9 - Only few grow below pH 2 and above pH 9  The optimal pH for growth represents the pH of the extracellular environment only - The intracellular pH must remain relatively close to neutral in order to prevent destruction of acid- or alkali-labile macromolecules  Growth media includes chemical buffers to prevent acid production - Some foods are preserved by acids produced by bacterial fermentation pH  Neutrophiles ----˃ optimal pH ranges 6-8 – Their intracellular pH is close to neutral  Acidophiles ----˃ live in acidic media – Have a pH optima of 3 or less – Fungi tend to be more acid-tolerant than bacteria *** Many fungi grow optimally at pH 5 or below, and few grow at pH as low as 2 – Some Bacteria and several Archea are acidophiles E.g. H. pylori that infects human stomach pH  Alkaliphiles ----˃ prefer basic environments – Grow best at high pH (8-10) – Typically found in soda lakes and high carbonate soils – Some alkaliphilic prokaryotes are also halophilic (saltloving) and most of these are Archea – The most well-studied alkaliphilic bacteria are the Bacillus species, such as bacteria in the intestine – Some are industrially important (they produce hydrolytic enzymes, proteases and lipases, which function at alkaline pH and are used as supplements for household detergents) pH Water (Moisture)  Cells consist of 70 to 95% of water  All living organisms need water to carry out their metabolic processes – Most of them will die in an environment without water  Only a few stages of the life cycle of certain microbes can live without water ----˃ survive desiccation – Desiccation is the state of extreme dryness, or the process of extreme drying – Bacterial spores & protozoan cysts can withstand desiccation Water (Moisture)  Osmotic pressure Is the pressure exerted on a cell membrane by solutions both inside and outside the cell Ideally, both pressures should be equal Water availability is expressed as water activity Salinity  When the concentrations inside and outside the cell are equal ----˃ cell is in a isotonic medium (or solution)  When the concentration of the solutes in the environment outside the cell is greater than the concentrations inside the cell ----˃ cell is in a hypertonic medium – In this case water leaves the cell by osmosis in an attempt to make the concentrations equal ----˃ leads to a case of shrinkage of the cell membrane ** Known as plasmolysis in cells that have a cell wall Salinity  If the concentration of solutes outside is less than the concentration of solutes inside the cell ----˃ cell is in a hypotonic medium – In this case, water enters the cell in an attempt to equalize the concentrations, resulting in cell rupture ** Known as plasmoptysis in cells that have a cell wall Salinity  Halophiles ----˃ organisms living in high salt concentration (ocean) – Mild halophiles (need 1-6% NaCl) – Moderate halophiles (7-15% NaCl) – Extreme halophiles require very high salt concentration (15-30% NaCl)  Halotolerants ----˃ can tolerate some reduction in a low water activity environment, but generally grow best without the added solute Salinity Hydrostatic pressure  Bacteria found in depths of the oceans require high pressure for growth, they inhabit environments 2500 m below sea level, where the pressure is more than 250 times atmospheric pressure.  Pressure dependent bacteria, called Barophiles (peizophilic), have been discovered that can grow at pressures between 500 and 1000 atmospheric pressure.  Barophilic microoorganisms require or grow more rapidly in the presence of increased pressure. They change membrane fatty acids to adapt to high pressures. Others  Osmophiles ----˃ organisms that are able to live in environments high in sugar  Xerophiles ----˃ organisms that are able to live in dry environments Bacterial Growth Requirements Physical  Temperature  pH  Osmotic pressure  Moisture & desiccation Chemical  Oxygen  Carbon source, nitrogen, hydrogen, phosphorus and sulfur  Sodium, potassium, chlorine, magnesium, calcium, and trace elements such as iron Aeration  Oxygen is easily reduced to toxic reactive oxygen species: superoxide radical, hydrogen peroxide, hydroxyl radical.  Aerobes produce protective enzymes: superoxide dismutase, catalase and peroxidase.  All strict anaerobic microorganisms lack or have very low quantities of superoxide dismutase and catalase. These microbes cannot tolerate O2. Aeration On the basis of oxygen requirements, bacteria are classified into:  Obligatory aerobic bacteria: that depend on aerobic respiration for energy needs and molecular O2 functions as a terminal oxidizing agent.  Obligatory anaerobic bacteria: that obtain energy by means of reactions that do not involve the utilization of molecular O2. ** These bacteria possess oxygen sensitive enzymes that can't function in presence of O2 , molecular O2 is a toxic substance. ** Anaerobes obtain their energy from oxygen-independent metabolism.  Instead of oxygen, they require another substance such as a hydrogen acceptor during the generation of metabolic energy and utilize fermentation pathways with distinctive metabolic products. Aeration  Aerotolerant anaerobes do not utilize oxygen but can survive and grow to a limited extent in its presence. These anaerobes are not harmed by oxygen, mainly because they possess alternate mechanisms for breaking down peroxide and superoxide.  Facultative anaerobic bacteria: are able to grow either in the presence or in the absence of molecular oxygen. ** They shift from a fermentative to a respiratory metabolism in the presence of air.  Microaerophilic bacteria: grow only over a very narrow range of oxygen concentrations.