Bacterial Growth and Requirements PDF

BACTERIAL GROWTH AND REQUIREMENTS  Objectives State metabolic needs for bacterial growth Identify Environmental Factors required for bacterial growth It focus on BACTERIAL GROWTH CURVE and its phases It also includes BACTERIAL SPORES and its Properties  Microorgan...

BACTERIAL GROWTH AND REQUIREMENTS  Objectives State metabolic needs for bacterial growth Identify Environmental Factors required for bacterial growth It focus on BACTERIAL GROWTH CURVE and its phases It also includes BACTERIAL SPORES and its Properties  Microorganisms are of diverse sizes, shapes and structures, and live in widely varied environments. they also differ widely in the details of their physiology, though their biochemical mechanisms  Water is The basic chemical composition making up about 80% of the cell’s weight. In addition to proteins, nucleic acids, polysaccharides and lipids. The main activity of the bacteria as a whole is reproduction, this requires the following: Nutritional requirments, Energy and Environmental Factors  BACTERIAL REQUIREMENTS 1- Nutritional 2-Energy 3-Environmental Factors pH Temperature Aeration Carbon Dioxide Moisture  A-Nutritional Requirements Nutrients in growth media must contain all elements necessary for the biologic synthesis of new organisms: Basic elements such as hydrogen, oxygen, carbon and nitrogen in large quantities with sulphur, and phosphorus in smalleramounts , while other elements such as sodium, potassium, iron, magnesium and manganese in traces.  Growth factors: These are essential constituents which cannot be synthesized by the organism and must be provided readymade. Examples of bacterial growth factors are: a) Amino acids. b) Bacterial vitamins  B-Energy The major mechanisms for generating metabolic energy are: aerobic respiration, Anaerobic respiration fermentation, photosynthesis  C-Environmental Factors 1- Hydrogen ion concentration: The majority of bacteria grow best at pH 7.2-7.6 (neutrophils), some bacteria as lactobacilli are acidophils and can grow at a pH of 4. Other species, e.g. Vibrio cholerae prefer a highly alkaline environment, pH 8.5-9.0 (alkaliphiles). 2- Temperature: Almost all bacteria which are pathogenic to man have an optimum temperature for growth at 37°C (body temperature) 3- Aeration: Two gases influence the growth of bacteria; oxygen and carbon dioxide. As regards oxygen, the bacteria can be classified into: Obligatory aerobes: These can grow only in the presence of oxygen, e.g. Mycobacterium tuberculosis and Pseudomonas aeruginosa. Facultative anaerobes: These organisms are able to grow in the presence or absence of oxygen though e.g. enterobacteria and staphylococci. Obligatory anaerobes: These organisms cannot grow in the presence of oxygen e.g. clostridia and anaerobic streptococci. Microaerophiles: These organisms (microaerophilic) grow best in the presence of little oxygen and high concentration of CO2, e.g. Campylobacter and Helicobacter.  Carbon Dioxide: All bacteria require CO2 for growth in normal atmosphere (0.04%). A higher concentration 5-10% is required for many parasitic species notably N. gonorrhoeae, N.meningitides and Brucella abortus. 4- ionic strength and osmotic pressure Most bacteria able to tolerate a wide range of external osmotic pressure and ionic strength Osmolarity is regulated by the active transport of k+ ions into the cell Organisms requiring high salt concentrations (Nacl  5-Moisture Four-fifths by weight of the bacterial cell consists of water (80%) as in the case of other organisms; moisture is absolutely necessary for growth. Drying in air is lethal to many bacteria, e.g. Gonococci and Treponema pallidum, common cold virus, whereas Mycobacterium tuberculosis, staph aureus, small pox virus may survive for weeks or months  BACTERIAL GROWTH Growth is the increase in the sum of all the components of an organism (increase in the number of the cells) A cell may double every 20 minutes, by simple binary fission  Most bacteria can grow on artificial culture media. However, some bacteria e.g. Mycobacterium leprae and Treponema pallidum cannot yet be grown in-vitro Other bacteria as chlamydiae and rickettsiae only replicate within host cells and are grown in tissue culture  BACTERIAL GROWTH CURVE When a small number of organisms are taken from a culture and inoculated into a fresh growth medium, the number of cells multiply a million- fold or more. If the number of cells present at different times after incubation is measured in relation to the time, the resultant is the growth curve 1- Lag phase 2- log phase 3- stationary phase 4- death phase  Lag phase: There is no multiplication of bacteria. It represents the time taken for the cells to adapt themselves in the new environment and prepare for division.  The duration of this phase varies from few hours to few days depending on the following factors: A-The nature of organism, E. coli has a short lag phase, may be less than one hour; Mycobacterium tuberculosis has a long lag phase may be few days. B-The size of inoculum, the bigger the size the shorter the lag phase. C-The more suitable the medium the shorter the lag phase.  Logarithmic phase (phase of accelerated growth) The cell divides by binary fission regularly at a constant maximal rate and in an exponential manner, In this phase the bacteria are more sensitive to the effect of antimicrobial agents.  Stationary phase: the rate of growth diminishes untill the number of cells remain constant where the number of newly formed cells becomes equal to that dying. This is due to one or more of the following factors: A- Exhaustion of nutrients. B-Accumulation of toxic metabolites. C-Development of unfavourable pH and O2 consumption.  Decline or death phase During this phase, the bacterial population declines due to death of the cells. The decline phase starts due to: 1. accumulation of toxic products and autolytic enzymes 2. exhaustion of nutrients.  This growth curve draws similarity to what happens to the human being during the course of a disease, the incubation period is similar to lag phase, the log and stationary phases to the acute and subacute stage of the disease and then the stage of decline represents convalescence  BACTERIAL SPORES Members of several bacterial genera are capable The two most common are Gram-positive rods: of forming endospores. the obligately aerobic and the obligately anaerobic genus Bacillus genus Clostridium Sporulation : the process of endospore formation within a vegetative (parent) cell It occurs when there is depletion of nutrients (carbon, nitrogen, or phosphorus). The spore is highly resistant to desiccation, heat, and chemical agents. When returned to favorable nutritional conditions and activated, the spore germinates to produce a single vegetative cell.  Spores play a very important part in the transmission of certain diseases like anthrax, tetanus, gas gangrene, and botulism  Properties of Endospores Spore wall, which is the innermost layer, it contains normal peptidoglycan which will become the cell wall of the germinating vegetative cell. Spore cortex is the thickest layer of the spore envelope. Spore coat confers on spores their relative resistance to antibacterial chemical agents. Exosporium  Chemical composition of spores The cytoplasm of the spore is in a state of almost complete dehydration. New constituents appear as for example calcium dipicolinate which is responsible for stabilization of enzymes against desiccation and heat.  Germination Endospores can remain dormant for thousands of years. An endospore returns to its vegitative state by a process called germination , which is triggered by physical or chemical damage of the spore coat. The enzymes of the endospore then break down the extra layers surrounding the endospore water enter calcium dipicolinateis lost ,and metabolism resumes and normal cell reappears. Morphology of bacterial spores.