mcb120 second exam.docx - MICROBIAL GROWTH Orderly increase...

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MICROBIAL GROWTH - Orderly increase of all the chemical constituents of a microorganism Cellular constituents Cellular structure Growth without cell division increase in size and weight of the cell For most microorganisms: growth followed by cell division and increase in cell number - Organisms that divide by binary fission – not possible to distinguish resulting cells from mother cell PHASES OF GROWTH 1. Lag phase (adaptation phase) - No immediate increase in cell number/mass - Cells are dormant but physiologically active - Synthesis of new protoplasmic constituents - Synthesis of new enzymes *varies considerably in length with the condition of the microorganisms and the nature of the medium 2. Exponential/Log phase - Growth and division at the maximal rate possible - Depends on: genetic potential of the organism nature of the medium growth conditions - rate of growth is constant - microorganisms divide and double in number at regular interval *cells nearly uniform in chemical composition and physiological characteristics 3. Stationary phase - No net increase/decrease in cell number (cryptic growth) - Slow metabolic activity - Cells enter this phase due to the ffg. - Reasons: Nutrient limitation Accumulation of toxic products *cells are smaller than those in log phase and don’t have constant composition 4. Death phase - Death rate > growth rate - Death is logarithmic but in most cases, slower than that of exponential growth MATHEMATICS OF GROWTH - Increase in cell number in an exponentially growing bacterial culture Geometric progression of the number 2 N f = N 0 x 2 n Where: N f = final cell number N = initial cell no. n = number of generations TIME TOTAL CELL NO TIME TOTAL CELL NO 0 1 4 256 0.5 2 4.5 512 1 4 5 1024 1.5 8 5.5 2048 2 16 6 4096 2.5 32 …. 1048526 3 64 10 1048526 3.5 128 *during exponential growth, cells increase in specific amount To solve number of generations (n) n = log N f – log N 0 0.301
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Generation time (g) – time required for population to double g = t/n where: t = number of hrs/mis of exponential growth k = 1/g (specific growth rate) - Graphical estimation of g by plotting data in semilogarithmic graphs MEASUREMENT OF MICOBIAL GROWTH CELL COUNT 1. Direct method: direct counting of individual cells - Advantages Simple and fast Requires minimum equipment Morphology of bacteria can be observed Makes use of counting chambers - Disadvantages Can’t be used for dilute suspensions Viable and non-viable cells can both be counted Petroff-Hausser Counting Chamber Cells/mL = ave # of cells x 25 squares x 1000 1/20 x 1/20 x 1/50 x 1/D Hemacytometer (count atleast 5 chambers Breed count – direct microscopic count of a stained film of a measured sample known volumple of sample -> spread on a known area -> fix and stain -> count cells in randomly selected fields cells/mL =ave # of organisms x # of microscopic fields per cm 2 x dilution factor / volume used # of microscopic fields per cm 2 = 1/ area of microscopic fields in cm 2 101 = 1.77 x 10 -4 cm 2 Electronic Particle Counter (Coulter Counter) - Electronically measures the number
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