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Lecture 13

MBIO 1010 Lecture 13: Lecture 13

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University of Manitoba
MBIO 1010
Christopher Rathgeber

5.6 The Growth Cycle (Continued…) Lag phase Interval between inoculation of a culture and beginning of growth (i.e. increase in cell numbers) Exponential phase Cells in this phase are typically in the healthiest state. Stationary phase Cells metabolically active, but growth rate of population is zero Either an essential nutrient is used up, or waste product of the organism accumulates in the medium. Death phase If incubation continues after cells reach stationary phase, the cells will eventually die 72 Microbiology 1010 – Lecture Notes Not all bacteria die, some bacteria form spores/cysts or dormant stages that allow a significant proportion of cells to survive for a long time. Continuous Culture Continuous culture – An open-system microbial culture of fixed volume Chemostat Most common type of continuous culture device Both growth rate and population density of culture can be controlled independently and simultaneously. Continuous Culture (Continued…) Dilution rate: rate at which fresh medium is pumped in and spent medium is pumped Concentration of a limiting nutrient controls the population size and the growth rate. Microscopic Counts Microbial cells can be enumerated by direct microscopic observations using a Petroff- Hausser counting chamber Each square corresponds to a calibrated volume. o But results can be unreliable… 73 Microbiology 1010 – Lecture Notes Microscopic Counts (Continued…) Limitations of microscopic counts Cannot distinguish between live and dead cells without special stains Small cells can be overlooked Precision is difficult to achieve (need a lot of counts) Phase-contrast microscope required if a stain is not used Cell suspensions of low density (<10 cells/ml) hard to count Motile cells need to immobilized Debris in sample can be mistaken for cells Cells may move (Brownian motion), some form clumps Based on random distribution and dispersal of the cells Microscopic Counts (Continued…) A second method for enumerating cells in liquid samples is use of a flow cytometer Uses laser beams, fluorescent dyes, and electronics Viable Counts Viable cell counts (plate counts): measurement of only living cells capable of growing to form a population Two main ways to perform plate counts: Spread-plate method o Pour-plate method 74 Microbiology 1010 – Lecture Notes Viable Count Issues Plate counts require significant amounts of preparation and manipulations (dilution tubes, agar plates), and incubation time (overnight or more) to get the measurements for a single culture. Plate counts can be highly unreliable when used to assess total cell numbers of natural samples (e.g., soil and water) Selective culture media and growth conditions target only particular species There is no medium that will grow every microbe! Can only count the types of bacteria that can grow in the medium you selected to use! They are inaccurate because sometimes colonies grow on top of each other...there is different anomalies that can cause numbers that are inaccurate. If you looking to culture bacteria…. You’re not going to have all microorganisms (there isn’t a media that grows all types of bacteria). Viable Counts The great plate anomaly: direct microscopic counts of natural samples reveal far more organisms than those recoverable on plates Modern genomic techniques suggest that only 1-10% of microbial diversity is culturable from most environmental samples (including the diver
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