MBIO 1010 - Lecture 16

• • Generation time
  • Definition is the time it takes for a generation to double
  • So - time/gen
  • Or just time
• • Growth rate is gen/time
• when growth is unlimited it is called exponential growth because it generates a curve whose slope increases continuously

  • growth rate (k) is the rate of increase in population number or biomass
  • since bacteria and archaea grow by binary fission, the growth rate is expressed as the number of doublings per hour
• growth can be looked as the time it takes for each cell to become 2 cells, this is called the generation time (g)
• the specific growth rate (k) can be calculated using the formula:$k=$$(log Nt - log No)/ 0.301 x delta t$
• where nt = the number of cells at time 2
• n0 is the number of cells at time 1
• delta t = time 2 - time 1
• • Growth rate
  • Inverse relation to generation time
• •  
  • Number of generations per hour or number of doublings per hour
• y axis always add 10^7
• growth rate is always in hours
• for each organism there is a specific growth rate that is the fastest growth rate in the best growth medium at optimal temperature

  • different for each organism
• clostridium perfringens can double in numbers every 10 minutes under optimal growth conditions
• Escherichia coli less than 30 minutes in a rich medium to grow
• mycobacterium tuberculosis cannot grow faster than one doubling every 24h
• batch culture: a closed-system microbial culture of fixed volume
• closed systems does not necessarily mean with a lid it means you get what you get - not adding anything, same number of resources
• typical growth curve for population of cells grown in a closed system is characterized by four phases

  • lag phase
  • exponential phase
  • stationary phase
  • death phase
• lag phase

  • interval between inoculation of a culture and beginning of growth
  • adjustment phase - settling
• exponential phase/log phase

  • cells in this phase are typically in the healthiest state
  • undergoing exponential/logarithmic growth
  • cells are at their healthiest
• exponential phase/log phase

  • cells in this phase are typically in the healthiest state
  • undergoing exponential/logarithmic growth
  • cells are at their healthiest
• 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
  • growing but growth of population is 0 - growing at the same rate theyre dying
  • rate of growth = rate of death
  • essential nutrient is used up or waste has accumulated and is taking up space or being toxic
• death phase

  • if incubation continues after cells reach stationary phase, the cells will eventually die
  • 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
  • higher death than growth rate
  • population declines
  • some growth but overall population is dying
• 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
  • dilution rate: rate at which fresh medium is pumped in and spent medium is pumped out
  • concentration of a limiting nutrient controls the population size and the growth rate
• microbial counts

  • can be enumerated by direct microscopic observations using a petroff-hausser counting chamber
  • each square corresponds to a calibrated volume
  • results can be unreliable
  • instead of incubating you would use this method if needed to know how many right then and there
• 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 suspension of low density (<10^6 cell/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
• flow cytometry

  • an alternative method that can be used to count the total number of cells
  • uses laser beams, fluorescent dyes and electronics
  • cell sorting
• viable = living, grow and divide to form a colony
• viable cell counts

  • measures only living cells
  • cells capable of growing to form a population
  • two main ways to perform a plate count
  • spread plate method
  • pour plate method
• Issues with viable cell counts

  • Requires lots of preparation (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 (Ex: soil and water)
  • Selective culture media and growth conditions target only particular species
  • A single medium will never grow every microbe
  • Can only count the types of bacteria that can grow in the medium you selected to use