culturing microorganisms

how can bacteria reperoduce

• most common is by asexual reproduction in which the bacterium divide in two (binary fission)

• once a bacterium reaches a predetmined size due to ratio of nuclar materia to cytoplasm, the DNA is replicated and the old cell wall begins to break down around the middle of the cell

• during this process the DNA appears to be associated with teh cell surface membrane. If a memsosome is present it may hold the DNA in position

• enzymes break open the circular piece of DNA, allowing the strands of DNA to unwind and be replicated

• new cross-walls are also formed between the two new daughter cells, and again the mesosomes appears to play a part in cells if it is present

• new cell membrane and cell wall material extend inwards, forming a septum which eventually divides the cell into two daghter cells, each contaiing a circular chromosome attached to the cell membrane

• plasmids often divide at the same time so the daughter cells usually each contain both a copy of the original genome and any plasmids present in the parent cell

what is generation time

the time between cell divisions. The small size and relative simplicity of the prokaryotic cells mean that they can undergo binary fission every 20 minutes when conditions are favourable but often take longer

can bacteria reproduce sexually?

in certain situations some types of bacteria can reproduce using what appear to be different forms of sexual reproduction

very rare and not true sexual reproduction and doesn tinclude formation and transfer of gametes

however they are methods by which genes cna be transferred bts bacteria not necessarily of the same species

why growth of bacterial colonies limited

• limited nutrients and an accumulation of waste products always slow down the rate of reproduction and growth

why logorithmic scale used

• as when considering the growth of bacteria the numbers invovled from difference from initial to descendantsare so enormous it becomes impossible to plot

• logorathmic scale the data become much easier to manage

stages in growth curve

• the lag phase when bacteria are adapting to their new environment and are not reproducing at their maximum rate

• the log phase/exponential phase when the rate of bacterial reproduction is close to or at its theoretical maximum, repeatedly doubling in a given time period

• the stationary phase when the total growth rate is zero as the number of new cells formed by binary fission is equal to the number of cells dying

• the death phase/ decline phase when reproduction has almost stopped and the death rate of cells is increasing

why does the rate of growth slows down?

-a reduction in amount of nutrients available

• at start plenty of nutrients but as numbers multiply exponentially in log phase of growth, the excess food is consumed

• if no fresh nutrients are added. the level of nutrients available will become insufficient to support further growth and reproduction so will limit the growth of the population of bacteria in the culture

• an accumulation of toxic waste productsand becomes sufficient to inhibit further growth and even to poison and kill the culture. In particular co2 whihc is produced by the respiration of the bacterial cells, accumulates and the pH of the colony falls to a point at which the bacteria can no longer grow