Mircobiology

Rhea Franklin

4-4-15

Micro Hw5&6

1. Bacterial growth is defined not in terms of cell size but as the increase in the number of cells, which occurs by cell division. Bacterial growth is measured by estimating the number of cells that have arisen by binary fission during a growth phase.

2. Serial dilution is a method of measurement in which successive 1:10 dilutions are made from the original sample. If you have made dilutions properly, you should get plates with a countable number of colonies. The accuracy of the serial dilution and plate count method depends on homogenous dispersal of organisms in each dilution.

3. Standard bacterial growth curve is a graph plotting the number of bacteria versus time and showing the phases of bacterial growth. Lag phase- first four major phases of the bacterial growth curve, in which organisms grow in size but do not increase in number. Log phase- second of four major phases of bacterial growth curve, in which cells divide at an exponential or logarithmic rate. Stationary phase- the third of four major phases of the bacterial growth curve in which new cells are produced at the same rate that old cells die, leaving the number of live cells constant. Decline phase – the fourth of the four major phases of bacterial growth curve in which cells lose their ability to divide and thus die.

4. Most species of bacteria can grow over a 30C temperature range, but the minimum and maximum temperatures for different species vary considerably. Seawater remains liquid below 0C, and organisms living in cold ocean waters can tolerate below freezing temperatures. According to their growth temperature range, bacteria can be classified as psychrophiles, mesophiles, or thermophiles. Psychrophiles – a cold-living organism that grows best at temperatures of 15 to 20C. Thermophile- a heat-loving organism that grows best at temperatures from 50 to 60C. Mesophile- an organism that grows best at temperatures between 25 and 40C including most...