Bacteria can grow and divide very rapidly, every 20 minutes for some species under ideal conditions, or as slowly as every 100 years for bacteria in deep underground environments. The generation time, or doubling time, is the amount of time it takes for the number of bacterial cells to double in a culture. Optical density measurements using spectrophotometry is a common way to indirectly measure bacterial growth and calculate doubling times by tracking increases in turbidity over time. Direct microscopic counting and viability assays that measure colony forming units are other methods to directly measure bacterial cell numbers.

1. Bacteria are known for their rapid growth, many of the enterics
will grow and then divide every 20 minutes under ideal conditions. Some
bacteria isolated from deep underground may divide once every hundred
years!
Generation Time (doubling time): the amount of time required to
double the number of cells in a culture.
Measuring doubling time:
theoretically: Nt = No x 2n
by direct measurement:
1.) Turbidimetry: Semi-log plot of optical density (OD) vs. time
2.) Direct microscopic count
3.) Viability count

2. Optical Density (OD)
--- a measure of the number of objects in a light path (beam),
generally also referred to as turbidimetry (requires a spectrophotometer)
--- Absorbance (OD measure) is the decrease in the amount of
light getting through the sample, usually the more sensitive measure
--- Transmittance is the total amount of light penetrating the
sample, doesn’t require as much optics as absorbance but is less
sensitive
--- OD (600nm): optical density taken with 600 nm light
--- be aware that absorbance over 2.0 is not measured accurately
by the spectrophotometer (sample will need to be diluted).
--- above OD 0.4 scattered light is no longer proportional
to cell density), very dense Cultures should be diluted by some known
factor before another reading is taken (1/10 dilution is common)

3. 1
1.0
0.1
0.01
5432
Time (hrs.)
OD(600nm) Semi-Log plot of Growth Data

4. Calculating doubling (generation) time from an
OD measurement (indirect method):

5. Other Means of Determining Cell Number
Direct Count & Viability Count (cfu)

6. Viability Count (Colony Forming Units, CFU)
Serial Dilution

7. Bacterial Growth Phases

8. Microenvironments within a Test Tube
What is the limiting factor to growth
of a strict aerobe in this system
other than O2?

9. Factors for Bacterial Growth
--- availability of nutrients (chapter 5)
--- accumulation of metabolic byproducts (wastes)
---pH (protein structure, electron transport)
--- oxygen (or lack there of)
toxicity of oxygen
--- temperature (protein structure)
--- availability of water (food storage)

10. Summary Chapter 6
Bacterial Growth
--- know each phase of the growth curve and what the
cells are experiencing in each phase
--- aseptic technique
-- culturing and media types
How do you measure cell density?
--- OD (growth curve)
--- Direct Count
--- Serial Dilution
--- Water sampling
Factors affecting bacterial growth
--- be able to list several and explain them
--- be able to define temperature classifications