Microbial Growth

Chapter 6

Objectives• Describe some techniques

used to grow microorganisms• Explain the requirements for

the growth of microorganisms• Explain the different phases of

bacteria growth

Growth of Microbes• Bacteria growth is an increase in number of

cells, not cell size– One cell becomes colony of millions of cells

• Bacteria divide by – Binary fission– Alternative means

•Budding (yeast)•Conidiospores (filamentous bacteria,

actinomycetes)•Fragmentation

• Control of growth important in control of infection

Figure 6.12a

Binary Fission

Generation Time• Time required for cell to divide, for

population to double• Average for bacteria is 1-3 hours• E. coli generation time = 20 min

–20 generations (7 hours), 1 cell becomes 1 million cells!

–Exponential growth

Figure 6.12b

Cell Division and Growth Curve

Phases of Growth• Lag phase:

– making new enzymes in response to new medium

• Log phase (exponential growth):– Most sensitive to drugs

and radiation during this period

• Stationary phase:– nutrients becoming

limiting or waste products becoming toxic

– death rate = division rate

• Death phase:– death exceeds division

Standard Growth Curve

Direct Measurement of Microbial Growth

• Several methods of measuring cell growth– Direct

• Plate count• Filtration• Microscopic count• Most probable number

– Indirect• Turbidity, Metabolic

activity, Dry weight

Figure 6.16

Serial Dilutions• Reducing concentration to manageable levels• After incubation, count colonies on plates with 25–250 colonies

(CFUs)• Measures viable cells; require 24 h or longer; assume that each

bacterium produces a single colony

Counting Bacteria by Filtration

• Water sample are passed through a thin filter whose pores are 0.22 m in size

• Used to detect coliform bacteria (indicators of fecal contamination) in drinking water

Most Probable Number (MPN)• M/os being counted usually grown in liquid media

– Usually liquid differential medium is used

• You are making a statement that there is a 95% chance the bacterial population falls within a certain range

Direct Microscopic Count of Bacteria

Estimating Bacterial #s by Turbidity

•A practical way of measuring bacterial growth in a spectrophotometer

•A beam of light is transmitted through a bacterial suspension to a light-sensitive detector

•As bacterial #s increase, less light will reach the detector

•The turbidity is expressed as absorbance or optical density in a scale of 0 to 2

Ex.• A piece of hamburger meat is left at room

temperature for 2 hrs. If the generation time for pathogenic E. coli (O157:H7) is 20 min, how many bacteria would be present at the end of the 2 and 7 hrs if you only had 2 bacterium at the beginning?

• A)128• B)4,194,304