Bacterial Growth



Lecture Outline:

1. A pure culture: A population of cells derived from a single cell. The study of bacteria involves the study of a population of cells rather than individual cells. In nature most bacteria are found living with other organisms, i.e., as a mixed culture, and not as pure cultures.

2. How are pure cultures obtained:

3. Nutritional requirements: In the simplest of terms, all organisms must have (1) a source of energy, (2) a source of carbon, and (3) certain essential elements. The major chemical elements in terms of amounts required for growth are carbon, hydrogen, oxygen, phosphate, nitrogen, sulfur, magnesium, and iron. In addition other elements, called trace elements, are needed in smaller amounts (manganese, zinc, cobalt, molydenum, nickel, and copper) and/or vitamins (biotin, B12, niacin). These elements can exist in many different chemical forms, and the ability to use these different compounds for growth defines the nutritional properties/characteristics of the bacterium.

4. The major groups of organisms based on their carbon and energy requirements

5. Types of media used to culture bacteria:

6. Bacterial cell division: An asexual process Fig. 4.1

7. Methods to monitor growth: Methods to monitor growth measure an increase in cell numbers. (WILL NOT COVER IN SUMMER QUARTER)

8. Phases of growth in a batch or closed system (a finite amount of nutrients are provided, and as bacteria grow conditions change): Cells are dividing asynchronously. Fig. 4.6

9. Factors that influence growth:

Learning Objectives:

 

Classification of Bacteria According to Their Source of Energy and Carbon
Name
Energy Source
Carbon Source

Photoautotrophs

Light
C02

Photoheterotrophs

Light
Various Organic Compounds

Chemolithotrophs

Inorganic compounds such as H2, NH3, N02-, Fe2+, H2S

C02

Chemoheterotrophs

Organic
Organic compounds

 

Methods to Determine Bacterial Growth:

A. Determination of cell number:

1. Total cell count methods:
a. Direct microscopic count- See fig. 4.5 pg. 91
Advantages- quick and easy

Problems- can not distinguish be live and dead cells, can not detect less than 106 bacteria/ml.

b. Coulter count (electronic count):

Advantages- very quick and easy

Disadvantages- same as above, count dust and debris, and apparatus very expensive.

2. Viable cell count method: See fig. 4.7 pg. 93

advantages- can count as few as 1 bacterium/ml, only count live cells

disadvantages- requires time for growth, may need to make dilution's of preparation and make dilution calculations (examples), error due to growth of bacteria in clumps and chains.

B. Determination of cell mass-

1. dry weight determination:
advantages- only way to determine growth of filamentous bacteria.

disadvantages- cumbersome, not very accurate, If cell numbers important must relate weight to total cell count number.

advantages- rapid and easy

disadvantages- does not give you cell numbers or increase in mass (must correlate to one of above), can not distinguish between live and dead cells, and must work within certain absorbency (more than 107 and less than 108).

C. Determination of cell constituents- measure increase in a specific cell material, i.e., DNA, RNA, Protein, or etc.

 

Dilution Problems:

(no. of bacteria/ml in original sample = no. of colonies on plate X 1/total dilution X 1/ volume sample plated)

1. You are interested in determining the number of bacteria in saliva. You spit into a tube, and then do four 1:10 dilution's. From the last dilution tube you plate 1.0 ml onto an appropriate medium, and observe 100 colonies on the agar surface after overnight growth. How many bacteria are present in the original sample?

2. A friend of yours tells you that there should be no bacteria in hamburger meat, and having had micro you say not true. To show him/her you do the following: You take 1 gram of meat and blend it in 100 ml of sterile water. You then do the following dilution: 1:10, 1:100, 1:10, and a 1:100. You then take 0.1ml from the last dilution, and plate onto an appropriate medium, and find that after 18 hours of growth that there are 125 colonies on the plate. How many bacteria were present in the original sample, per ml of blended material and per gram of hamburger meat?

 

Enzyme Content of Bacteria With Different Requirements (sensitivity) for Oxygen
Name
Enzyme Content for O2 detoxification
Strict aerobe
catalase

H2O2 ---> H2O + O2

superoxide dismutase

O2- + 2H+ --> O2 + H2O2 --> H2O + O2

Facultative anaerobe
catalase and Superoxide dismutase
Strict anaerobe

neither catalase nor superoxide dismutase

Microaerophile
small amounts of catalase and superoxide dismutase
Aerotolerant
superoxide dismutase

 6/14/09