Growing Microorganisms in the Lab

Defined media

known exact composition of bacteria

Complex media

unknown composition of bacteria

Selective media

inhibits the growth of unwanted microorganisms

Differential media

creates visible difference in media or colonies

Enriched media

for fastidious bacteria

Anaerobic media

oxygen free

Transport media

prevents specimen contamination and infection of staff

Blood agar

differential for hymolysis

hymolysis

ability to break down blood

MacConkey agar

selective and differential for lactose fermenting bacteria

What are the phases of microbial growth?

1. Lag phase

2. Log(exponential) phase

3. Stationary phase

4. Death(decline) phase

During the lag phase:

cells adapt to the environment and prepare for division

During the Log phase

cells divide by binary fission resulting in rapid population growth

During the stationary phase:

cell growth rate is equal to death rate; population stabilizes

During the death phase

population declines due to nutrient depletion

Plate count

(DM) manually counting each colony on petri dish, best for 25-250 colonies

Serial dilutions

(DM) used for plate counts with 250+ colonies; pond water

Filtration

(DM) used for low bacterial count; potable water

Most Probable Number Method(MPN)

(DM) statistical estimate of number of bacteria present; shellfish bed water

Turbidity

(IM) how much light passes through culture suspension

Metabolic Activity

(IM) how much acid, CO2, etc. has been produced

Dry weight

(IM) when fungus is filtered, dried, and weighed

Streak plate

used to isolate bacterial colonies by progressively diluting the sample across an agar surface

Pour plate

(DM) designed to distribute microbes throughout molten agar, making it easier to count bacterial colonies; pool water