Bacteria: Growth and Reproduction

Bacterial Growth

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Bacteria grow and divide at astonishing rates (sometimes as often as every 20 seconds)! We can culture bacteria in a lab by providing them with optimal environments within a petri dish or a liquid broth.

  1. petri dish contains agar (gel with food/nutrients as well a surface to grab onto). Bacteria grow in collections, or colonies on the petri dish.
  2. Liquid broth is full of nutrients/food and bacteria are suspended within it.

Bacterial Reproduction

Two types of reproduction are seen in bacteria: binary fission and conjugation.

Binary Fission

In binary fission, a bacterium doubles its size by elongating. It also replicates its DNA before dividing in half, producing 2 identical cells. This is asexual reproduction. Take a look at binary fission below:

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Stop and Think: How is binary fission similar to eukaryotic mitosis? Examine the diagrams below.  (Answer: They both produce identical cells.)

Conjugation

Conjugation is a process where two bacteria can transfer genetic material from one cell to another. They do this by using pili (singular: pilus) that link one bacterium to the other. The genetic material (called an F plasmid) is copied and then transferred; the F plasmid is a ring of circular DNA that frequently contains antibiotic resistance genes. Therefore, conjugation will double the number of resistant bacteria in a population.

Examine the steps of conjugation below:

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Review conjugation with this interactive.

Stop and Think: In the interactive above, a plasmid was passed on that had an antibiotic resistance gene. Any bacterium with this plasmid would be able to survive that antibiotic. Here is a plasmid that allows the bacterium to survive in the presence of the antibiotic ampicillin. Predict what would happen if bacteria with or without this plasmid were grown on different types of agar.  (Answers: Yes in all but the bottom right corner.)

  Petri Dish with Nutrient Agar Petri Dish with Nutrient Agar containing antibiotic Ampicillin
Bacteria with amprPlasmid Will you see bacteria grow? Will you see bacteria grow?
Bacteria without Plasmid Will you see bacteria grow? Will you see bacteria grow?

Bacterial Dormancy: Endospores

Bacteria sense and react to their environment. When food is scarce, they may move to look for it, or some species of gram positive bacteria may use an extreme survival strategy: endospores.

Dormancy is a state of inactivity in some organisms where growth and metabolism temporarily stop. A few species of gram positive bacteria can enter a state of dormancy and form endospores if they experience environmental stress, such as lack of nutrients.

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Endospores are incredibly resistant to destruction; they can survive boiling, UV light, freezing, drying and disinfectants, all of which would normally kill bacteria. How does an endospore form? The bacteria divides within in its cell wall and one of the two cells engulfs the other.

Endospores can be revived after decades of dormancy; they will start to grow quickly when conditions are favorable. Pasteurization is a process of heating and cooling food products repeatedly so that the endospores which break out of dormancy into regular bacterial form during cooling will be killed if heated again.

 

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