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What Is Bacterial Growth & Generation Time? - Curves, Phases & Stages

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  • 0:05 Impossible Bacterial Growth
  • 1:33 Graphing Bacterial Growth
  • 2:52 Phases of Bacterial Growth
  • 4:51 Calculating…
  • 6:09 Lesson Summary
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Lesson Transcript
Instructor: Angela Hartsock

Angela has taught college Microbiology and has a doctoral degree in Microbiology.

Bacteria are able to reproduce at an incredibly rapid pace. In this lesson, we will examine the different phases of bacterial growth and how to calculate population numbers based on generation times and culture size.

Impossible Bacterial Growth

It's likely many of you out there have seen this video. If not, let me describe what's happening. This is a video of a single E. coli cell dividing into many, right before your eyes. Now, actual binary fission, which is the process where the cell divides in two, doesn't occur nearly this fast. In reality, it takes about 20 minutes for one E. coli cell to split into two. To put it another way, every 20 minutes, the population of E. coli can double. If you started with a single E. coli cell and let it grow unchecked for 36 hours, you would end up with enough cells to fully cover the surface of the Earth. Let it grow unchecked for 48 hours, and you would have a mass of E. coli cells that weighed as much as 4,000 Earths! Pretty impressive, especially when you consider that a trillion cells weigh only one gram.

In addition to being an infamous cause of food poisoning, E. coli is a common inhabitant of the gastrointestinal tracts of mammals. So if E. coli is able to double every 20 minutes and cover the Earth in a day and half, why don't we literally have E. coli coming out of our, well, we'll just say ears? Because bacterial growth is not infinite with a constant rate. In this lesson, we will examine the major characteristics of bacterial growth and discover why we're not constantly walking through giant slimy puddles of bacterial cells.

Graphing Bacterial Growth

I mentioned that bacterial growth is not infinite and constant. In fact, bacterial growth is quite complex, influenced by any number of variables, including the species, temperature, pH, available nutrients, toxin concentrations, and competition between organisms. In order to illustrate what is happening during the life of your average bacteria, let's examine another infamous bacteria: Staphylococcus aureus. This common skin bacteria is often implicated in deadly bacterial infections. The reason I've chosen Staph. aureus is that under ideal conditions, it has a generation time of 30 minutes, a nice round time for performing calculations. A generation time is simply the time it takes for one cell to become two. So, if we start with one Staph. aureus cell, in 30 minutes there will be two. In another 30 minutes, there should be four, and so on to 8, 16, 32, 64, indefinitely. If we graphed this relationship, it would look like this, a perfect exponential graph.

Bacterial Growth Generation Time
Bacterial Graphing Growth

In reality, the graph of Staph. aureus growth will look like this. You can see a small portion that resembles the exponential graph, but before and after it looks a bit strange. Fortunately, we can break this graph into four sections, called phases.

Phases of Bacterial Growth

The first phase is called the lag phase. This phase is characterized by very little to no bacterial growth. During this time, the bacteria are adjusting to the environment, growing in size, synthesizing biochemicals, and storing up nutrients. The length of the lag phase depends on the environmental conditions the bacteria are in and the overall health of the cells.

The second phase should look familiar. This is the log phase, characterized by the bacterial cells doubling at a constant, exponential rate. During this time, our Staph. aureus is hitting its stride, doubling in number every 30 minutes. You can see how the population can increase very rapidly. In pathogenic species, it is usually during the log phase that disease symptoms occur as the rising population inflicts greater and greater tissue damage in the host.

The third phase is the stationary phase, when population growth levels off as the rate of cell death begins to equal the rate of cell division. After several hours of rapid cell division, two things are bound to occur: nutrients will be consumed, and toxic byproducts will be released. In either case, cell division can't continue at the same rate if it becomes harder to find food, or you can't survive the toxins.

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