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created 3 years ago by Carmelle_Rodney_Norvilus
When bacteria are inoculated into a new sterile nutrient broth, their numbers don’t begin to increase immediately. Instead, there is a lag phase that may last for an hour or even several days. Why don’t bacterial numbers increase immediately?
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When bacteria are inoculated into a new sterile nutrient broth, their numbers don’t begin to increase immediately. Instead, there is a lag phase that may last for an hour or even several days. Why don’t bacterial numbers increase immediately?

The bacteria must adjust to the nutrient content in the new medium, synthesizing necessary amino acids, growth factors, and enzymes.

A new medium may not have the same nutrients that were available in the medium from which an inoculum was taken. The bacteria may have to synthesize different amino acids, growth factors, or enzymes to enable them to grow in this new medium. Once those are synthesized, the growth rate is likely to increase, and the cells will move into log (exponential) growth phase.


What Is Happening during Stationary Phase?

After a period of rapid growth (log phase), bacterial growth rates will slow and enter the stationary phase. The number of viable cells no longer increases, but instead stays constant. In this activity you will indicate the statements that correctly describe what is happening during stationary phase.Select each statement that accurately describes what is happening to bacterial cultures during stationary phase.

1.-The cells are likely running out of nutrients.
2.-The number of cells that are dying is balanced by the number of new cells that are being formed.
3.-Harmful waste products may be accumulating.

During the stationary phase, the number of viable cells does not increase and remains constant, or stationary. This phase begins as the nutrient supply is depleted and/or waste products begin to accumulate at a toxic level.


Prolonging Exponential Growth

A chemostat is continuous culture system that is designed to promote and prolong exponential growth and prevent bacteria from entering stationary phase. How might this work?

Chemostats provide a continued source of fresh nutrients and remove wastes and dead bacterial cells.


Calculating Population Increases during Exponential Growth.
This activity asks you to calculate the size of a bacterial population during exponential growth.
A broth medium has been inoculated, and microbial numbers will be counted periodically to generate a bacterial growth curve. At 2 hours after inoculation, the culture has progressed through lag phase and is now in log phase. At this point, the population size is 1 million cells. The generation time is 30 minutes. Assuming the continuation of log growth, how many cells would there be at 2 hours of growth in log phase?

There would be approximately 16 million cells. In the 2-hour interval of log growth, there are four 30-minute generations. This is four doublings of the 1 million organisms present at hour 2.

1 million cells x 24 = 16 million cells


Growth Rate and Generation Time

This question asks you to make comparisons regarding the slope of an exponential growth curve, based on generation time.Each of the three graphs shown below includes data collected during exponential growth of a species of bacteria grown in three different growth conditions. Which growth condition resulted in the longest generation time?

There are a number of ways to count or estimate numbers of bacteria; some are relatively convenient, and others are quite labor intensive. Some counting methods count only those cells that are living (viable count, such as a plate count), whereas other methods count the number of all cells without discriminating whether they are living or dead (such as a direct microscopic count). Microbial numbers can also be estimated indirectly by turbidity measurements, using a spectrophotometer.


Counting Bacterial Populations to Prepare a Growth CurvePart complete

This activity asks you to consider various methods of counting or estimating the size of bacterial populations and to select the method that will provide the most accurate data for plotting a bacterial growth curve.

Which of the following methods would be most appropriate for gathering data to plot a bacterial growth curve throughout the four phases?

A plate count will provide the number of living cells. These data are most appropriate for generating a plot of a bacterial growth curve


The Lake of the Ozarks is a human-made lake, so it collects runoff from coal strip-mining, fertilizers, resort wastewaters, and septic drainages. The average lake temperature is between 10 C and 21 C. Consider the physical requirements for growth and multiplication that would allow fecal coliforms to “blossom” in the Lake of the Ozarks. Which of the following would accurately describe these organisms?

Facultative halophiles

The bacterial growth process depends on the microorganism’s ability to adapt to environmental conditions. Being able to grow to larger numbers increases the chances for survival and spread to new hosts. Requirements for growth have led us to develop descriptive terms for the organisms that refer to temperature range [Chapter 6, section 6-1], pH range [Chapter 6, section 6-2], and pressure range [Chapter 6, section 6-3].


In addition to physical requirements, bacterial growth and multiplication also entails chemical requirements. Which of the following statements would accurately describe chemical requirements and conditions associated with growth in the Lake of the Ozarks environment?Select all of the following statements that are true regarding fecal coliforms in the Lake of the Ozarks.

Fecal coliforms will likely contain enzymes to counteract toxic oxygen forms.

- Fecal coliforms will need to acquire trace elements for growth in the lake environment.

- The lake environment will support the growth of facultative anaerobes.

With chemical and physical requirements met, bacteria are able to replicate in their environment and potentially increase numbers enough to find or invade a new host.


Bacteria naturally prefer company instead of solitude for growth. Examples of this kind of communal growth pattern can be found everywhere, from surfaces of the teeth and the intestines, to the surface of a slimy rock in the lake, to the thick floc that clogs water pipes. These examples of communal bacterial growth are known by what name?


Biofilms are a large collection of mixed populations of organisms that form a matrix of “slime” for protection and assemble a complex, coordinated, and functional community on a surface. Biofilms often provide a safe haven for bacterial growth, often among “friendly” microorganisms. The complex architecture of biofilms gives the cells benefits they would not have in an individual lifestyle as a planktonic cell. As such, biofilms can play an extremely important role in colonizing a host and leading to a disease state.


Sampling lake water for microscopic organisms is not as easy as it sounds when you want to both count and identify species. In a given water sample, your bacteria of choice can be rare or difficult to culture, particularly given the mixture of bacteria in a biofilm. Which of the following culture media would be best for growing fecal coliforms when they are relatively less abundant than other bacteria in the lake?

enrichment culture medium

Growth of microorganisms for identification is not a trivial exercise. To accurately and efficiently grow bacteria in culture, you must consider the bacteria’s relative abundance, metabolic and chemical needs, and best growth conditions, as well as the cost and overall goals of the experiment. Enrichment culture medium is a good choice for environmental samples whose desired populations are low compared to the rest of the microbes present. From a sample of large numbers of organisms, complex medium favors the growth of only certain organisms.


Cultures of the Lake of the Ozarks water samples were grown on enrichment media for identification. However, these organisms are present to some degree in all samples. An important question from a clinical perspective is whether or not there is a significant increase in the fecal coliform populations that would put people’s health at risk. If you were part of the team investigating this outbreak, how would you best enumerate the fecal coliform threat?

filtration count culture

This method will trap organisms and keep them on the surface of the filter. Transferring the organisms to a plate will then produce an accurate count.

Fecal coliform bacteria were counted by direct filtration CFUs. Some of the samples taken at the beaches exceeded the recommended maximum levels established by the Environmental Protection Agency (EPA) for direct sampling: greater than 235 E. coli per 100 ml of water, or a geometric mean of greater than 126 E. coli per 100 ml of water. The beach samples were 178.2 E. coli per 100 ml and 1299.7 E. coliper 100 ml, with a geometric mean of 173.2 E. coli per 100 ml.