Reading Guide
Chapter 4: Microbial Growth
This chapter focuses on the principles of prokaryotic growth, the requirements
(both chemical and physical) for growth, and methods used to enumerate/count bacteria.
Use my lecture notes as an additional guide to the most important ideas/figures in the chapter as well as this reading guide/summary.
Bacterial cells multiply by the process known as binary fission. After a cell has increased in size, replicated all cellular components, it divides. Microbial growth is referred to as an increase in cell numbers, not an overall increase in cell size. Of interest to microbiologists is the time it takes for a population of cells to double, this is the generation time. The generation time varies with each organism as well as with the specific conditions in which the organism is grown. For example, E. coli has a generation time of 20 minutes under optimal growth conditions while Mycobacterium tuberculosis has a generation time of 12 hours. The generation time is also important from a health perspective. If you have undercooked chicken that is left at room temperature for hours, the greater number of cells present on the food product can lead to food poisoning. To calculate the number of cells in a population you need to use the following formula
N t
= N
0
X 2 n
Where N t
is the number of cells in a given population at a certain time (t)
N
2 n
0
is the number of cells in the population
is the number of cell divisions the cells have undergone during the time (n)
The bacterial growth curve is used to identify the stages of growth in a closed system, one in which nutrients are not replaced and wastes are not removed. This growth curve is a more true reflection of the stages of growth seen with a population of bacteria grown in broth culture. Bacteria that grow on an agar surface in colonies is different since the position of the cells within the colony dictates what type of metabolism and the amount of nutrients that are available. The stages of growth in the bacterial growth curve are
1.
lag phase
2.
log phase
3.
stationary phase
4.
death phase
Review what happens at each phase of growth and be able to draw the bacterial growth curve. At what point in the growth curve are antibiotics made? At what point are cells that can produce endospores more likely to do this?
There are many different environmental factors that influence microbial growth.
In your text there is a summary of these key features in Table 4.2, p 90. You should be familiar with the following features that influence microbial growth:

1.
Temperature a. What is meant by the optimum growth temperature b. the five groups that prokaryotes are divided into the following groups based on temperature preferences: psychrophile psychrotrophs mesophiles thermophiles hyperthermophiles
2.
Oxygen requirements a. How can you determine the oxygen requirements of an organism? b. These groups are based on oxygen preferences, what is the preferred condition for each? obligate aerobe obligate anaerobe facultative anaerobe microaerophile aerotolerant anaerobe c. Why can some organisms grow in the presence of oxygen and others not? Hint:
What is the function of the following enzymes, superoxide dismutase, catalase, and peroxidase?
3.
What pH range do bacteria prefer? Can you identify any examples of bacteria found in either extremely acidic or extremely alkaline environments?
What pH range do molds/fungi prefer?
4. What requirements do bacteria have regarding water, or rather what osmotic environment can they grow in? Remember that the osmotic environment can severely impact bacterial cells, especially hypertonic environments.
Keep in mind that bacterial cells need specific environmental factors to grow (things like temperature, pH, oxygen) as well as chemical nutrients. The basic chemical elements required by all cells are
Carbon
Oxygen
Hydrogen
Nitrogen
Sulfur
Phosphorous
Potassium, magnesium, and calcium
Iron

Use Table 4.4 to review the function of these basic elements. You should be able to identify some organic compounds and/or bacterial structures that are made with each of these elements. For example, carbon is needed by the cell to make any organic compounds. It constitutes almost 50% of the entire cell weight and organisms obtain carbon from either other organic compounds (these are known as heterotrophs) or CO2
(these organisms are known as autotrophs).
There are different types of “foods” made to support the growth of bacteria. Generally speaking the food to support the growth of bacteria is called media and is prepared in either a liquid form (called a broth) or a solid form due to the addition of agar. The agar media can be dispensed into tubes or Petri dishes, forming agar slants, agar deeps, and agar plates. Complex media refers to a kind of media in which there is an abundance of nutrients (an unknown amount of growth factors) and the growth of many organisms is supported. Chemically defined media refers to media that has a known concentration of each nutrient or growth factor. This type of media is useful for determining the requirements of an organism for growth since you can slowly add growth factors and see if the organism grows. For example, GSA (Glucose Salts Agar) is a chemically defined media since we know the exact concentration of each nutrient in the food. (See Table
4.7). Organisms that can grow on GSA have to synthesize all the necessary growth factors from glucose and inorganic salts. What are some examples of necessary growth factors? What organism can grow easily on GSA?
There are further classifications of agar media based on how the addition of chemicals either selects for a group of organisms to grow (selective media) or allows identification of a particular metabolic pathway or enzyme that an organism utilizes (differential media). What are some examples of selective and differential media? Some meida has
BOTH selective and differential features combined. What is an example if this type of media?
It is also important to be able to count bacteria and determine the concentration of cells present in a solution. There are many different ways to count bacteria. The easiest method is to perform a direct cell count. While this is quick and easy to set up, you need to have a special slide called a cell counting chamber. The number of cells counted by this method includes both living and dead cells in a population. Anther method to count both living and dead cells is to measure the turbidity of a sample in a spectrophotometer. This method only works if you have performed a standard curve with the organism you are working with and know what the concentration of cells is at a given spectrophotometer reading. Other methods to determine cell numbers are the viable cell counts such as the plate count and membrane filtration methods. We will do the viable cell count method in lab next week!