Prescott’s Microbiology, 9th Edition
Chapter 7 –Microbial Growth
GUIDELINES FOR ANSWERING THE MICRO INQUIRY QUESTIONS
Figure 7.1 In addition to chromosomes, what other cytoplasmic contents must be equally
distributed between daughter cells?
Multiple other contents, including plasmids, ribosomes, inclusion bodies, etc These have a
variety of mechanism to ensure inclusion in each daughter cell..
Figure 7.3 Why is it important that the origin of replication migrate to the center of the cell prior
to replication?
Binary fission produces two identical daughter cells, so with the origin of replication in the
center of the cell DNA replication progresses in both directions from the origin resulting in the
chromosomes moving toward opposite ends of the cell for division.
Figure 7.4 What would be the outcome if FtsZ formed a Z ring that was not in the center of the
cell? Consider both the morphology of the daughter cells and the partitioning of the
chromosome.
These E. coli are normally rod-shaped. A Z ring on one side of the cell would lead to one
elongated daughter cell and one more coccoid. The two chromosomes may not be evenly
partitioned, which is typically lethal.
Figure 7.7 Which step in the development of rod-shaped cells is essential in determining cell
morphology?
Step BI, sidewall elongation, is essential. Since cell wall synthesis occurs along the sides but not
at the poles, this elongates the shape. This is confirmed since, as the text describes on this page,
deletion of the mreB gene results in spherical cells, and the MreB protein guides this process.
Also, mreB homologs are present in rod-shaped bacterial species but not cocci.
Figure 7.9 What is the difference between halophilic and halotolerant?
Both can survive in high salt concentrations. Halophilic bacteria require higher salt
concentrations to grow, whereas halotolerant do not.
Figure 7.13 Why do facultative anerobes grow best at the surface of the tube while aerotolerant
microbes demonstrate a uniform growth pattern throughout the tube?
Aerotolerant anaerobes only carry out anaerobic metabolism, they cannot utilize oxygen.
Oxygen does not kill them, thus they are aerotolerant. Their metabolic rate, and thus amount of
growth in the tube, is the same in the presence or absence of oxygen. However, facultative
anaerobes possess multiple modes of metabolism, being able to utilize oxygen when it is present,
but also carrying out other metabolic modes (such as fermentation) when oxygen is not present.
Aerobic metabolism is the most efficient however, therefore there is more growth in the tube
where oxygen is present.
Figure 7.16 What biomolecules make up the extracellular polymeric matrix, and what functions
does the matrix serve?
The first EPS studied were made of polysaccharides, such as alginate made by Pseudomonas.
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© 2014 by McGraw-Hill Education. This is proprietary material solely for authorized instructor use. Not authorized for sale or distribution in
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Prescott’s Microbiology, 9th Edition
However, we have now discovered that the chemical composition of the EPS varies by species;
some using polymerized nucleotides, other proteins, other glycolipids. Yet the functions of the
EPS are the same in biofilms. EPS serves to help attach the bacteria to a surface, to give the
bacteria a substrate in which to be embedded, to protect the bacteria from predators (such as
protozoa) and toxins, and to hold secreted enzymes to catalyze extracellular reactions for nutrient
acquisition, detoxification, cell signaling, etc. Note that in most bacterial species, the polymer
that is utilized as their biofilm matrix is the same as the polymer they use as a capsule when as
individual or small clumps of bacteria.
Figure 7.27 Identify the regions of the growth curve in which (1) nutrients are rapidly declining
and (2) wastes accumulate.
At the end of exponential phase in a closed system, the bacteria have used up the nutrients and
wastes begin to build up, which then leads to stationary phase. Thus, while exponential phase is
about rapid growth, the stationary phase is about stress survival. So, in stationary phase,
nutrients are rapidly declining and wastes are accumulating. One major reason why cells die
during the death phase is because the wastes have accumulated to toxic levels.
Figure 7.35 Why is it important to have no more than about 250 colonies on a plate when
counting colonies to determine population size?
One the number of colonies raises above this number, they are no longer individual, and begin to
be touching each other, thus the ability of the microbiologist to accurately distinguish from each
other is lost, and the count becomes unreliable. The recommendation for an accurate CFU count
is to includes plates with between 20 to 250 colonies. Below 20, the numbers become to low for
statistical validity.
Figure 7.39 Why does the cell density stay relatively constant over such a large range of dilution
rates?
Cell density remains fairly constant across a number of dilution rates because only a limited
amount of an essential nutrient is available so the nutrient and subsequent energy is used for cell
maintenance not cell growth.
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© 2014 by McGraw-Hill Education. This is proprietary material solely for authorized instructor use. Not authorized for sale or distribution in
any manner. This document may not be copied, scanned, duplicated, forwarded, distributed, or posted on a website, in whole or part.