Answers to Tell Me Why Questions
CHAPTER 1 A Brief History of Microbiology
p. 7: Some people consider Leeuwenhoek the “Father of Microbiology.” Explain why
this moniker makes sense.
p. 17: Some people consider Pasteur or Koch to be the Father of Microbiology rather
than Leeuwenhoek. Why might they be correct?
Pasteur was first in many fields of microbiology. He disproved the theory of spontaneous
generation, showed that microbes could travel in air, demonstrated that yeast ferments
p. 21: Why are so many modern questions in microbiology related to genetics?
CHAPTER 2 The Chemistry of Microbiology
p. 30: Electrons zip around the nucleus at about 5 million miles per hour. Why don’t
some fly off?
p. 35: Chlorine and potassium atoms form ionic bonds, carbon atoms form nonpolar
covalent bonds with nitrogen atoms, and oxygen forms polar covalent bonds with phos–
phorus. Explain why these bonds are the type they are.
The bond between K and Cl is an ionic bond. Chlorine is much more electronegative than
Answers to Tell Me Why Questions
p. 36: Why decomposition reactions are exothermic (release energy)?
p. 39: Why does the neutralization of an acid by a base often produce water?
p. 52: Why do the cell membranes of microbes living in Arctic water likely contain more
unsaturated fatty acids than do membranes of microbes living in hot springs?
Cell membranes must be fluid to function properly. At low temperatures, saturated fatty acids
CHAPTER 3 Cell Structure and Function
p. 58: The smallest free-living microbe – the bacterium Mycoplasma – is non–motile.
Why is it alive even though it cannot move?
The bacterium Mycoplasma has metabolic activity, grows, reproduces, and responds to the
p. 59: In 1985, an Israeli scientist discovered a single-celled microbe called Epulopiscium
fishelsoni. This organism is visible with the naked eye. Why did the scientist think
Epulopiscium was eukaryotic? What discovery revealed that the microbe is really a gi-
ant bacterium?
p. 65: Why is a pilus a type of fimbria but a flagellum is not a type of fimbria?
p. 67: Why is the microbe illustrated in Figure 3.2 more likely a Gram-positive bacte-
rium than a Gram-negative one?
p. 73: E. coli grown in a hypertonic solution turns on a gene to synthesize protein [sic]
that transports potassium into the cell. Why?
Instructor’s Manual for Microbiology with Diseases by Body System, 5e
p. 76: The 2001 bioterrorist attack in the U.S. involved Bacillus anthracis. Why is B. an-
thracis able to survive in the mail?
Bacillus anthracis is capable of forming endospores. Endospores are not metabolically active
p. 77: Why do scientists consider bacterial and archaeal flagella to be analogous rather
than evolutionary relations?
While both bacterial and archaeal flagella function similarly, they are composed of different
p. 78: Why did scientists of the 19th and early 20th centuries think that archaea were
bacteria?
p. 78: Why do some scientists consider archaea, which are prokaryotic, more closely re–
lated to eukaryotes than they are to the other prokaryotes – bacteria?
p. 79: Why are eukaryotic glycocalyces covalently bound to cytoplasmic membranes,
and why don’t eukaryotes with cell walls have glycocalyces?
p.80: Many antimicrobial drugs target bacterial cell walls. Why aren’t there many
drugs that act against bacterial cytoplasmic membranes?
p. 87: Colchicine is a drug that inhibits microtubule formation. Why does colchicine in–
hibit phagocytosis, movement of organelles within the cell, and formation of flagella and
cilia?
Answers to Tell Me Why Questions
CHAPTER 4 Microscopy, Staining, and Classification
p. 97: Why do scientists use metric units rather than English units?
p. 106: Why is magnification high but color absent in an unretouched electron micro-
graph?
p 112: Why is a Gram-negative bacterium colorless but a Gram-positive bacterium
purple after it is rinsed with decolorizer?
The ethanol and acetone mixture of the decolorizer disrupts the outer lipid layer and the thin
p. 119: Why didn’t Linnaeus create taxonomic groups for viruses?
CHAPTER 5 Microbial Metabolism
p. 132: How can oxidation take place in an anaerobic environment, that is, without
oxygen?
p. 142: Why do electrons carried by NADH allow for 50% more production of ATP
molecules than do electrons carried by FADH2?
p. 143: Why does catabolism of amino acids for energy result in ammonia and other
nitrogenous wastes?
Instructor’s Manual for Microbiology with Diseases by Body System, 5e
p. 149: An uninformed student describes the Calvin-Benson cycle as “cellular
respiration in reverse.” Why is the student incorrect?
The Calvin-Benson cycle is a cycle in which reactants are regenerated, but it does not share
p. 151: Why is nitrogen required for the production of amino acids by amination?
p. 152: Why is feedback inhibition necessary for controlling anabolic pathways?
Feedback inhibition prevents excessive production of biological molecules beyond what is
CHAPTER 6 Microbial Nutrition and Growth
p. 169: Why should cardiac nurses and respiratory therapists care about biofilms?
Biofilms on teeth (dental plaque) can result in cardiovascular system infections like
p. 177: Why do clinical laboratory scientists keep many different kinds of culture
medium on hand?
A large variety of microbes are capable of colonizing the body and causing disease. They
p. 185: Some students transfer some “gunk” from a two-week-old bacterial culture into
new media. Why shouldn’t they be surprised when this “death phase” sample grows?
The term death phase culture means the rate of cell death exceeds the rate of new cell
CHAPTER 7 Microbial Genetics
Answers to Tell Me Why Questions
p. 202: DNA replication requires a large amount of energy, but none of the cell’s ATP
energy supply is used. Why isn’t it?
p. 216: In bacteria, polypeptide translation can begin even before mRNA transcription
is complete. Why can’t this happen in eukaryotes?
In bacteria, transcription and translation are not compartmentalized, and mRNA is not
p. 223: Changes in RNA resulting from poor transcription of DNA to RNA are not as
deleterious to cells as changes to its DNA resulting from mutations. Why is this the
case?
There are typically large numbers of copies of the same RNA molecule present in the cell so a
single damaged RNA molecule will be unlikely to result in the production of a significant number
p. 229: Why is the genetic ancestry of microbes much more difficult to ascertain than
the ancestry of animals?
Animals only transmit genes vertically, that is, to their offspring. Many microbes, especially
CHAPTER 8 Recombinant DNA Technology
p. 238: Why aren’t the terms genetic engineering and biotechnology synonymous?
Biotechnology is the use of microorganisms to make useful products, including many
p. 242: Why did the discovery and development of restriction enzymes speed up the study of
recombinant DNA technology?
Instructor’s Manual for Microbiology with Diseases by Body System, 5e
p. 247: Why wasn’t polymerase chain reaction (PCR) practical before the discovery of
hyperthermophilic bacteria?
The temperature used for DNA denaturation far exceeds the temperature maximum of
p. 254: Why don’t doctors routinely insert genes into their patients to cure the common
cold, flu, or tuberculosis?
Gene therapy technology is primarily useful in correcting genetic disorders, not infectious
p. 255: Why don’t scientists who work with recombinant DNA know all the long–term effects of
their work?
CHAPTER 9 Controlling Microbial Growth in the Environment
p. 264: Why does milk eventually go “bad” despite being pasteurized?
Pasteurization does not kill heat-tolerant or heat-loving prokaryotes. The heat-tolerant
p. 266: Why are BSL-4 suits pressurized? Why not just wear tough regular suits?
In the event of an accident in which the suit is damaged, the pressurization reduces the
p. 273: Why are Bacillus endospores used as sterility indicators? (See Figure 9.8.)
The endospores of Bacillus bacteria are the living microbes most resistant to heat and other
p. 280: Many chemical disinfectants and antiseptics act by denaturing proteins. Why
does denaturation kill cells?
Answers to Tell Me Why Questions
CHAPTER 10 Controlling Microbial Growth in the Body:
Antimicrobial Drugs
p. 287: Why aren’t antibiotics effective against the common cold?
p. 296: Some antimicrobial drugs are harmful to humans. Why can physicians safely use
such drugs despite the potential danger?
Many pathogens have higher metabolic rates than their human hosts, so they are more
p. 300: Why don’t physicians invariably prescribe the antimicrobial with the largest
zone of inhibition?
The antimicrobial with the largest zone of inhibition may be too toxic for use in the patient, or
p. 302: Why is it incorrect to say a particular bacterium develops resistance in response
to an antibiotic?
CHAPTER 11 Characterizing and Classifying Prokaryotes
p. 326: Why does binary fission produce chains of cocci in some species but clusters of
cocci in other species?
In some species of cocci, binary fission always occurs in the same plane, producing daughter
p. 326: Why are taxonomic names and categories in our current taxonomic scheme
different from those in 1990?
The current taxonomic scheme is based primarily on degree of relatedness as reflected in the
Instructor’s Manual for Microbiology with Diseases by Body System, 5e
p. 329: Why is it that archaea don’t seem to cause disease of humans, plants, or
animals?
p. 345: Why are bacteria all in the same domain (Bacteria) despite their widely divergent
oxygen tolerance, sizes, shapes, and nutritional requirements?
CHAPTER 12 Characterizing and Classifying Eukaryotes
p. 355: Why is it incorrect to call mitosis “cell division”?
p. 361: Why did early taxonomists categorize such obviously different microorganisms
as parabasalids, diplomonads, euglenozoa, alveolates, rhizaria, and amoebozoa in a
single taxon, Protozoa?
These organisms are all single-celled eukaryotes without a cell wall, characteristics which
p. 370: Why isn’t a fungal dikaryon—with its two haploid (n) nuclei—considered a
diploid?
p. 373: Why aren’t there large numbers of pathogenic algae?
p. 374: Why are water molds more closely related to brown algae than to true molds?
p. 376: Why are large eukaryotes like mosquitoes and ticks considered in a
microbiology class?
CHAPTER 13 Characterizing and Classifying Viruses, Viroids, and
Prions
Answers to Tell Me Why Questions
p. 387: Why are naked icosahedral viruses able to crystalize?
p. 388: What characteristics of the genomes of parvoviruses and reoviruses make them
very different from cells?
p. 397: Why are lysogenic and latent viral infections generally longer lasting than lytic
infections?
p. 394: Why are DNA viruses more likely to cause neoplasias than are RNA viruses?
DNA virus genomes are more likely to become integrated into the host genome, and insertion
p. 401: HIV replicates only in certain types of human cells, and one early problem in
AIDS research was culturing those cells. Why are scientists now able to culture HIV?
p. 401: Why are viruses seemingly alive yet not alive?
Viruses seem alive because they have genomes and within host cells, they direct complex
p. 404: Why did scientists initially resist the idea of an infectious protein?
The concept of nucleic acid as the repository of heritable information is now thoroughly
CHAPTER 14 Infection, Infectious Diseases, and Epidemiology
p. 415: Why is absolute commensalism difficult to prove?
It is always difficult to prove a negative. Commensalism is defined as a relationship in which
Instructor’s Manual for Microbiology with Diseases by Body System, 5e
p. 417: Why might animal reservoirs be involved in more diseases than are human
reservoirs?
There are large numbers of animal species that might harbor pathogens capable of causing
p. 419: Why does every infection start with contamination but not every contamination
results in an infection?
Contamination refers to the presence of microbes on the body, without regard to whether they
p. 427: Why is mutated Streptococcus pneumoniae, which cannot make a capsule, una-
ble to cause pneumonia?
A capsule prevents phagocytes from engulfing and destroying bacteria. A mutated form of
p. 427: Why is the tube leaving the bladder (the urethra) more likely to be a portal of
exit than a portal of entry?
p. 429: Why can’t we correctly say that all arthropod vectors are reservoirs?
p. 430: Why is an acute disease with a high rate of mortality unlikely to be associated
with a pandemic?
p. 438: Why are all iatrogenic infections healthcare associated, but not all healthcare-
associated infections are iatrogenic?
CHAPTER 15 Innate Immunity
Answers to Tell Me Why Questions
p. 444: Why aren’t the body’s skin and mucous membrane barriers significant factors
in your resistance to infection with hyperthermophiles?
p. 448: Some strains of Staphylococcus aureus produce exfoliative toxin, a chemical that
causes portions of the entire outer layer of the skin to be sloughed off in a disease called
scalded skin syndrome. Given that cells of the outer layer are going to fall off anyway, why
is this disease dangerous?
p. 462: Why are pathogen-associated molecular patterns (PAMPs) necessary for Toll-
like receptors (TLRs) to fully function?
CHAPTER 16 Adaptive Immunity
p. 470: Why are the activities of B and T cells called adaptive?
B and T cell responses are called adaptive because these responses develop only on exposure to a
p. 485: Why are exogenous epitopes processed in vesicles instead of endoplasmic reticulum,
as endogenous epitopes are?
Exogenous epitopes are derived from extracellular antigens that have been endocytosed or
p. 484: Why did scientists give the name perforin to this molecule secreted by TC cells?
p. 487: Plasma cells are vital for protection against infection, but memory B cells are not.
Why not?
p. 488: Why is passive immunity effective more quickly than active immunity?
Instructor’s Manual for Microbiology with Diseases by Body System, 5e
response provides active immunity but requires many days to develop. Memory responses are
faster but still require a few days to produce sufficient antibody to be effective.
CHAPTER 17 Immunization and Immune Testing
p. 507: Vaccines have drastically reduced the number of cases of many diseases, such as
measles and whooping cough. Why should parents have their children vaccinated given that
there are so few cases?
p. 516: A diagnostician used an ELISA to show that a newborn had anti- bodies against HIV
in her blood. However, six months later, the same test was negative. How can this be?
During pregnancy IgG antibodies from the mother cross the placenta into the fetus. So at birth, a
child born to a HIV positive mother will show presence of IgG antibodies that had crossed the
placenta – from the mother into the fetus. The acquired immune system is not mature at birth and
CHAPTER 18 Immune Disorders
p. 534: During the war in Afghanistan in 2012, an army corporal with type AB blood re-
ceived a life-saving blood transfusion from his sergeant, who had type O blood. Later, the
sergeant was involved in a traumatic accident and needed blood desperately. The corporal
wanted to help but was told his blood was incompatible. Explain why the corporal could re-
ceive blood from but could not give blood to the sergeant.
The corporal has type AB blood, which means he has both A antigen and B antigen on his red
p. 536: Why can’t scientists use the postulates of Robert Koch to determine the specific
cause of Graves’ disease?
Koch’s postulates are useful where a causative organism for a disease has been identified and can
Answers to Tell Me Why Questions
p. 544: Why is it difficult for a medical treatment to rid the body’s cells of HIV?
CHAPTER 19 Microbial Diseases of the Skin and Wounds
p. 554: Why is the normal skin microbiome considered beneficial to a person?
See p 553: column 2: Microbiome of the skin
p. 568: Why do most cases of Rocky Mountain spotted fever occur in May, June, and July?
p. 581: Why has the number of cases of measles increased dramatically, especially in
Europe?
Answer: p 580, 1st column, 2nd paragraph
p. 586: Onychomycoses (on’i–k–m–k’ss) are nail infections that can be caused by several
species of fungi. Why are these mycoses so difficult to treat? Why is it generally necessary to
treat patients with antifungal agents for long periods of time?
CHAPTER 20 Microbial Diseases of the Nervous System and Eyes
p. 598: Why is it important that the cells forming the blood vessels of the brain and spinal
cord be tight against one another, forming a blood-brain barrier?
The blood-brain barrier prevents prevents most microbes and large molecules in the blood from
p. 610: Why is the incubation period of wound botulism more than twice as long as the
incubation period of foodborne botulism?
It is the toxins released from the bacteria Clostridium botulinum that cause the illness called
Instructor’s Manual for Microbiology with Diseases by Body System, 5e
p. 618: The word enterovirus literally means “intestine poison,” yet enteroviruses do not
cause intestinal diseases. Why these viruses are called enteroviruses?
p. 619: Why is penicillin ineffective in treating cryptococcal meningitis?
A: Causative agent for cryptococcal meningitis is Cryptococcus neoformans, a yeast (fungus).
Penicillin is an antibacterial. It is produced by fungus, but is itself not an antifungal drug.
p. 621: Why has the number of cases of primary amebic meningoencephalopathy increased
dramatically as societies have become more developed?
Because the causative agents, namely Acanthamoebae and Naegleria are found in artificial water
p. 622: Why is infectious CJD called variant:
Infectious CJD is called variant CJD to distinguish it from the genetic disorder described by
p. 624: Doxycycline—one of the tetracyclines—is the treatment for most adults infected with
Chlamydia trachomatis; however, it is not recommended for pregnant women or babies.
Why not?
Because it gets deposited in teeth and bone. The deposits stain teeth permanently. Patients are at
CHAPTER 21 Microbial Cardiovascular and Systemic Diseases
p. 630: Bacteria infecting the mouth can enter the blood through small cuts resulting from
normal brushing and flossing. Why would these bacteria reach the right atrioventricular
valve before the other heart valves?
p. 644: Health departments recommend that outdoor enthusiasts wear light-colored pants
while hiking in areas where Ixodes ticks are endemic. Why?
p. 654: Whereas many doctors are convinced that Epstein-Barr virus causes chronic fatigue
syndrome, others deny the association between EBV and the syndrome. Why is the etiology
of chronic fatigue syndrome debated even though Epstein-Barr virus is present in patients?
Answers to Tell Me Why Questions
p. 663: Why can people who avoid cats get infected with Toxoplasma anyway?
CHAPTER 22 Microbial Diseases of the Respiratory System
p. 674: Why do patients with methicillin-resistant Staphylococcus aureus (MRSA) as part of
their normal nasal microbiota pose a risk to other patients in a hospital?
Nasal secretions containing MRSA can easily contaminate both equipment and personnel in the
p. 678: Why must diphtheria immunization be boosted every 10 years?
The diphtheria vaccine is an inactivated toxin extract with few antigenic determinants. Few
antigenic determinants do not generate lasting immunity.
p. 680: Why is it inappropriate to treat a cold with penicillin, erythromycin, or
ciprofloxacin?
The common cold is caused by viruses. These antibiotics interfere with metabolic processes of
p. 690: Mycoplasma pneumoniae is resistant to penicillin, though Mycoplasma does not
synthesize an enzyme to break down penicillin. Explain why Mycoplasma is resistant to
penicillin.
p. 697: Why do epidemiologists think there will be a major flu pandemic in people caused by
bird influenza viruses?
Recent type A bird influenzas carry antigens similar to those of influenzas responsible for global
p. 703: Outbreaks of blastomycoses have occurred in Latin America even though the
organism itself is not normally found there. Why might a few cases of blastomycosis appear
outside of endemic areas?
Fungal spores are hardy and lightweight enough to become suspended in the air on dust particles.
CHAPTER 23 Microbial Diseases of the Digestive System
p. 711: Why is the digestive system an important portal of entry for microorganisms?
Instructor’s Manual for Microbiology with Diseases by Body System, 5e
p. 711: Why does the use of antibacterial drugs over an extended period of time increase the
likelihood of oral candidiasis (thrush) and so-called C. diff diarrhea?
The bacteria of the microbiota limit the populations of potentially harmful inhabitants of the
p. 725: Why is the elimination of sucrose sugar from the diet not enough to prevent the
formation of all dental caries?
Eliminating sucrose sugar from the diet will reduce the rate of dextran and acid production, but
other foods can contribute to the conditions that promote the formation of plaque and damage to
p. 737: In areas of poor sanitation, which form of hepatitis would you expect to be most
common—infectious hepatitis, serum hepatitis, or chronic hepatitis? Why?
p. 742: Why does the visually distinctive appearance of Giardia trophozoites improve the
success of medical treatment of giardiasis compared to that for amebic infections?
A moderate level of skill with the microscope is sufficient for a technician to be able to recognize
Giardia trophozoites in fecal samples, and living trophozoites are not required for recognition.
p. 746: Tapeworm infestation of an intermediate host requires consumption of eggs in
fecally contaminated food. Why is it possible for humans become accidental intermediate
hosts for T. solium?
CHAPTER 24 Microbial Diseases of the Urinary and Reproductive
Systems
Answers to Tell Me Why Questions
p. 750 Why are newborn girls less likely to contract vaginal infections than are three-year–
olds?
p. 751 Why does insertion of a urinary catheter increase the likelihood of cystitis?
p. 757 Why does Candida albicans, which is a member of the normal microbiota, sometimes
cause disease?
p. 760 Why have STIs and STDs become pandemic over the last 50 years?
STIs and STDs have become pandemic for many reasons. The segment of the population under 25
p. 766 Why is erythromycin substituted for tetracycline in treatment of chlamydial
infections in children? Why are penicillins and cephalosporins useless against Chlamydia?
Children are not given tetracycline because it disrupts proper calcification of bones and teeth.
p. 769 Why are DNA viruses like herpesviruses and papillomaviruses more likely to cause
recurrent diseases and cancers than are RNA viruses?
DNA viruses typically enter the nucleus of a cell to replicate, and some, including herpesviruses
p. 770 Why might a course of antibacterial drugs trigger a case of trichomoniasis in a female
patient?
Treatment with antibacterial drugs may reduce the bacterial members of the vaginal microbiota,
Instructor’s Manual for Microbiology with Diseases by Body System, 5e
CHAPTER 25 Applied and Industrial Microbiology
p. 791: Even though microbes are naturally present in raw milk, raw milk is not generally
used for the production of cheeses. Why not?
Raw milk contains a variety of microorganisms, some of which may ferment the milk to produce
p. 801: In a polluted lake, the microbes reproduced prolifically, died, and then sank to the
bottom, “feeding” the anaerobes in the sediment. Even though the surface water looks clear,
why is it still unsafe to drink the lake water?
CHAPTER 26 Microbial Ecology and Microbiomes
p. 810: A blogger stated that microorganisms are dangerous and should be avoided in all
cases. Why is this idea wrong?
The vast majority of microbes are harmless or beneficial, only a small percent are dangerous to
humans. The idea is totally impractical as well because we are surrounded by and inhabited by
p. 813: Compare human, animal, and plant pathogens that could be used as biological
agents in terms of environmental survivability. Why are animal and plant pathogens more
common in environmental reservoirs than are human pathogens?
The pathogen with the greatest level of environmental survivability is anthrax: The endospores of
the animal pathogen Bacillus anthracis are one of the most resistant biological entities known. The
endospores of Clostridium botulinum are equally hardy, and the toxin is heat resistant, but unlike