Chapter 7: QUICK-CHECK answers
Chapter 7
Answers QUICK-CHECK questions
1 Do all pathogens that enter an organism cause disease?
No. If a pathogen that is capable of causing a particular disease enters an organism but does not cause damage to the host cells of that organism then, in that case, the micro-organism concerned is not acting as a pathogen.
2 What are spongiform encephalopathies?
Spongiform encephalopathies are degenerative diseases of the brain. The name derives from the fact that the diseases destroy brain cells and the individual is left with spaces or ‘holes’ in the brain tissue rather like a sponge.
3 What techniques are used to examine viruses?
Because of their extremely small sizes, viruses are typically examined using an electron microscope.
4 Describe the general structure of a virus. In what ways do the structures of different viruses differ from each other?
A virus has one or more outer layers of protein and various viruses have different kinds of proteins in this outer coat and different active sites. Inside the protein coat is nucleic acid, the genetic material. Some viruses have DNA as their genetic material while other kinds of viruses, known as retroviruses, have RNA as their genetic material.
5 List the steps involved in the reproduction of a virus.
The sequence of reproduction in a virus is:
· The virus attaches to the outer membrane of a host cell.
· The nucleic acid of the virus is injected into the host cell.
· Viral nucleic acid uses material within the host cell to produce viral components.
· Viral parts are assembled.
· The host cell lyses, releasing new viral particles.
6 Name three virus disorders.
The many viral disorders include:
· German measles, caused by the rubella virus
· influenza, caused by the influenza virus
· poliomyelitis, also called polio or infantile paralysis, caused by the polio virus.
(Note: Many other viral diseases exist in humans and other organisms, and reports appear in the news from time to time. These include the human immunodeficiency virus (HIV), which causes AIDS; Hendra virus, routinely harboured by fruit bats, but which occasionally is transmitted to horses and, from infected horses, to their human caretakers; rabbit calicivirus; Epstein-Barr virus, which causes glandular fever; and rhinoviruses, which cause the common cold in humans.)
7 Explain the difference between acute and chronic infections.
An acute infection is one in which the agent responsible for the infection reproduces rapidly, resulting in the rapid destruction of host cells with the likely outcome being the death of the host.
A chronic infection is one in which the infective agent reproduces slowly without the host organism being killed by the infection.
8 Explain why it has been difficult to develop drugs against viruses.
Viruses cause damage inside cells. Drugs that are capable of destroying virus particles would also destroy the cells that the viruses have invaded.
(Note: Recent advances include the development of drugs that prevent entry of virus particles into cells.)
9 What are the differences between the structure of a virus, a viroid and a prion?
Prions are abnormal protein molecules that cause degenerative neurological diseases.
Viroids are short pieces of naked RNA, thought to act as pathogens in plants.
Viruses comprise a protein coat containing a nucleic acid: DNA in some viruses but RNA in others.
10 What are the three basic shapes of bacteria and what is the general name given to each?
Coccus — round-shaped bacteria
Bacillus — rod-shaped bacteria
Spirochaete — spiral-shaped bacteria
11 Name two different ways in which bacteria cluster.
Bacteria may gather in various ways including:
· formation of grape-like clusters
· linear formation or line of cells
· arrangement in pairs.
12 What are flagella and what would their presence on a bacterium indicate?
Flagella are thin, whip-like appendages that originate just below a bacterial wall. Their presence on a bacterial cell indicates that the bacterium is able to move (swim), particularly in a fluid environment.
13 Name two specialised structures found in some bacteria.
Specialised structures found in some bacteria include:
· flagella — thin, whip-like appendages that enable a bacterium to move
· spore — special reproductive structure resistant to heat and drying out
· capsule — a layer of slimy gelatinous material outside the cell wall of a bacterium.
14 Why do microbiologists routinely use the Gram stain to classify bacteria?
Use of the Gram stain allows classification of bacteria into two groups that vary in their susceptibility to certain drugs.
Gram-positive bacteria take up the violet stain. Gram-negative bacteria appear pink. The difference in staining in the two groups relates to differences in wall structure.
15 What name is given to bacteria that grow in the presence of oxygen? In the absence of oxygen?
Aerobic bacteria (aerobes) grow in the presence of oxygen.
Anaerobic bacteria can live in the absence of oxygen.
16 How would you classify bacteria that use light as an energy source?
Bacteria that use light as their energy source can be termed photosynthetic.
17 How would you classify bacteria that oxidise ammonia as an energy source?
Bacteria that oxidise ammonia as their energy source can be termed chemosynthetic. These bacteria oxidise inorganic compounds to provide their energy for living.
18 How would you classify bacteria that are pathogenic to humans on the basis of their energy and carbon source?
Human pathogenic bacteria are chemosynthetic; they use energy released from the oxidation of organic compounds. These pathogens also use organic compounds as their carbon source.
19 What is meant by the virulence of a bacterium?
The virulence of an organism is the degree to which it is capable of causing a disease.
20 Describe three different ways in which bacteria can be transmitted from host to host. What measure would prevent such transfer in each case?
Bacteria can be transmitted between hosts by:
· contamination of food: An infected person who is careless in handling food when cooking may transmit bacteria to the consumer of the food. This can be prevented by washing hands thoroughly and using a face mask.
· contamination through air: An infected person sneezing or coughing can transmit bacteria carried in fine air-borne droplets. This can be prevented by avoiding association with infected people.
· contamination of water: Water of unknown quality may be contaminated by human or other animal material carrying bacteria, such as faeces. This can be prevented by providing your own drinking water or using a filter with water of uncertain quality.
21 Explain what is meant when a person is a ‘carrier’ of an infectious disease.
A ‘carrier’ is a person who carries the infective agent responsible for an infectious disease but does not have the symptoms of the disease that normally result from the infective agent.
22 Why is it important to have laws relating to the preparation and handling of food?
Laws relating to the preparation and handling of food are necessary to ensure that the incidence of diseases that can be transmitted via food is as low as possible.
23 Identify six ways in which an infective agent can be transmitted from one host to another.
Infective agents may be transmitted by:
· physical contact, such as kissing or shaking hands with another person
· eating contaminated food prepared by a cook with a particular disease
· eating food that has been contaminated by flies or other organisms
· being in a situation where a person sneezes without attempting to prevent droplet spread
· engaging in sexual intercourse with a person infected with a sexually transmitted disease
· sharing needles for intravenous drug use.
24 List one difference between endotoxins and exotoxins.
Exotoxins are released into the environment by bacteria as they grow.
Endotoxins are released only after the bacterial cell breaks open.
25 List two ways in which exotoxins interfere with normal metabolism of their host’s cells.
The exotoxin in cholera alters the permeability of intestinal cells, one effect being the production of very watery diaorrhea.
The exotoxin in diphtheria prevents protein synthesis.
26 What is chemotherapy?
Chemotherapy is the use of chemicals in the treatment of a disease.
27 What is an antibiotic?
Antibiotics are naturally occurring compounds, extracted from fungi or bacteria, used against some disease-causing microorganisms.
28 What is the difference between broad-spectrum and narrow-spectrum antibiotics?
Broad-spectrum antibiotics act against a wide range of disease-causing organisms. Narrow-spectrum antibiotics act against a limited variety.
29 Why is penicillin effective against Gram-positive bacteria in particular?
The effectiveness of penicillin against Gram-positive bacteria is due to the fact that these bacteria lack an outer lipid layer such as that found in Gram-negative bacteria. The lipid layer present in the wall of Gram-negative bacteria enables them to resist action by penicillin.
30 If tests show two different antibiotics are equally effective against a particular bacterial species, why might a doctor prefer one antibiotic over the other for a patient?
Reasons for preferring one antibiotic over another include:
· The action of the antibiotic may not combine well with the action of another crucial antibiotic the patient may already be taking for a chronic condition.
· The side effects of the preferred drug may be less severe.
· The patient may be allergic to one of the drugs.
31 Name two diseases caused by protozoans. How are they transmitted from host to host?
Malaria is caused by the protozoan Plasmodium falciparum.
Giardiasis is caused by the protozoan Giardia lamblia.
32 If the Anopheles mosquito could be eliminated, explain the impact this might have on the incidence of malaria.
Provided no other vector replaced the role of the Anopheles mosquito, the elimination of Anopheles would eliminate malaria. This is because Anopheles transmits the gametocyte stage of Plasmodium falciparum between hosts when it bites an uninfected human after it bites a human with malaria.
33 Explain why most fungal infections are not serious infections.
Most fungi obtain their nutrients from decomposition of dead organic matter. Most cause superficial diseases on skin, nails or hair and are not generally serious.
34 Describe two common fungal diseases.
Ringworm, caused by the fungus Microsporum canis, is characterised by the development of circular scaly lesions on the body or scalp. Hair loss may be a characteristic if disease is on the scalp.
Tinea of the groin, also called ‘jock itch’, is caused by the fungus Epidermophyton floccosum and is characterised by red, scaly and itchy lesions.
35 What is an ectoparasite? Give three examples.
An ectoparasite lives on the outside of its host. Examples include:
· Trichophyton rubrum, a fungus that causes tinea of the nails, characterised by thickened and discoloured nails
· Phytophthora infestans, a plant pathogenic fungus that causes late blight of potatoes. Over one million Irish died in the 1840s when late blight destroyed the potato crop, because potatoes were the staple diet of the population.
· Pediculus human capitis, a small Arthropod animal called the head louse. It infests human hair and is also called ‘nits’.
36 What factors lead to the spread of threadworm from one host to others?
Threadworm, also called pinworm, is the worm Enterobius vermicularis. Although these worms live in the large intestine of some children, females move outside the anus to lay their eggs. Material on the eggs irritates the skin around the anus and causes itching. If a child scratches this area, eggs readily transfer to the child’s hand and fingers. This child may then transfer some of these eggs to other children either directly by touching or indirectly by transfer to objects or material that other children later handle.
37 What is meant by the phrase ‘a human becomes the intermediate host instead of in the life cycle of Echinococcus granulosis’?
There are two distinct stages in the life cycle of the hydatid tapeworm, Echinococcus granulosis: a maturing worm stage and a hydatid cyst stage. These stages take place in different animals, one in the primary host and the one in the intermediate host. The primary host is generally a dog, and the intermediate host is generally a sheep. If a human handles a dog carrying adult worms and accidentally ingests some of the mature eggs, hydatid cysts are likely to form. In such a case, the human becomes the intermediate host in the life cycle of the worm.
38 Name one STD caused by:
a a bacterium
Treponema pallidum, a bacterium, causes the infectious disease syphilis, also known as the pox. It causes lesions of the skin and mucous membranes.
b a virus
Herpes simplex virus type 2 causes genital herpes. This is one of the most common STDs in developed countries and produces blister-like lesions and ulceration.
c a fungus
Candida albicans, a fungus, causes vaginal thrush, also called candidiasis. It results in a thick, cream-coloured vaginal discharge.
d protozoan.
Trichomonas vaginalis, a protozoan, causes trichomoniasis, characterised by a smelly discharge and itching of the vaginal area.
39 Identify two measures that are taken to prevent entry of pathogens into Australia.
Measures to prevent entry of pathogens into Australia include:
· All legal ports of entry to Australia are manned and monitored by officials of AQIS (Australian Quarantine and Inspection Service). These men and women, assisted by specially trained dogs, check incoming passengers and their luggage for evidence of food, wood or other goods that may carry pests capable of being pathogens of Australian flora and fauna.
· Entry of animals into Australia must meet strict quarantine regulations. These include consideration of the previous location of an animal and the diseases prevalent in those countries; whether the animal has had appropriate vaccinations; and the potential impact on our native flora and fauna if the animal is infected.
40 Identify two measures that are taken to prevent the spread of pathogens within Australia.
Measures to prevent the spread of pathogens within Australia include:
· humans and animals being given appropriate vaccinations to prevent the spread of diseases
· presence of checkpoints at some state borders to prevent entry of produce that may be infected by a parasite that is absent from that state.
© John Wiley & Sons Australia, Ltd 2