Betsy T., Keogh J. Microbiology Demystified

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BLOOD AND TISSUE PROTOZOA

Hemoflagellates are protozoa that are carried by blood-feeding insects and are

transmitted into the blood stream by the insect’s bite. Here are commonly found

hemoflagellates:

•

Trypanosoma gambiense: T. gambiense is transmitted in the saliva of the

tsetse fly as a result of a bite and causes trypanosomiasis, which is better

known as African sleeping sickness.

•

Trypanosoma cruzi: T. cruzi is carried by the Reduviid bug or “kissing

bug,” which is commonly called the kissing bug because it bites the face. T.

cruzi causes Chagas’ disease. Chagas’ disease is thought to have made

Charles Darwin sick during his voyage on the H.M.S. Beagle. Typically

there aren’t any symptoms for months after the bite. During this time T.

cruzi spreads through organs of the body, weakening the heart, intestines,

and esophageal. It also cause, both eyes to swell (Romaña’s sign).

CILIATES

Ciliates (Fig. 11-3) are protozoa that have shorter hair-like structures called

cilia that are found in rows on the outer surface of the cell. These cilia are used

to move the protozoa through the environment and are used to bring food into

the cell.

An example of a ciliate is Balantidium coli. It is the only ciliate that causes

disease in

humans. When ingested, it enters the large intestine, causing severe

dysentery.

Fig. 11-3. Ciliates are protozoa that have cilia.

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APICOMPLEXANS

Apicomplexans are protozoa that live and grow inside another living organism

(obligate intracellular parasites) and cannot move by themselves (nonmotile).

Apicomplexans have an apical complex of organelles that form an apex (tip).

This tip contains enzymes that enable it to penetrate the tissues of a host.

Here are common Apicomplexans:

•

Plasmodium: Plasmodium lives in the female Anopheles mosquito and

causes malaria when the mosquito bites a human. Symptoms of malaria

include severe chills and fever or “rigor” (a sudden chill or coldness that is

followed by fever).

•

Bebesia microti: B. microti lives in ticks and causes bebesiosis when the

tick bites someone. B. microti then enters red blood cells where it multi-

plies quickly. At first there aren’t any symptoms (asymptomatic). However,

soon afterwards there is a high fever, headache, and muscle pain as B.

microti destroys red blood cells. This causes the person to become anemic

(insufficient hemoglobin because of the reduction in the number of red

blood cells) and show signs of jaundice (an increase in bile, causing the

skin and eye sclera—the white part of the eye—to yellow).

•

Toxoplasma gondii, also known as T. gondii, lives in cat feces and raw

meat and causes lymphadenitis (infection of the lymph nodes), which can

have a devastating effect on people who are immunocompromised, such as

AIDS patients. T. gondii causes congenital infections in a fetus because it

can pass from the mother to the fetus through the placenta.

Helminths

Helminths are parasitic, multicellular eukaryotic animals. The majority of these

animals belong to the phyla Platyhelminthes and Nematoda. There are free-

living members of these phyla; however, in this section, only the parasitic organ-

isms are discussed.

Many parasitic helminths do not have a digestive system and instead absorb

nutrients from the food that is consumed by their host organism, the host’s body

fluids, and the host’s tissues. Parasitic helminths have a very simplistic nervous

system because they have to respond to very few changes in their host’s environ-

ment. They lack or have reduced means of locomotion because they are transferred

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from one host to another. Parasitic helminths have a very complex reproductive

system that produces fertilized eggs (zygotes) that infect the host organism.

LIFE CYCLE OF HELMINTHS

The life cycle of parasitic helminths that go through a developmental larval stage

involves an intermediate host.

Dioecious adult helminths are of one sex. That is, one individual has a male

reproductive system and another has a female reproductive system. When these

two adult helminths with different sex organs occupy the same host organism,

sexual reproduction can occur. Monoecious adult helminths are hermaphroditic

(an organism that has both female and male reproductive organs). Some of

these monoecious helminths can fertilize themselves while others may fertilize

each other.

FLATWORMS

Flatworms, also known as platyhelminths, are mostly parasitic, aquatic organ-

isms that range in size from 1 millimeter to 10 meters, as in the case of a tape-

worm. There are more than 15,000 known species of flatworms. A flatworm has

both male and female reproductive parts (monoecious). Most but not all of their

oxygen and nutrients is absorbed through their body wall.

There are two types of flatworms:

•

Flukes: Flukes are flat, leaf-shaped bodies that have an oral and a ventral

sucker that are used to hang on to the body of a host. Flukes live is inside

the intestines or on tissues of humans. Three common flukes are:

• Schistosoma: This genus of flukes causes the disease schistosomiasis, a

debilitating disease that causes portal hypertension and liver cirrhosis.

• Paragonimus westermani: P. westermani causes paragonimiasis, which

is the result of the fluke’s depositing eggs into the bronchi of the lung.

• Clonorchis sinensis: C. sinensis, also known as Chinese liver fluke,

causes clonorchiasis, which occurs when the fluke latches inside the

liver.

•

Tapeworms: Tapeworms have a knob-like “head,” called a scolex, with

hooks that allow it to attach to the wall of the intestine of vertebrate ani-

mals (including humans). Tapeworms have a series of flat, rectangular

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body units called proglottids (compartments that contain reproductive

organs). Proglottids eventually break away from the tapeworm and are

excreted in feces. However, new proglottids take their place. A tapeworm

continues to grow as long as its scolex and neck are intact.

•Taenia saginata: T. saginata, also called beef tapeworm, lives in raw or

poorly cooked beef and can cause taeniasis. T. saginata can grow to a

length of 25 meters in the intestines of a human, leading to an intestinal

blockage and malnutrition as the tapeworm absorbs nutrients intended

for the person.

•Tania solium: T. solium, also called the pork tapeworm, lives in raw or

poorly cooked pork and can cause taeniasis. T. solium can grow to a

length of 7 meters in the intestines leading to an intestinal blockage and

malnutrition.

• Echinococcus granulosus: E. granulosus is a tapeworm that is spread to

humans through contact with an infected dog and is transmitted when a

dog licks a person. This can lead to infection, anaphylactic shock, and

death if the tapeworm enters the body. E. granulosus can lay eggs that

produce cysts called hydatid cysts in the lungs, liver, and brain.

• Hymenolepis nana: H. nana is a tapeworm that lays eggs in cereals and

foods that are contaminated with infected parts of insects. When some-

one ingests the cereal or food he or she also ingests the tapeworm. The

tapeworm then attaches to the intestines, leading to diarrhea, abdominal

pain, and convulsions.

• Diphyllobothrium latum: D. latum is a broad fish tapeworm that lives in

raw or poorly cooked fish. The tapeworm attaches to the intestines of

the fish where it then lays eggs. While attached, the tapeworm absorbs

large quantities of vitamin B

from the intestine eventually causing the

person to develop vitamin deficiency anemia. This is also, called perni-

cious anemia because there is insufficient vitamin B

to make red

blood cells.

ROUNDWORMS (NEMATODES)

Roundworms are also known as nematodes and live in soil, fresh water, and salt-

water. Most of the over 80,000 species of roundworms are parasites and live in

plants or animals such as insects. They have a primitive body that consists of a

cylindrical tube that has tapered ends and is covered with a thick protective layer

called a cuticle.

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Common roundworms are:

•

Ascaris lumbrieoides: A. lumbrieoides is a roundworm that is transmitted

by contaminated human fertilizer, food, or water. It causes ascariasis,

which is an infection of the small intestine.

•

Strongyloides stercoralis: S. stercoralis is a roundworm whose larvae pen-

etrates human skin and spread into the small intestine where it causes

strongyloidiasis, which is an infection of the small intestine.

•

Trichinella spiralis: T. spiralis is a roundworm whose larvae cause trichi-

nosis and live in undercooked meats, mainly pork. These juvenile worms

that are in the ingested meat mature in the small intestines of the host organ-

ism. The mature females burrow through the wall of the small intestines

and release their offspring (juveniles) into the blood of the host, where skele-

tal muscle is soon infected. It is these juveniles that burrow into the skeletal

muscle of the host. The larvae travel to the muscle where they form into a

sack (encyst), causing muscle pain and fever; this results in a large number

of eosinophilic leukocytes (eosinophilia). An eosinophilic leukocyte is a type

of white blood cell that increases with allergies and infections.

•

Wuchereria bancrofti: W. bancrofti is a roundworm that lives in mosqui-

toes and causes elephantiasis when the infected mosquito bites a human.

The mosquito injects the larvae into the skin where they then migrate to the

lymph nodes, causing blockages.

•

Onchocerca volvulus: O. volvulus is a roundworm that lives in the black

fly and causes river blindness when the black fly bites a human.

•

Dracunculus medinensis: D. medinensis is a roundworm that lives in lob-

sters, crabs, shrimps, and other crustacea. When the infected crustacea is

ingested, this roundworm’s larvae migrate from the person’s intestines

through the abdominal cavity to subcutaneous tissue where they mature.

D. medinensis releases a toxic substance that creates a skin ulcer, which is

the symptom of dracunculosis disease.

•

Hookworms are roundworms that have tiny hooks that are used to attach

it to a host, which is typically the intestine. Here are some common hook-

worms:

•

Necator americanus: N. americanus, also known as the New World

hookworm, lives in the lower intestine and is the second most common

hookworm infection. Its eggs are passed into the feces. Once it comes

into contact with a human, it penetrates the skin and spreads into the

heart, lungs, and eventually the small intestine where it grows into an

adult. This can lead to severe blood loss and anemia.

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•

Ancylostoma duodenale: A. duodenale, also known as the Old World

hookworm, is similar to N. americanus, but is native to Southern Europe,

North Africa, Northern Asia, and parts of western South America.

•

Ancylostoma braziliense: A. braziliense is a hookworm that exists in

cats and dogs and causes cutaneous larva migrans, which is also known

as creeping eruption. Its eggs are passed in feces and the larvae develop

in the soil. The larvae can tunnel into the epidermis of humans and can

cause an infection to develop.

•

Ancylostoma caninum: A. caninum is a hookworm that exists in dogs

and frequently infects puppies. It eats away at the tissues in the small

intestine and sucks blood from the dog. This can result in diarrhea,

weight loss, anemia, and death. The larvae can tunnel into the epidermis

of humans and can cause an infection to develop.

PINWORMS

Pinworms can be up to 10 millimeters long and live in the large intestine.

Female pinworms crawl out the anus to lay eggs on the perianal skin.

Afterwards, she dies. Pinworms infect about 10 percent of humans, although

the person may not know he or she is infected: Pinworms cause few or no

symptoms besides a mild gastrointestinal upset and perianal itching, which can

lead to bacterial infections. Pinworms are highly contagious and can be trans-

mitted in bed linens and clothing that has been contaminated with eggs. A com-

mon pinworm is Enterobius vermicularis. Also known as E. vermicularis, it

is the most common pinworm in the United States. E. vermicularis causes

enterobiasis, in which the skin around the anus is so itchy that a person might

not be able to sleep.

QUIZ

1. Helminths are

(a) fungi

(b) algae

(b) worms

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2. Fungi are different from plants because

(a) fungi have chlorophyll

(b) plants have chlorophyll

(d) plants absorb nutrients from organic wastes

3. Mushrooms are

(a) fungi

(b) protozoa

(d) algae

4. The body of a mold or fleshy fungus is made up of long, loosely packed

filaments called

(a) soma

(b) hyphae

(d) mycelium

5. Fungi imperfecti can reproduce

(a) sexually

(b) do not reproduce

(d) both (a) and (b)

6. These are unicellular algae that have a hard, double outer shell made of

silica

(a) diatoms

(b) chrysophytes

(d) phaeophyta

7. What are hemoflagellates?

(a) Protists

(b) Nematodes

(d) Proglottids

8. What are platyhelminths?

(a) Ringworms

(b) Nematodes

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(d) Hookworms

9. An example of a fluke would be

(a) H. ana

(b) D. latum

(d) E. vermicularis

10. Some protists use cilia to move food into a mouth-like opening called

(a) pseudopods

(b) a cytosome

(d) exocytosis

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CHAPTER

185

Viruses, Viroids,

and Prions

In this chapter, you will learn about viruses, how they work, and the diseases

they cause. You will also learn about viroids and prions. A viroid is a small

“naked” infectious RNA molecule that has similar properties of a virus. Prions

are particles of proteins and cause infections.

Viruses

In 1889, Dutch plant microbiologist Martinus Beijerinck described the concept

of viruses through his studies of Tobacco Mosaic Disease. Nobel prize–winner

Sir Peter Medawar described how microbiologists feel about viruses when he

said, “A virus is a piece of bad news wrapped in a protein.”

Viruses are strands of nucleic acids that are encased within a protein coat,

making them difficult to destroy. A microorganism needs both DNA and RNA in

order to reproduce. A virus cannot express genes without a host, because a virus

has either DNA or RNA but not both.

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A virus (Latin for “poison”) is an obligate intracellular parasite that can only

replicate inside a living host cell. Once inside the living host cell, a virus

becomes integrated in the metabolism of its host, making a virus difficult to con-

trol by chemical means. You cannot kill a virus with antibiotics. Drugs that

destroy the host’s ability to be used by a virus for replication tend to also be

highly toxic and have a negatively and sometimes deadly effect on the host cell.

Before a virus enters a cell, it is a free virus particle called a virion. A virion

cannot grow or carry out any biosynthetic or biochemical function because it is

metabolically inert. Viruses are not cells. They vary in size from 20 nanometers

(polio virus) to 300 nanometers (smallpox virus) and cannot be seen under a

light microscope.

In 1933, microbiologist Wendell Stanley of the Rockefeller Institute for Medical

Research showed that viruses could be regarded as chemical matter rather than

as living organisms.

VIRAL STRUCTURE

The major components of a virus are:

•

Nucleic acid core. The nucleic acid core can either be DNA or RNA that

makes up the genetic information (genome) of the virus. RNA genomes

only occur in viruses.

•

Capsid. A capsid is the protein coat that encapsulates a virus and protects

the nucleic acid from the environment. It also plays a role in how some

viruses attach to a host cell. A capsid consists of one or more proteins that

are unique to the virus and determine the shape of the virus.

•

Envelope. An envelope is a membrane bilayer that some viruses have out-

side their capsid. If a virus does not have an envelope, the virus is called a

naked virus. Examples of diseases caused by naked viruses are chickenpox,

shingles, mononucleosis, and herpes simplex. A naked virus is more resist-

ant to changes and is less likely to be affected by conditions that can dam-

age the envelope. Environmental factors that can damage the envelope are:

• Increased temperature

• Freezing temperature

• pH below 6 or above 8

• Lipid solvents

• Some chemical disinfectants (chlorine, hydrogen peroxide, and phenol)

Naked viruses are more resistant to changes in temperature and pH. Examples

of diseases caused by naked viruses include poliomyelitis, warts, and the com-

mon cold.

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