Microbiology: Principles and Explorations

Sixth Edition

Chapter 11:Eukaryotic Microorganisms

and Parasites

Copyright © 2005 by John Wiley & Sons, Inc.

Jacquelyn G. Black

Principles of Parasitology

• Parasite: an organism that lives at the expense of another organism (host)

• Parasites that cause disease are called pathogens

• Parasitology is the study of parasites

• Historically, in the development of the science of biology, parasitology came to refer to the study of protozoa, helminths, and arthropods that live at the expense of other organisms

Parasites in Relation to Their Hosts

• Ectoparasites live on the surface of other organisms (e.g. ticks and lice)

• Endoparasites live within the bodies of other organisms (e.g. protozoa and worms)

• Most parasites are

1. obligate parasites: must spend at least some of their life cycle in or on a host

2. facultative parasites: normally are free-living, but can obtain nutrients from a host

• Parasites are also categorized acording to the duration of their association with their hosts:

1. Permanent parasites (tapeworms): remain in or on a host once they have invaded it

2. Temporary parasites (biting insects): feed on and then leave their hosts

3. Accidental parasites (ticks): invade an organism other than their normal host

4. Hyperparasitism (malaria): refers to a parasite itself having parasites

• Vector: agents of transmission, of many human parasitic diseases

• An organism that transfers a parasite to a new host is a vector

• Biological vector: A vector in which the parasite goes through part of its life cycle (malaria mosquito is both a host and a biological vector)

• Mechanical vector: A vector in which the parasite does not go through any part of its life cycle during transit (flies that carry parasite eggs, bacteria, or viruses from feces to food)

Hosts• Definitive hosts: harbor a parasite while it reproduces

sexually

• Intermediate hosts: harbor the parasite during some other developmental stages

• Mosquito is the definitive host for the malaria parasite because that parasite reproduces sexually in the mosquito; the human is an intermediate host

• Reservoir hosts are infected organisms that make parasites available for transmission to other hosts

Many parasites have one or more of the following mechanisms for evading host defense mechanisms:

1. Encystment

2. Changing the parasite’s surface antigens

3. Causing the host’s immune system to make antibodies that cannot react with the parasite’s antigens

4. Invading host cells, where the parasites are out of reach of host defense mechanisms

Characteristics of Protists

• Members of the kingdom Protista

• Diverse assortment of organisms

• Unicellular, eukaryotic organisms

• True nuclei and membrane-enclosed organelles

• Microscopic and vary in diameter from (5um – 5mm)

Certain species of Gonyaulax, Pfiesteria piscicida and some other dinoflagellates produce toxins. When these marine organisms appear seasonally in large numbers, they cause a bloom known as a red tide

Representative algae, or plantlike protists: Euglena, a euglenoid

Usually have a single flagellum and a pigmented eyespot called a stigma

Representative algae, or plantlike protists: The diatom Campylodiscus hibernicus

These protists usually have cell walls surrounded by a loosely attached test that contains silicon or calcium carbonate. Most reproduce by binary fission. Some lack flagella.

Representative algae, or plantlike protists: Gonyaulax, a dinoflagellate that causes red tides

Usually have two flagella, one extending behind the organisms like a tail, and other lying in a transverse groove. Some have a theca which contains cellulose.

The Funguslike Protists

• Also called water molds (Oomycota) and slime molds (saprophytes)

• Have some characteristics of fungi and some of animals

• Water molds, mildews and plant blights produce flagellated spores, called zoospores

• Slime molds are commonly found as glistening, viscous masses of slime on rotting logs; they also live in other decaying matter or in soil

• Plasmodial slime molds form a multinucleate, amoeboid mass called a plasmodium

• Cellular slime molds produce pseudoplasmodia, fruiting bodies, and spores with characteristics different from plasmodial slime molds

• Pseudoplasmodium is a slightly motile aggregation of cells that produces fruiting bodies, which in turn produce spores

Representative funguslike protists: A plasmodial slime mold of the genus Hemitrichia on a decaying log

Representative funguslike protists: Pseudoplasmodia of a cellular slime mold, Dictyostelium discoideum

Animal-Like Protists (protozoa)

• Heterotrophic, mostly unicellular organisms

• Most are free living and some are commensals (live in or on other organisms without harming them)

1. Mastigophorans have flagella, few are free-living, but most live in symbiotic relationships

2. Sarcodines are usually amoeboid and move by means of pseupodia. A few have flagella at some stage.

3. Apicomplexans are parasitic and immobile. Sporozoites, merozoites, trophozoites, and gametocytes.

Representative protozoa (animal-like protists): Trichonympha, a mastigophoran, an endosymbiont from a termite gut. Particles seen inside the body are ingested wood particles

Representative protozoa (animal-like protists): Amoeba proteus, a sarcodine, free-living inhabitant of ponds

Representative protozoa (animal-like protists): Plasmodium vivax (inside red blood cells), an apicomplexan, one of the parasites that cause malaria

Representative protozoa (animal-like protists): Paramecium caudatum, a ciliate

Pfiesteria piscicida, a dinoflagellate that is responsible for destruction of over 1 billion fish in North Carolina estuaries

The life cycle of the malaria parasite Plasmodium

Ciliates

• Largest group of protozoans

• Have cilia over most of their surfaces for movement and assist in food gathering

• Balantidium coli is the only ciliate that parasitizes humans and causes dysentery

• Have several highly specialized structures1. Contractile vacuole which regulates fluids2. Trichocysts tentacles used to capture prey3. Conjugation structure for genetic exchange

Characteristics of Fungi

• Fungi, studied in the specialized field of mycology, are a diverse group of heterotrophs

• Many are saprophytes that digest dead organic matter and wastes

• Some are parasites that obtain nutrients from the tissues of other organisms

• Most fungi, such as molds and mushrooms, are multicellular, but yeasts are unicellular

Structural Components of Fungi

• Thallus: The body of a fungus

• The thallus of most multicellular fungi consists of a mycelium

• Mycelium: a loosely organized mass of threadlike structures called hyphae

• Mycelial cells release enzymes that digest substratum

• Cell walls of a few fungi contain cellulose, but most contain chitin

• Chitin: a polysaccharide also found in the exoskeletons of arthropods

The mycelium of a typical fungus. The mold Aspergillus niger consists of filamentous hyphae, the cells of which can be multinucleate and separated by pore-containing septa.

Lichen Housemates: A lichen is not a single organism but a fungus living in symbiosis with a photosynthetic organism (e.g. cyanobacterium or green alga)

Reproduction

• Many fungi reproduce both sexually and asexually and only a few have only asexual reproduction

• Asexual reproduction always involves mitotic cell division, which in yeast occurs by “budding”

• Sexual reproduction occurs in several ways:1. plasmogamy: haploid gametes unite, and their

cytoplasm mingles2. Dikaryotic: if the nuclei fail to unite, a “two-nucleus”

cell forms3. Karyogamy: Eventually the nuclei fuse in this process

to produce a diploid cell

Budding Yeast: Circular scars seen on the surface of the cell on the right represent sites of previous budding

One method of sexual reproduction in fungi

The formation of asexual spores (conidiospores)

A. Brushlike clusters of chains of spores of the fungus Penicillium

B. Spores of the rose rust fungus Phragmidium

Parasitic Fungi

• These fungi have three requirements for invasion:

1. Proximity to the host

2. Ability to penetrate the host

3. Ability to digest and absorb nutrients from host cells

Dutch elm disease: American elms (Ulmus americana) killed by Dutch elm disease

Classification of Fungi

• Fungi are classified according to the nature of the sexual stage in their life cycles

• Such classification is complicated by two problems:

1. No sexual cycle has been observed for some fungi

2. It is often difficult to match the sexual and asexual stages of some fungi

• Dimorphism: the ability of an organism to alter its structure when it changes habitats

Dimorphism in Fungi

Hyphae of Mucor Yeast form of Mucor

The black bread mold, Rhizopus nigricans: Sexual zygospores are the result of joining and fusion of genetic materials at the tips of special hyphal side branches. The zygospores germinate to produce a sporangium that, in turn, produces many asexual spores

The Life Cycle of an Ascomycete (sac fungi)

Spore Prints: Mushroom identification requires knowledge about the spores of your unknown specimen. Spores range in color from black to white, tan and even pink

Club Fungi

• Include mushrooms, toadstools, rusts, and smuts

• Rusts and smuts parasitize plants and cause significant crop damage

• Have hyphae to form mycelia and club-shaped sexual structures called basidia (Basidiomycota)

• Basidiospores: sexual spores in a typical basidiomycete life cycle

Mushroom Spores

The gills on the bottom of a mushroom (Leucoagaricus naucinus) cap have microscopic, club-shaped structures called basidia

Each basidium of Psilocybe mexicana produces four ballonlike structures called basidiospores

Fungi Imperfecti (Deuteromycota)

• No sexual stage has been observed in their life cycles

• Without information on the sexual cycle, taxonomist cannot assign them to a taxonomic group

• By their vegetative characteristics and production of asexual spores, most of these fungi seem to belong with the sac fungi

Are Fungi the Biggest and Oldest Organisms on Earth?

Characteristics of Helminths

• Helminths, or worms, are bilaterally symmetrical

• They have left and right halves that are mirror images

• Also has a head and tail end

• Tissues are differentiated into three distinct layers:1. Ectoderm2. Mesoderm3. Endoderm

• Helminths that parasitize humans include flatworms and roundworms

Helminths

• Flatworms (Platyhelminthes) are primitive worms which lack a coelom and have a simple digestive tract with a single opening. Most flatworms are hermaphroditic.

• Roundworms (nematodes) share many characteristics with flatworms, but they have a pseudocoelom. They have cylindrical bodies with tapered ends and are covered with a thick, protective cuticle

Parasitic Helminths

1. Flukes

2. Tapeworms

3. Adult roundworms of the intestine

4. Roundworm larvae

Flukes

• Two types of fluke infections occur in humans:1. Involves tissue flukes, which attach to the bile ducts,

lungs, or other tissues2. Involved blood flukes, which are found in blood in

some stages of their life cycle• Parasitic flukes have a complex life cycle often

involving several hosts• Miracidia: free-swimming forms• Sporocysts: life form after penetration of molluskan or

snail hosts and divide to form rediae• Rediae give rise to free-swimming cercariae which

penetrate another arthropod host to encyst as metacercariae

Clonorchis sinensis, the Chinese liver fluke. It infests the gallbladder, bile ducts, and pancreatic ducts, where it causes biliary cirrhosis and jaundice

The life cycle of a bloodfluke, Schistosoma japonicum

Tapeworms• Consist of a scolex, or head end with suckers that attach to the intestinal wall

and a long chain of hermaphroditic proglottids

• Proglottids: body components that contain mainly reproductive organs of both sexes

• The life cycle of tapeworms usually includes the following stages:

1. Embryos develop inside eggs and are released from proglottids

2. Proglottids and eggs leave the host’s body with the feces

3. Another animal ingests vegetation or water contaminated with eggs and eggs hatch into larvae, which invade the intestinal wall

4. A larvae can develop into a cysticercus (bladder worm), or it can form a cyst

5. A cyst can enlarge and develop many tapeworm heads within it (hydatid cyst) and if an animal eats flesh containing this, each scolex can develop into a new tapeworm

Head (scolex) of a tapeworm. The hooked spines and suckers are used for attachment to intestinal surfaces

The life cycle of the beef tapeworm Taenia saginata

Adult Roundworms

• Most roundworms that parasitize humans live much of their life cycle in the digestive tract

• Usually enter the body by ingestion with food or water, but some penetrate the skin (e.g. hookworm)

• The life cycles of intestinal roundworms show considerable variation

Mouth of the Old World hookworm Ancylostoma duodenale. The muscular pharynx of this roundworm pumps blood from the intestinal lining of its host

The life cycle of the roundworm Trichnella spiralis

The life cycle of the roundworm Wuchereria bancrofti. This roundworm produces microfilariae and causes elephantiasis of the legs and scrotum

The microfilarial (minature laral) stage of the heartworm Dirofilaria immitis, in a sample of dog blood, is transmitted by mosquito bites

The life cycle of guinea worm (Dracunculus medinensis)

Characteristics of Arthropods

• Constitute the largest group of living organisms

• 80% of all animal species belong to phylum Arthropoda

• Characterized by:

1. jointed chitinous exoskeletons

2. Segmented bodies

3. Jointed appendages

4. Have a true coelom

5. Small brain and an extensive network of nerves

6. Sexes are distinct and females lay many eggs

Classification of Arthropods

• Certain members of three subgroups (classes) of arthropods are important either as parasites or as disease vectors

1. Arachnids2. Insects3. Crustaceans

• Properties (Table 11.4)

An Arachnid: A wood tick, Dermacentor andersoni. Characterized by four pairs of legs and two body regions

1. Cephalothorax

2. Abdomen

Insects

• Have three body regions

1. Head

2. Thorax

3. Abdomen

• Three pairs of legs

• Highly specialized mouth parts

The pubic louse, Phthirus pubus, also known as a crab louse, clinging to a human pubic hair. Lice suck blood, feeding about five times a day

The housefly, Musca domestica, can carry microbes on its body

The Aedes mosquito

A flea, Ctentocephalidis canis

Crustaceans

• Generally aquatic arthropods• Typically have a pair of appendages

associated with each segment• Appendages include:1. Mouthparts2. Claws3. Walking legs4. Appendages that aid in swimming or

copulation