Jacquelyn G. Black
Microbiology: Principles and
Explorations
Sixth Edition
Chapter 11:
Eukaryotic Microorganisms
and Parasites
Copyright © 2005 by John Wiley & Sons, Inc.
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)
•
1.
Most parasites are
obligate parasites: must spend at least some of their life
cycle in or on a host
facultative parasites: normally are free-living, but can
obtain nutrients from a host
2.
•
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
Temporary parasites (biting insects): feed on and
then leave their hosts
Accidental parasites (ticks): invade an organism
other than their normal host
Hyperparasitism (malaria): refers to a parasite itself
having parasites
2.
3.
4.
•
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
Sarcodines are usually amoeboid and move by means
of pseupodia. A few have flagella at some stage.
Apicomplexans are parasitic and immobile.
Sporozoites, merozoites, trophozoites, and
gametocytes.
2.
3.
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
•
1.
2.
3.
Have several highly specialized structures
Contractile vacuole which regulates fluids
Trichocysts tentacles used to capture prey
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”
•
1.
Sexual reproduction occurs in several ways:
plasmogamy: haploid gametes unite, and their
cytoplasm mingles
Dikaryotic: if the nuclei fail to unite, a “two-nucleus”
cell forms
Karyogamy: Eventually the nuclei fuse in this process
to produce a diploid cell
2.
3.
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
•
1.
2.
3.
Tissues are differentiated into three distinct layers:
Ectoderm
Mesoderm
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
•
1.
2.
•
•
•
•
Two types of fluke infections occur in humans:
Involves tissue flukes, which attach to the bile ducts,
lungs, or other tissues
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
•
1.
2.
3.
The life cycle of tapeworms usually includes the following stages:
Embryos develop inside eggs and are released from proglottids
Proglottids and eggs leave the host’s body with the feces
Another animal ingests vegetation or water contaminated with eggs and eggs
hatch into larvae, which invade the intestinal wall
A larvae can develop into a cysticercus (bladder worm), or it can form a cyst
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
4.
5.
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
•
1.
2.
3.
4.
5.
6.
Characterized by:
jointed chitinous exoskeletons
Segmented bodies
Jointed appendages
Have a true coelom
Small brain and an extensive network of nerves
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. Arachnids
2. Insects
3. 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
•
1.
2.
3.
Have three body regions
Head
Thorax
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
•
•
•
1.
2.
3.
4.
Generally aquatic arthropods
Typically have a pair of appendages
associated with each segment
Appendages include:
Mouthparts
Claws
Walking legs
Appendages that aid in swimming or
copulation