Week 6: Cell Biology Of Eukaryotes And How They Cause Disease (MICRO)

Microbial eukaroytes typically classified as

fungi, protoza, algae,

Traits of Specific Eukaryotic Microbes

Fungi

 Grow in chains called hyphae or a single celled yeasts

Traits of Specific Eukaryotic Microbes

Amebas (Amoebas)

– Single celled protists of highly variable shape that form pseudopods

Traits of Specific Eukaryotic Microbes

Ciliates

Single celled protists with short, whip-like motility organelles called cilia

Traits of Specific Eukaryotic Microbe

Trypanosomes and metamonads

Flagellated protists with complex parasitic life cycles involving

developmental stages within hosts

Traits of Specific Eukaryotic Microbes

Algae

 Protists containing chloroplasts that conduct photosynthesis

Amebas

Single-celled protists of highly variable

shape that form Pseudopods

Pseudopods

are locomotor

extensions of cytoplasm enclosed by

the cell membrane

pseudopods 2 major groups

Lobed ameba with large, bulky pseudopods

– Free living in soil or water

• Filamentous ameba with thin, needlelike

pseudopods

Amebas are voracious predators which

Consume other protists and bacteria

Intestinal amebiasis

Entamoeba histolytica

• Serious cause of illness in the developing world,

leading to malnutrition and death

where can Intestinal amebiasis be found

Can be found in the water supply of certain regions

• Ingested in the form of cysts, which are dormant cells

encased in a touch coating

• Can reach the intestines, where they “excyst” and

develop into trophozoites

▪ Trophozoites phagocytose red blood cells (RBCs)

• Typically become cysts again and get excreted with

the host feces

• In severe cases, they can invade the liver, lungs, and

other organs

▪ Must be diagnosed using microscopy

• Wheatley’s trichrome stain stains RBCs red, and the

parasites green or blue

Other Diseases Involving Amebas: Primary amebic

meningoencephalitis

Infection of the brain

• Can be picked up swimming

– Naegleria fowleri

Other Diseases Involving Amebas: Corneal infection (keratitis)

Can come from contaminated

contact lens solutions

– Acanthamoeba

Predators and Parasites – How do they catch their prey?

Ciliates

Ciliates

are covered with cilia

(singular, cilium), short, hairlike

organelles composed of

microtubules.

• Beating of cilia propel a stream

of prey into the oral groove

(mouth)

• Some can swallow prey of their

own size

• Cilia utilized for cell propulsion

and food acquisition

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what are cilates

 cilia-covered, heterotrophic protists that consume algae and

smaller protists and are in turn consumed by amebas.

• Some can cause disease in humans

– Balantidium coli is a cause of dysentery

where do cilates grow

in aqueous solutions with salt and organic

concentrations lower than those of their cytoplasm

• Causes water to enter the cell, which is expelled by a

contractile vacuole

– Helps maintain osmotic pressure/resist osmotic

shock and lysis

how many nucleuis do cilates have?

2

micronucleus

macronuclues

Both nuclei chromosomes are replicated during asexual

reproduction

Micronucleus

genes maintain genetic fidelity for sexual

reproduction

Macronucleus

Contains multiple gene copies that express gene

products throughout the large cell

 Apicomplexans

form a major group of

parasites of humans and other animals.

• obligate intracellular parasites

Apicomplexans structure

Apical complex is a specialized structure that

facilitates entry of the parasite into a host cell.

Some Apicomplexan life cycles involve multiple

hosts

definite host

intermediate host

definite host

Apicomplexans

is the host where the parasite

matures and may reproduce sexually

intermediate host

Apicomplexans

is an alternate host that

support asexual proliferation of the parasite

and is required for transmission back to the

next definite host

Types of Apicomplexans

malarias

toxoplasmosis

babesiosis

waterborne apicomplexans

Malaria

Plasmodium falciparum or Plasmodium

vivax

• Definite host is mosquitos, intermediate

host is humans

• Mosquitos transmit the parasite to humans

through bites

– Endemic disease in some regions

malaria life style

The merozoite form of P. falciparum invades a

red blood cell. The apical complex (structure at

the penetrating tip of the cell) facilitates

invasion and then dissolves as the merozoite

transforms into an intracellular form becoming

invisible to the immune system until new

progeny break out of the cell

Toxoplasmosis

Toxoplasma gondii which is a parasite carried by cats and is transmissible to humans

• 30% of the world is infected with this parasite, mostly asymptomatically

– Can harm a fetus after crossing the placental barrier

– Can occasionally enter the brain and alter host behavior

o Potentially a link to some cases of schizophrenia

Babesiosis

Blood parasite Babesia microti spread by deer ticks

– Also spread by blood contact and transfusions

– Replicates in RBCs like malaria

– Often misdiagnosed as Lyme disease

Waterborne Apicomplexans

• Often cause intestinal disease

Trypanosomes

are vector-borne parasites.

• Traditionally extracellular parasites

– Obligate parasites with an elongated cell and single flagellum

• Sleeping sickness (T. brucei), Chagas’ disease (T. cruzi), and leishmaniasis (L.

donovani)

• Uses multiple forms to maintain infection in a host and spread to new hosts

Metamonads

 Parasites or endosymbionts with highly degenerate cells

• Flagellated parasites

– Example: Giardia lamblia

▪ Commonly found in infant day-care centers where caregivers

are carriers

▪ Endemic in wildlife and contaminates freshwater streams

▪ Have an anaerobic metabolism and lack mitochondria

▪ Some types are the cause of sexually transmitted infections

• Trichomonas vaginalis grows in the urethra and in the

vagina

• Typically asymptomatic in men but cause pain in women

and can complicate pregnancies

Algae

are protists that conduct photosynthesis by using

chloroplasts.

Green algae

Chlorophyta

• Photosynthetic autotrophs

– Grow using photosynthesis and absorption of minerals

• Unicellular and multicellular

Red algae

• Rhodophyta

– Colored red due to the pigment phycoerythrin

• Photosynthetic

• Can colonize deeper marine habitats compared to green

algae

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Algae Derived from Secondary Endosymbiosis

dinoflagellates

dinoflagellates

 are a major group of marine

algae, essential to marine food chains.

▪ Responsible for “red tide”

▪ Some can release neurotoxins to fish that eat

them, which then can get transferred to humans

who eat infected fish

▪ Have an armor-plated appearance

▪ Highly motile with two flagella

▪ Can inhibit other organisms as mutualists

helminths

 Parasitic worms

Invertebrate parasites

• Multicellular animals

• Commonly cycle through more than onehost

• Only eggs are sometimes microscopic!

• Can have a definite host, indefinite host,

or incidental host (host in which the

parasite may grow and cause morbidity

but cannot be transmitted to other hosts)

Three major types Helminths

Nematodes

Trematodes

Cestodes

Cestodes

 (tapeworms) are parasitic

flatworms, absorbing nutrients through

their skin. The sucker head structure grows

a long “tail” of segments containing male

and female reproductive structures.

Trematodes

(flukes) are oval-shaped

flatworms, with a digestive tube that ends

in a cecum (no outlet). They are

hermaphrodites.

Nematodes

(roundworms) are cylindrical

(hence the term “roundworm”), with a

digestive tube that ends in an anus.

Helminths: Pinworms

Small, white nematodes of

the species Enterobius

vermicularis

• Infect only humans

• Can cause malabsorption of

nutrients and anorexia

related to the host’s immune

response

• Can also cause secondary

bacterial infection

• All family members exposed

in a house must be treated

due to the eggs

Helminths: Hookworms

Common in the southern United States and in

tropical regions around the world

– Larvae grow in the soil until the penetrate host

skin

• Ancylostoma duodenale and Necator

americanus

– Travel through blood to lungs

– Penetrate the alveoli in the lungs

– Host coughs and parasite moves to the throat

and gets ingested.

– In the small intestine they latch onto the

intestinal lining and suck blood.

– Offspring exit in the host feces

Helminths:Roundworms

Can grow to relatively large sizes within the human digestive tract

• Ascaris lumbricoides

– Eggs ingested from soil.

– Worms increase in size in the digestive tract.

• Worms may also invade the lungs or other

organs, causing life-threatening damage.

• Feces of infected individuals can lead to

transmission to other hosts

Trematodes (flukes)

Flatworms (Platyhelminthes)

– Internalized mouth

– Pharynx

– Digestive tube, but the tube ends in one or more pouches called caeca.

• Because the worm lacks an anus, it must expel its wastes back out of the

mouth.

– Usually has two suckers, one near the mouth and one on the ventral side of

the body

• Infect many kinds of animals

– Humans can become infected when eating contaminated animals

Cestodes: tapeworms

 Transmitted as larvae in uncooked meat

• Different species are found in pork (Taenia

solium), beef (Taenia saginata), and fish

(Diphyllobothrium).

• Composed of a head and successive

segments

– Each segment contains male and female

reproductive organs, which can

cross-fertilize with each other or other

segments

• May grow 2–15 meters in length!

• Can grow for many years undetected until

offspring migrate to the muscle, brain, or

liver

Arthropods

 Invertebrate animals with an exoskeleton and jointed

appendages

• Many free-living, some are ectoparasites

– Suck host blood or inject eggs into hosts

– Ectoparasites may be vectors of infectious diseases.

• Include insects and arachnids (spiders and mites)

types of Arthropods

Arachnids

• Have eight legs

• Mites

– Mange (in animals)

– Scabies (in animals or humans)

• Ticks

– Lyme disease and other bacterial

infections

– Babesiosis (protozoan)

– Flaviviruses

Arthropods: Scabies

Sarcoptes scabiei attaches to skin, burrows, and

lays eggs.

• Eggs hatch to larvae, which attach to hair

follicles and feed, causing inflammation.

Arthropods: Lyme disease and other tick-borne diseases

Ixodes scapularis is a vector for many forms of

borreliosis (Lyme disease is a borreliosis caused

by Borrelia burgdorferi) and several other

bacteria, protozoa, and viruses that cause

diseases in humans.

• Disease transmission occurs when ticks are feeding

Arthropods:insect parasites

are six-legged,

and with or without wings.

• Lice are wingless

ectoparasites.

Arthropods: bedbugs

(Cimex lectularius)

• Ectoparasite that lives on the

external surface of their host

and feeds on their blood

Protozoa

are heterotrophic eukaryotic microbes that are usually motile

• Different than fungi

▪ Can cause diseases like Malaria, sleeping sickness, amebic dysentery, etc.

• Damage from protozoan species due to persistence of the organism in tissues

Immune avoidance by protozoa

Antigenic masking: some protozoans coat themselves in host antigens to avoid detection by the immune system.

• Antigenic variation: change their surface antigens during an infection, allowing them to escape immune response

Protozoan Pathogenesis: Intracellular location

Intracellular lifestyle by some protozoan

species helps delay the immune system

from recognizing these organisms

– Only a temporary refuge from the immune system

due to the proteins needed on the surface of these

cells to continue to grow

Protozoan Pathogenesis: Immunosuppression

some protozoans induce secretion of

anti-inflammatory cytokines to reduce the

innate immune response.

– Delays detection and the ability of the immune

system to kill the pathogens

Fungal Pathogenesis

Fungal infections are of concern in medical settings

• Prominent in immunocompromised people!

• Can be opportunistic

▪ Fungi are heterotrophic eukaryotes with chitinous cell

walls and are non-motile

▪ Medically important fungi include species that produce

superficial mycoses of skin, hair, and nails, as well as species

that cause systemic infections

• Systemic infections include serious respiratory, central

nervous system (CNS), GI tract, and tissue diseases

• Some systemic infections have high mortality rates!

▪ Two types of fungi medically important

Molds that grow as filaments

• Single-celled yeast

▪ Most medically relevant fungi replicate

asexually in nature by forming spores

How Do Fungal Pathogens Cause Disease?

growth

entering the host

avoiding the immune system

injuring the host

How Do Fungal Pathogens Cause Disease?: Growth

Medically important fungi are typically dimorphic, usual growth form depends on

temperature of the host

– Hyphae 25°C, Yeast Cells 37°C

o Hyphae important for penetrating tissues, yeasts can travel easily in the body

How Do Fungal Pathogens Cause Disease?: entering the host

Usually acquired from the external environment rather than by

person-to-person transmission

– Example: inhaling mold

• Can cause disease after introduction to host due to host injury

How Do Fungal Pathogens Cause Disease?: avoiding the immune system

Healthy people have immune systems that are effective against most fungal

infections, but immunocompromised people can be vulnerable

• Some species of fungi can coat their outer cell layer with alpha-glucans, which is

undetectable to the immune system

• Some cells can break out of immune cells that have engulfed them and spread to

vulnerable body sites

How Do Fungal Pathogens Cause Disease?: injuring the host

Some fungi produce mycotoxins that can affect human health when ingested

– Not necessarily part of the pathogenesis of these microbes, usually collateral

damage from the immune response

virus

is a non cellular particle with a genome contained by a capsid

(protein coat).

▪ The term virus refers to the particles and their replication cycle

▪ The term virion refers specifically to the structure of the virus particle

▪ The virus must take over a cell to manufacture progeny (offspring).

unique abt virus

Unlike cells, some viruses (such as HCV) have RNA-based genomes. Other viruses (such as the virus that causes herpes) have DNA-based genomes.

▪ RNA viruses have high error rates during replication and therefore high rates of mutation

▪ They are not considered to be prokaryotic or eukaryotic because they are acellular

▪ Every kind of cellular organism known (including bacteria) can become infected by viruses! 

Viruses are ubiquitous, infecting every taxonomic group of organisms

including bacteria, eukaryotes, and archaea.

Viruses can act as

predators and sequester significant amounts of nutrients

in their structures

• They “leech” off their host!

Viruses cause important human diseases like

influenza, the common cold,

and COVID-19.

• Their role in disease and mortality among populations have directly influence

human behaviors and politics

Viruses can be used for scientific/research

purposes: bacteriophages

are used routinely as cloning

vectors, small genomes into which foreign

genes can be inserted and cloned for gene

technology.

Bacteriophages

are viruses that can infect

bacteria but can’t infect human hosts

natural enviorment virus

In natural environments, viruses fill important

niches in all ecosystems.

• Help decide which organisms survive in an

environment

History of the Virus Concept

Our concept of a virus has developed over the last

century

▪ First suspected when infectious agents were passing

through a filter too small to permit bacterial cells

▪ Virus particles originally thought to be an

infectious “fluid”

▪ Instead, we know now that the “fluid” is small

infections particles called virions

▪ Viruses are typically very small particles, but some

viruses larger than some bacteria are now being

discovered! 

3 Theories/models for how viruses may have originated

Virus-first model

Reduction Model

Escape Model

No universally accepted theory for the origin of viral particles exists

Virus-first model

Ancient viruses were cellular life forms that predated cellular organisms

o Pro: Viruses contain many genes found in no living cells

o Con: No viruses have ever been found to replicate out of a host cell

Reduction Model

Viruses might have evolved from parasitic cells by genome reduction

Escape Model

Some viruses (particularly smaller ones) may have arisen from cell parts that

“escaped” and evolved the ability to infect other cells

The effects of viral disease arise from the replication of the viruses

within host cells

Destroys or debilitates cells

• Viruses can alter host genomes to cause cancers like leukemia and

cancers caused by human papillomavirus (HPV)

The human gut teems with

bacteriophages (phages), viruses that infect

bacteria.

Within the bacterial cytoplasm, the phage genome directs the replication

of progeny virion, completed viral particles.

virion

are released into the host when the cell lyses (ruptures)

In the lab, phages can be observed in

 Petri dish as a plaque, a clear spot

against a lawn of bacterial cells.

Bacteriophages have been proposed as a

method for treating against

antibiotic resistant bacteria

• Bacteriophages can’t cause disease in a human host

Viruses Replicate in Host Cells- Humans: measles

 has an envelope that is derived from

the host cell plasma membrane when the virus exits the

host cell.

• When the virus enters a new cell this envelope fuses

with the host cell plasma membrane releasing the

viral contents into the cell's cytoplasm

• After replicating within the infected cell, newly

formed virions become enveloped by host

membrane as they bud out of the host cell,

continuing the replication cycle

• Spreading virus causes a rash of red spots on the

infected patients

▪ Many different viruses can infect and cause disease in

human hosts in specific ways