BIOL 2421 Microbiology Lecture Notes: Protozoans Dr. Weis
Protozoa means “first animal” and are a diverse group of eukaryotic single celled cells that have different modes of mobility and can still be categorized based on those modes.
Amoebas
These protozoans move by extending blunt like projections of the cytoplasm called pseudopods. This group is characterized by a feeding and dividing trophozoite stage that can form a temporary resistant cyst stage.
Flagellates
These protozoans have a whip-like structure (flagella) that allows them to be pulled through the medium. The flagellum is present in the trophozoite stage and infection arises from the ingestion of cysts. Some produce toxins.
Ciliates
These protozoans possess cilia, which are tiny hair like structures that beat in a rhythmic fashion and are found in rows covering the surface of the cell.
Sporazoans
The sporozoans do not have structures for movement.
Life Cycle
Protozoa exist as a motile feeding stage called a trophozoite. Many protozoans convert into a cyst, a dehydrated, protective form of the organism when they encounter harsh environmental conditions. In a cyst form, the organism is able to survive a wide variety of environments and thus allows the organism to spread more easily. Reproduction may be through asexual means: binary fission or schizogony or sexual reproduction involving the fusion of gametes. Cysts rupture releasing the trophozoite that is the more motile, feeding, and replicating stage. Cysts are formed and passed in the environment.
New groupings have been based on sequencing RNA rather than motility.
A. Alveolates
B. Amoebae
C. Euglenozoa
2. Kinetoplastids: mitochondrial DNA = kinetoplast
D. Archaezoa
2. Parabasala
Taxonomy:
Kingdom Protista
Phylum Sarcomastigophora [the protozoa]
Subphylum Mastigophora [Flagellates]
Order Trichomonadida (Archaezoa)
Genera: Trichomonas
Order Diplomanadida (Archaezoa)
Genera: Giardia; Microsporidium
Order Kinetoplastida (Euglenoid)
Genera: Trypanosoma
Genera: Leishmania
Subphylum Sarcodina [Amoebae]
Genera: Entamoeba, Naegleria, Acanthoamoeba
Phylum Apicomplexa (Apicomplexans)
Subclass Coccidia
Order Eucoccidiorida
Suborder Eimeriina
Family Eimeriina
Genus: Isospora
Genus: Eimeria
Family Sarcocystidae
Genus: Toxoplasma
Family Cryptosporidiidae
Genus: Cryptosporidium
Suborder Haemosporoina
Genus: Plasmodium
Subclass Piroplasmasina
Family Babesiidae
Genus: Babesia
Phylum Ciliophora (Ciliates)
Class Litostomatea
Order Vestibuliferida
Genera: Balantidium
Ecological Niches in the Human Body:
Skin: Leishmania
Eye: Acanthamoeba
GI: Giardia, Entamoeba, Cryptosporidium, Isospora, Balantidium, Leishmania
Urinary: Trachomonas
Circulatory: Plasmodium, Trypanosoma
Lymphatic: Leishmania
Muscle: Trypanosoma cruzi
CNS: Trypanosoma, Naegleria, Toxoplasma, Plasmodium
AMOEBA
Entamoeba histolytica
Causative agent of amoebic dysentery which can spread to the liver causing abscesses.
The organism is characterized by a trophozoite and cyst stage. Humans are the definitive host. These amoeba have a pathogenic phase and a non pathogenic phase.
Ingestion of cysts in sewage contaminated water or unwashed vegetables / fruits. Excystation occurs in the distal small intestine and colon. Trophozoites feed and multiply several times to cause disruption [ulcer , necrosis] of the lining. Trophozoites move by extending a finger like pseudopodium and pulling the rest of the organism forward. Amoeba may be carried to the liver via the hepatic portal vien. Cysts pass out with the feces and can be in the environment for 2-5 weeks.
DX: Id cysts or trophozoites in feces or tissues
TX: Metronidizole
Prevention: clean water
Acanthoamoeba castellanii, A. culbertsoni
Free
living amoeba causing opportunistic infection. Amoeba causes corneal
ulcers. Occasional
death through invasion of the CNS.
Naegleria fowleri
Free living amoeba. Opportunistic infection. Enters the nasal passages from water.
Invasion of the CNS causes fatal meningitis.
CILIATE
Balantidium coli
Similar lifecycle to E. histolytica with transmissions by cysts. Zoonotic infection acquired from pigs. Also spread by infected food and water. May invade and cause ulceration of the colon resulting in severe diarrhea since trophozoites will invade the intestinal epithelium. Cysts can remain in intestinal wall and cause chronic disease.
DX: Id organism in feces
TX: Metronidizole
Prevention: Hygiene
FLAGELLATES
Giardia lamblia
Single
celled protozoan that moves with the aid of 5 flagella. It is the most
frequent non-bacterial cause of diarrhea in the
Children are most often infected along with HIV individuals who will exhibit chronicity.
Life long immunity post infection in normal individuals.
Clinical Syndromes
Asymptomatic
Acute, self limiting diarrhea with nausea and upper abdominal discomfort, cramps
Chronic: Damage to upper small intestinal wall that can lead to malabsorption syndrome resulting in diarrhea that can last for several weeks and weight loss.
DX: Id organism in stool (multiple samples) looking for cysts or trophozoite state
ELISA detects excretory products of the organism
IFA
TX: Metronidizole
Prevention: Clean water sources
Trichomonas vaginalis
STD affecting the vagina in females and urethra, prostate, epididymis in males and causes a white purulent discharge with burning /itching.
Life cycle consists only of a trophozoite stage which is transmitted by direct contact during sexual contact. Trophozoites live closely associated to the epithelium of the urogenital tract, where they replicate by binary fission. Organisms have four flagella and a single nucleus.
DX: Demonstration of trophozoites in secretions
Culture organism if not seen in secretions
TX: Metronidizole
Prevention: Safe sex
Trypanosma brucei gambiense and T. brucei rhodesciense
Causative agent of African
sleeping sickness. The disease
is an arthropod (insect) born infection that is spread by the bite of
the tsetse fly found in rural Africa. Zoonotic infection as animals
can act as reservoir hosts of the disease. The Glossina fly
takes up a trypanosome with bloodmeal. These
trypanosomes multiply and undergo a developmental cycle first in the
gut and then in the salivary glands of the fly. Infective forms (metacyclics) develop here and pass out in the saliva during
feeding. In the host, the first stage is multiplication of the trypanosomes
at the site of the bite causing a chancre. The trypanosomes gain access
to the blood causing fever and malaise. The final stage is invasion
of the CNS causing a comatose state and the classical sleeping sickness
syndrome.
Trypanosoma cruzi
Causative agent of Chagas disease. Zoonosis as animals can act as a reservoir.
The intermediate host in this case are triatomid bugs that feed off the blood man.
Bug
takes a blood meal that takes up the trypanosome. Multiplication occurs
in the midgut and
moves down the GI tract of the bug until metacyclic forms
are in the rectum. Infection results from the inoculation of the bug’s
feces that contains the organism in the bite wound. The organism
circulates in the blood and then invade cells. Individuals who
survive the acute stage of the disease are frequently left with chronic
and progressive neuronal and muscle lesions in the heart and GI tract due
to amastigote forms that multiply in these areas and form a pseudocyst. Trypanosome form will occur if the cyst ruptures
and the trypanosome form will circulate in the blood to continue the cycle. Human
Clinical syndromes include: Megaesophagus, megacolon,
arrhythmia, cardiac dialation, sudden death.
Leishmania species: L. donoviani, L. tropica, L.mexicana, L. braziliensis
Causative agent of visceral and cutaneous leishmaniasis. Zoonotic infections due to the reservoir in dogs and rodents. The disease is spread to vertebrate hosts by the bite of sandflies. In man, the promastigotes are inoculated when a sandfly bites. These promastigotes are ingested by macrophages and multiply within the WBC to become amastigotes which are released and ingested by further macrophages. Amastigotes are picked up in circulation by the fly and will multiply asexually in the gut of the vector to create promastigota and migrate forward to be reinoculated at the next bite.
Cutaneous leishmania will occur as a localized open sore in the dermis. If it spreads, it can occur in the nose, mouth and palate causing destruction and called mucocutaneous leishmaniasis.
Visceral leishmania can produce systemic disease. Affected macrophages can be in the liver, spleen, bone marrow and other organs. The parasites continue to multiply in macrophages and if the disease is untreated, it is usually fatal.
APICOMPLEXA
Cryptosporidium parvum
Small coccidian organism. Zoonotic potential as it is enzootic in young calves and is passed to main in fecal contaminated water containing oocysts of the organism. Oocysts are ingested and hatch in the instestine into sporozoites. These invade intestinal cells and undergo 2 asexual reproduction cycles to form gametes, an oocyst containing 4 sporozoites. Oocyts then pass out with the feces. Organism causes a self limiting diarrhea in infants, small children. In AIDS patients it can cause severe and life-threatening diarrhea.
DX: Oocysts in feces
TX: antiprotozoal drugs
Prevention: Sanitation, especially in agricultural
“run off” areas
Plasmodium species: P. falciparum, P. vivax, P. malariae, P.ovale
Cauative agent of malaria. The disease is spread by mosquitoes inhabiting fould, stagnant water. Humans are the definitive host.
In the stomach of the female anopheline mosquito, the male and female gametocytes of the parasite to form a zygote. This in turn forms a motile ookinete that penetrates the midgut wall and develops into an oocyst within which are many thousands of sporozoites. When mature, the sporozoites rupture and the oocyst penetrates the salivary glands. The female mosquito inoculates sporozoites during feeding. These sporozoites first invade human hepatocytes to form pre-erythrocytic schizonts which create daughter cells called merozoites. These merozoites enter the RBCs to start the asexual intraerythrocytic cycle (schizogony) to form new gametocytes. The asexual red cell stages (invasion, rupture, and reinvasion) are responsible for the pathological changes that occur in malaria: fever, chills, anemia, hepatomegaly, encephalitis, renal damage and death. The new male (micro) and female (macro) gametocytes are taken up by the mosquito to continue the life cycle.
DX: clinical symptoms and microscopic examination of a blood smear.
TX: antimalarial drugs
Prevention: Mosquito control
Toxoplasmosis gondii
A coccidian organism which is the causative agent of toxoplasmosis. The domestic cat is the definitive host from which man and other mammals can become infected. Cats are exposed by ingesting paratenic (intermediate) hosts such as mice, birds, or acquire disease transplacentally. The life cycle includes two phases called the intestinal (enteroepithelial) and extraintestinal phases. The intestinal phase occurs in casts only and produces “oocysts.”
Infection commonly arises from the consumption of cysts either through contamination with cat feces or arises from the consumption of bradyzoites under cooked meat. Cysts rupture in the intestine, releasing sporozoites which penetrate gut epithelial cells. After several cycles of multiplication (endodygeny), tachyzoites enter circulation and infect various nucleated cell types in the extraintestinal phase.
Finally tissue cysts are formed in brain and muscle, which contain many slowly dividing bradyzoites and lasts for years.
In the healthy adult is usually asymptomatic. Infectin is problematic in the fetus or AIDS patients. The most devastating form of toxoplasmosis is seen in congenital infection when a pregnant mother passes the organism to the fetus. This can result in severe abnormalities at birth such as miscarriage, stillbirth, or severely handicapped child, both mentally and physically.
In AIDS patients, cysts can develop in the brain.
Other signs: rash, high fever, chills, trouble breathing, and fatigue. Inflammation of organs a can cause heptatitis, pneumonitis, mycarditis, meningoencephalitis.
Three main life forms of T. gondii occur
a) Oocyst produced from the sexual cycle that occurs in the GI tract of cats
oocysts contain sporozoites
b) Tachyzoite is produced from the asexual cycle found in the secondary host
c) Tissue cyst contains bradyzoites.
DX: Ab titer; CT/MRI ; biopsy
TX: Antibiotic {TMPS}
Prevention: Freeze or thoroughly cook meats
Pregnant
women should not clean out litter boxes
Coccidiosis
Two genera (Eimeria and Isospora) are referred to as the “coccidia,” which after infecting a number of species can have a variety of disease syndromes. Life cycles for both are similar.
The host is infected when it ingests an oocyts that have been passed in the feces of another host. The oocyst then excysts in the new host’s small intestine and the sporozoites contained within the oocyst are liberated. The sporozoites penetrate the cells lining the intestine and reproduce asexually for several generations, each time producing multiple merozoites. The merozoites are released from the infected cell and are now able to reinfect other cells, resulting in massive tissue distruction that will eventually result in the death of the host organism. Some merozoites that enter the host’s cells transform into gametocytes à gametes à fuseà zygote à oocyst. The unsporulated oocyst escapes from the host’s cells and is passed in the host’s feces. The unsporulated oocyst will continue its development cycle in the environment to sporulate and produce sporozoites inside to become the infective stage.
DX: Oocysts in the feces of the host (sporulated oocysts allows for Genera Id)
TX: Ab [Sulfa drugs]
Prevention: Sanitation
Disease
is self limiting as parasite stops dividing after several generations
Babesia bigemina
Piroplasm organism that have two host life cycles involving ticks and mammals. Infection of red blood cells transmitted by a hard tick [Boophilus sp]. Babesia lives inside RBCs (usually in pairs) that eventually destroys the RBCs, producing fever, headache, and muscle aches. Anemia may result from the breakdown of red blood cells as the organism undergoes asexual reproduction results in red urine (due to hemoglobin).
People are rarely infected, unless their spleen has been removed, and then the risk of death is high. Clinical signs resemble malaria and include: high fever, anemia, jaundice, dark urine, and kidney failure.
DX: Id organism in RBC blood smear
TX: Ab [Clindamycin, Azithromycin]
Prevention: Tick control
OTHER Organisms
Pneumocystis jiroveci [Formally P. carinii]
Debate as to whether this pathogenic organism is a fungus or protozoan.
RNA analysis indicates that this organism is related to yeasts / fungi.
Source: Found in Human respiratory system in the lining of the alveoli
Transmission: Respiratory droplet as early as 2 years of age.
Clinical Course:
Thick walled cyst in the alveolar lining that contains 8 intracystic bodies created via sexual reproduction. Cyst ruptures releasing the 8 bodies that develop into trophozoites.
Trophozoites reproduce asexually by fission or sexually as an encysted stage.
Susceptible individuals: immunocompromised [HIV, cancer, transplant patients]
Dz: Pneumonia, alveoli fill with frothy exudate, can be fatal
DX: Id cysts found in sputum
PCR
TX: Ab [TMPS]
Prevention: Prophylaxis treatment in susceptible patients.
NOTE: See chapter table in book for overall comparision of pathogenic protozoa.