Microbiology and Immunology

Sporozoa WORLD OF MICROBIOLOGY AND IMMUNOLOGY

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Spirochetes have varied habitats and growth require-

ments. Some of the bacteria require oxygen for their survival,

while others do not tolerate the presence of oxygen.

In terms of human health, spirochetes are noteworthy

because of the disease causing members of the group.

Treponema pallidumis the cause of syphilis and Borrelia

burgdorferiis the cause of Lyme disease, which can produce

a chronic infection that can result in arthritis, damage to the

central nervous system, and even heart failure. Borrelia

burgdorferican convert to a metabolically dormant cyst in

natural environments and even in humans. The cyst form

allows the bacterium to survive inhospitable conditions and to

elude host immune defense mechanisms.

In ruminants, spirochetes are beneficial. Their chemical

activities help the cow or other ruminant digest food.

Spirochetes also live in harmony with mussels and oysters,

where the bacteria help in feeding by acting as cilia to sweep

food into the mollusk.

A spirochete known as Aquaspririllum magneto-

tacticumis of interest to microbiologists because it is one of a

number of bacteria that possess magnetic particles. These par-

ticles allow a bacterium to orient itself in the water in relation

to Earth’s magnetic field.

See alsoBacteria and bacterial infection; Bacterial movement;

Magnetotactic bacteria

SPONTANEOUS GENERATION THEORY•see

HISTORY OF MICROBIOLOGY

SSporozoaPOROZOA

The fifth Phylum of the Protist Kingdom, known as

Apicomplexa, gathers several species of obligate intracellular

protozoan parasitesclassified as Sporozoa or Sporozoans,

because they form reproductive cells known as spores. Many

sporozoans are parasitic and pathogenic species, such as

Plasmodium (P. falciparum, P. malariae, P. vivax),

Toxoplasma gondii, Pneumocysts carinii, Coccidian, Babesia,

Cryptosporidum (C. parvum, C. muris),and Gregarian.The

Sporozoa reproduction cycle has both asexual and sexual

phases. The asexual phase is termed schizogony (from the

Greek, meaning generation through division), in which mero-

zoites (daughter cells) are produced through multiple nuclear

fissions. The sexual phase is known as sporogony (i.e., gener-

ation of spores) and is followed by gametogony or the pro-

duction of sexually reproductive cells termed gamonts. Each

pair of gamonts form a gamontocyst where the division of

both gamonts, preceded by repeated nuclear divisions, origi-

nates numerous gametes. Gametes fuse in pairs, forming

zygotes that undergo meiosis (cell division), thus forming new

sporozoites. When sporozoites invade new host cells, the life

cycle starts again. This general description of Sporozoan life

cycle has some variation among different species and groups.

Sporozoans have no flagellated extensions for locomo-

tion, with most species presenting only gliding motility,

except for male gametes in the sexual phase, which have a

flagellated stage of motility. All Sporozoa have a cellular

structure known as apical complex, which gave origin to the

name of the Phylum, i.e., Apicomplexa. Sporozoa cellular

organization consists of the apical complex, micropore, longi-

tudinal microtubular cytoskeleton, and cortical alveoli. The

apical complex consists of cytoskeletal and secretory struc-

tures forming a conoid (a small open cone), polar wings that

fix the cytoskeletal microtubules, two apical rings, and secre-

tory vesicles known as micronemes and rhoptries. The apical

complex enables Sporozoans to invade the host cells.

Plasmodiumspecies are the causing agents of malariain

humans and animals and affects approximately 300 million

people around the world, with an estimative of one million

new cases each year. They are transmitted by the female

anopheles mosquito (infecting vector) that injects Plasmodium

sporozoites present in the salivary glands of the mosquito into

the host’s blood stream. Once in the blood stream,

Plasmodiumsporozoites invade erythrocytes (red blood cells)

and migrate to the liver to infect the hepatocytes, where their

asexual reproductive phase starts. When the merozoite stage is

reached, they are released into the circulation again, where

they become ring-like trophozoites that undergo schizogony,

forming new merozoites that invade the erythrocytes, thus

repeating the reproductive cycle. Female anopheles mosqui-

toes ingest merozoites together with the host’s blood. Ingested

merozoites form zygotes in the guts of the vector mosquito,

later developing into oocysts, from which new sporozoites

will be formed and migrate to the anopheles’ salivary gland,

ready to contaminate the next host. Malaria can also be trans-

mitted through infected blood transfusions.

The vectors for Babesiaare ticks, causing fever, periph-

eral capillary hemorrhage, and anemia. Contaminated cats are

Toxoplasma gondiidirect vectors to humans, through the

ingestion of oocysts present in cat feces. However, this para-

site is also present in birds and other mammals, and humans

can be infected by ingesting raw or poorly cooked contami-

nated meat. Pregnant women may have miscarriages when

infected, or can transmit toxoplasmosisthrough the placenta

to the fetus, leading to blindness and/or mental retardation of

the child. Periodic fecal tests of the house cat and adequate

treatment may prevent transmission, as well as avoidance of

half-cooked meat in the diet.

Pneumocysts carinii causes interstitial plasma cell

pneumonia when the cysts containing trophozoites are

inhaled. Cryptosporidium parvum is usually transmitted

through the ingestion of water or foods contaminated with its

oocysts, causing intestinal infection and, in immunodepressed

patients, diarrhea can be chronic, accompanied by fever.

Coccidianspecies infect epithelial tissues of both vertebrates

and invertebrates whereas Gregarianspecies are found in the

body cavities of invertebrates, such as earthworms.

See alsoGastroenteritis; Malaria and the physiology of para-

sitic infections; Microbial taxonomy

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