Chapter 1-3: Cysts, Hosts, and Reproduction in Parasites

Cysts, Encystment, and Excystment in Protozoa

• Encystment is the process of cyst formation around a parasitic protozoan; cysts protect it from harsh environmental conditions.
• Cysts are the infective stage of most protozoa; they can survive outside the host for several months and are relatively resistant to chlorination, UV exposure, and freezing.
• Cysts are excreted in the host's feces.

Transmission, Excystment, and Life Stages

• Cysts are ingested by consuming contaminated food or water or through the fecal-oral route.
• When cysts are ingested, the low pH of the stomach acid triggers excystment; the activated flagella break through the cyst wall, and excystment releases the trophozoite.
• Trophozoites are the feeding and growing stage of the protozoa; they reproduce asexually and either float freely or attach to the mucosa of the intestine.
• Some trophozoites encyst in the small intestine (i.e., form cysts again); both the cyst and the trophozoite are passed in feces and are infectious immediately or shortly after.
• While in the cyst, the trophozoid can actually undergo multiplication so that more than one organism is released in existent. [Note: This sentence reflects the transcript; biologically, trophozoites reproduce in the host (often by binary fission) and cysts are the dormant environmental stage; multiplication inside cysts is not typical.]
• Although arthropods are not microbes themselves, they can act as vectors; some suck human or animal blood and transmit microbial diseases while doing so.
• Vectors: arthropods that carry pathogenic microorganisms.

Hosts and Reproduction: Intermediate vs Definite Hosts

• The human is considered an intermediate host in this context, meaning there is asexual reproduction (binary fission) within the host.
• Binary fission is a form of asexual reproduction where a eukaryotic cell duplicates its body, DNA is replicated, and two daughter cells are formed. Steps described: the body duplicates, DNA replication occurs, and the two cells separate.
• After a vector bite, the organism can become infected and the vector may serve as a host.
• It becomes a definitive host when sexual reproduction occurs within the vector or the definitive host.
• The final stage transfers the organism back to the intermediate host, and the cycle starts again.
• Helmints (helminths) often take advantage of multiple hosts, which can complicate transmission dynamics and control.

Beef Tapeworm Lifecycle (Taenia saginata)

• Humans are the definitive host for the beef tapeworm; feces contain mature proglottids (segments) with a complete sexually mature reproductive system.
• The mature segment also contains thousands of eggs.
• It is the eggs that are transferred or ingested by the intermediate host (cattle);
• Larvae hatch from the eggs and bore through the intestinal wall, migrate to the muscle, and form cysts (cysticerci).
• Meat contaminated with cysticerci is consumed by humans; the worm attaches to the new host's intestine.
• In the intermediate host, reproduction is asexual; sexual reproduction occurs in the definitive host (human).
• The cycle continues when eggs in feces infect a new intermediate host, and cysticerci form in the meat of that host.

Fungi: Reproduction

• Fungi can reproduce both sexually and asexually.

Asexual Reproduction in Fungi

• Fragmentation: a filamentous fungus can reproduce asexually by fragmentation of the hyphae; cells break off and, if they land in a favorable environment, they germinate.
• If conditions are not ideal, they will sporulate and release spores and wait for more favorable conditions to grow.

Sexual Reproduction in Fungi

• Two types of sexual spores: donor spores denoted with a plus sign (+) and recipient spores denoted with a minus sign (-).
• Plasmogamy: a haploid nucleus from a donor cell penetrates the cytoplasm of the recipient cell.
• Karyogamy: the nuclei fuse to form a diploid zygote nucleus; denoted as a diploid nucleus before meiosis.
• Meiosis: the diploid nucleus gives rise to haploid nuclei or sexual spores.
• Some species may be more genetically diverse due to this process (meiotic recombination).
• Notation: mating types are often represented as + and - to indicate donor and recipient compatibility for sexual reproduction.

Key Notations and Concepts

• Haploid state: n
• Diploid state: 2n
• Meiosis yields haploid products from a diploid parent, increasing genetic diversity.
• Asexual reproduction vs sexual reproduction implications for diversity and dissemination.
• Transmission routes: fecal-oral, contamination of food and water, and animal vectors.
• Host roles: intermediate host (asexual stage) and definitive host (sexual stage).
• Cyst vs trophozoite stages: cysts for environmental survival and transmission; trophozoites for feeding and growth within the host.
• Public health relevance: cysts’ resistance to chlorination, UV, and freezing necessitates robust water treatment and meat safety practices to prevent infections.