Microbiology and Parasitology

• Protozoal & helminthic infections are more important in tropical countries than in countries like the U.S
• Parasitic diseases are becoming more prevalent in the U.S. as more infected people move there and due to immune deficiencies such as AIDS making people more susceptible to certain parasites
• The immune system responds to these parasites, but is seldom able to rid the body of them
• Protozoa are unicellular eukaryotes limited to a moist environment because they lack a cell wall
• Protozoa are heterotrophs and reproduce asexually by fission and budding; some undergo schizogony
• Protozoa have complex life cycles, requiring multiple hosts and changing their morphology
• Protozoa have an active, motile, feeding stage called trophozoite and a resistant, inactive stage called cyst
• Mastigophora or Zoomastigophora move by means of flagella
• Trypanosoma gambiense infects blood and tissue fluids, causing African sleeping sickness and is transmitted by the tsetse fly
• Giardia lamblia causes waterborne dysentery and forms cysts
• Trichomonas vaginalis causes vulvovaginitis and has numerous flagella
• Sarcodina move by means of pseudopodia or "false feet" temporary extensions
• Fungi are eukaryotic cells that are nonmotile and heterotrophic
• Fungi prefer more acidic conditions than bacteria and can tolerate higher osmotic pressure and lower moisture
• Fungi are larger than bacteria and have more cellular and morphologic detail
• Fungi cannot tolerate high temperatures like bacteria can
• Fungi are important decomposers in ecosystems and some are parasites causing diseases
• Fungi are classified by how they reproduce, either sexually or asexually
• Yeasts are nonfilamentous, unicellular organisms that reproduce asexually by budding
• Molds are filamentous, multicellular organisms with a vegetative structure called a mycelium
• Zygomycetes produce sporangiospores and zygospores, with an example being Rhizopus nigricans
• Ascomycetes include molds with septate hyphae and some yeasts, producing ascospores and conidiospores
• Basidiomycetes form basidiocarps and produce basidiospores, with an example being Amanita
• Deuteromycetes are called imperfect fungi and grow as yeasts or molds
• Dimorphic fungi switch between a single-celled yeast phase and a mycelial phase
• Fungal diseases in humans are usually acquired from nature and are not highly contagious
• Antibiotics like Penicillins are produced by Penicillium and Cephalosporins by Cephalosporium
• Protozoa:

  1. Mastigophora (move by means of flagella)

  a. Giardia lamblia - causes giardiasis; acquired by drinking contaminated water

  b. Trypanosoma brucei - causes African sleeping sickness; transmitted by tsetse flies

  c. Trichomonas vaginalis - causes vulvovaginitis; numerous flagella

  2. Sarcodina (move by means of pseudopodia or "false feet")

  a. Amoeba proteus - freshwater; not pathogenic

  b. Entamoeba histolytica - causes amoebic dysentery; acquired by consuming fecally contaminated water or food
• c. Naegleria fowleri - causes amoebic meningioencephalitis; usually seen in swimmers

  d. Acanthamoeba polyphaga - accumulates on the water surface of contaminated hot tubs; causes ulceration of the eyes and skin

  3. Ciliophora (move by means of cilia)

  a. Paramecium caudatum - freshwater; not pathogenic

  b. Balantidium coli - causes diarrhea of large intestine; rare except in the Philippines

  4. Apicomplexa or Sporozoa or Haemosporina

  a. Plasmodium vivax - causes malaria; transmitted by mosquitoes
• b. Toxoplasma gondii - causes toxoplasmosis; acquired by consuming cysts in infected animals or ingesting material contaminated by cat feces

  c. Cryptosporidium - causes enteritis & diarrhea; resistant to chlorine

  d. Pneumocystis carinii - causes pneumocystis pneumonia; common in AIDS patients
• Helminths - Flatworms & Roundworms

  A. Platyhelminthes (Flatworms = Trematodes + Cestodes)

  1. Trematoda (Flukes)

  a. Clonorchis sinensis (Chinese or Human Liver Fluke) - adults live in bile ducts of humans; life cycle involves snails and fish

  b. Schistosoma mansoni (Blood Flukes) - adults live in circulatory system; transmitted through skin penetration

  2. Cestoda (Tapeworms)

  a. Taenia solium (pork tapeworm) - humans infected by consuming cysts in pork; larvae develop in human digestive tract
• b. Taenia saginata (beef tapeworm) - humans infected by consuming larvae in beef; life cycle similar to T. solium

  c. Echinococcus granulosus (dog tapeworm) - humans can get hydatid cysts from ingesting eggs passed in dog feces

  d. Dipylidium caninum (dog & cat tapeworm) - larvae develop in fleas; humans infected by ingesting the flea

  e. Hymenolepis nana (dwarf tapeworm) - humans can ingest eggs in cereals and foods containing insects

  B. Nematoda (Nematodes)

  1. Ascaris lumbricoides (intestinal roundworm) - largest intestinal nematode in humans; disease called ascariasis
• 2. Enterobius vermicularis (pinworm) - parasitizes large intestine of humans; causes itching in perianal region

  3. Ancylostoma caninum & Necator americanus (hookworms) - larvae penetrate skin and mature in small intestine

  4. Trichinella spiralis (pork roundworm) - larvae encysted in pig muscle tissues; causes trichinosis

  5. Wucheria bancrofti - adults live in lymph nodes; transmitted by mosquitoes

  6. Dirofilaria immitis - causes heartworm disease in dogs; transmitted by mosquitoes
• 7. Trichuris trichiura (whipworm) - adults embed in the mucosa of the large intestine; each adult produces thousands of eggs per day