Infection and Immunity

Exam 1: Study Guide

Medical Protozoa:
- Pathogenic protozoa
o Plasmodium falciparum
- Opportunistic parasites
o Not normally pathogens
o Become pathogenic due to impairment of host resistance
o Clinical importance in the AIDS epidemic
 Cryptosporidium parvum
- Basic Structures
o Cytoplasm
 Ectoplasm: locomotion, ingestion
 Endoplasm: metabolism
o Nuclear
 Vesicular or compact form
o Locomotive organelle
 Pseudopodium, flagellum, cilia
- Life Cycle Patterns
o One stage
 Trophozoite
 Take food, mobile, multiply
o Two-stage form
 Trophozoite and cyst
 Not mobile with cyst wall
o Two-host form
 Mammals – mammals
 Mammals – insect vectors
- Hosts for Parasites
o Intermediate
 Eggs/early larva develop into next stage
 Larval or intermediate stages respectively
o Trophozoite → Gametocyte
o Definitive
 Larval stages mature into sexually mature adult parasites
 Gametocyte → Gametes → Zygote → Trophozoite
o Reservoir
 Essential for survival of parasite
 Means of transmission to other organisms
- Types of Vectors
o Mechanical
 Transmits pathogen but not essential to its life cycle
 Housefly and E. coli
o Biological
 Required for proper development of pathogen
 Anopheles mosquitoes and malaria
- Parasite Pathogenesis
o Exhibit wide range of pathogenic effects
 Typically do not have high virulence levels and may not replicate in host
o Pathogenicity of parasites related to:
 Infectious dose of organism
 # of organisms acquired over time
 Chronic nature of most parasitic diseases
 Immunopathologic effects
 Invasion of host and attachment to host cells/tissues
 With damage thereof
- Parasite Portals of Entry
o Protozoan and helminthic infections almost always exogenous
 Entry into the host becomes critical event
 Most commonly used portals of entry:
o Ingestion
 Contamination of food/water with human/animal
waste
 Applicable to most parasitic organisms
o Direct penetration
 Arthropod bites (typically protozoa)
 Transplacental penetration (limited)
 Organism-directed penetration (typically helminths)
- Adherence and Replication of Parasites
o Species and tissue tropisms usually important
 Nonspecific – mechanical process
 Specific – cell-surface molecules interact with adhesins on parasite’s
surface
 A variety of cell-surface molecules useful to parasite
o Glycoproteins (fibronectin, Duffy antigen, etc)
o Complement receptors
o Complement proteins
- Cell and Tissue Damage from Parasites
o Toxins produced not on same level as bacterial toxins
o Damage results from:
 Degradative enzymes
 Proteases, phospholipases, etc
 Function as toxins
 Mechanical tissue damage
 Blockages
o Intestinal, bile duct, blood or lymphatic vessels
 Migration through tissues
 Immunopathology
 Hypersensitivity
 Autoimmunity
 Carcinogenic effects from chronic inflammation
 Degradation of immunoglobulins by proteases
- Groups of Medically Important Protozoa
o Amoebas(Sarcodina)
 Amoeboid movement/pseudopodia
 Entamoeba, Acanthamoeba, and Naegleria
o Flagellates (Mastigophora)
 Giardia, Trichomonas, Trypanosoma, and Leishmania
o Ciliates (Ciliata)
 Balantidium coli
o Apicomplexa (Sporozoans)
 Typically use TWO hosts
 Gliding motility
 Apical complex that facilitates entry into host cells
 Plasmodium, Toxoplasma, Pneumocystis, Cryptosporidium
- Medically important Phyla and Organisms in the Kingdom Protozoa
o Metamonada (flagellates)
 Giardia, Chilomastix
o Parabasala (flagellates)
 Dientamoeba, Trichomonas
o Euglenozoa (flagellates)
 Leishmania, Trypanosoma
o Percolozoa (amoeba)
 Naegleria
o Amoebozoa (amoebas)
 Acanthamoeba, Balamuthia, Entamoeba
o Sporozoa (sporozoans/apicomplexans)
 Cryptosporidium, Cyclospora, Toxoplasma, Babesia, Plasmodium
o Ciliophora (ciliates)
 Balantidium coli
- General Characteristics of Protozoa
o Single-cell eukaryotes
o 2-100 μm in size
o Motility important for classification/pathogenicity
o Phagocytosis/pinocytosis OR simple diffusion (some cases)
o Facultative anaerobes
o Majority free-living and not pathogenic
o Many can change surface antigens frequently
 Immunoevasive tactics
- Reproduction in Protozoa
o Asexual
 Simple binary fission
 Schizogony
 Nuclei repeatedly divide
o Cytoplasm divides
 Nuclei separate
 Merogony if daughter cells merozoites
 Sporogony if daughter cells sporozoites
 Gametogony if daughter cells gametes
o Sexual
 Syngamy
 Two cells fuse
 Conjugation
- Protozoan Forms
o Trophozoites
 Motile
 Feeding form
 Fragile
 Cannot live long outside body
 Rarely withstand stomach acid
 Noninfectious form mostly
o Cysts (Diagnostic)
 Resting/dormant stage
 Infectious form
 Common in fecal-oral transmission
 Survive longer than trophozoites
 Survive exposure to stomach acid
 Protozoa without cyst form
 Transmitted by direct contact or arthropod vector only
- Fungal Characteristics
o Kingdom Fungi/Myceteae
o Eukaryotic cells
 Cell wall: contains chitin and glucan
 Cell membrane: contains ergosterol
o Lifestyles
 Saprobes, Symbionts, Commensal, Parasites
o Most aerobic
 Facultative and strict anaerobes are possible as well
o Produce primary and secondary metabolites
 Many extreme economic value
o Very slow growth rates
 Generation times in hours (instead of minutes)
- Fungal Morphologies
o Yeasts
 Unicellular
 Budding or Fission reproduction
 Nuclear fission → two cells
 Budding → Ballistoconidia
 Pseudohyphae
 Germ tube → Pseudohyphae
 Important in pathogenic yeast (i.e. Candida albicans)
o Molds
 Hyphae
 Grow by apical extension
 Coenocytic vs Septate
o Septate = septum
o Coenocytic = no septum
o Septate hyphae with clamp connections
 Aerial vs Vegetative
o The portion of the mycelium that anchors mold and absorbs
nutrients is vegetative
o Portion that produces asexual reproductive spores is aerial
o Dimorphic Fungi
 Two morphologies
 In response to temperature or nutrient supply
o Thermal dimorphism
 Typically yeast forms seen in human tissues
 Mold forms in the environment
 “Yeast beasts in body heat; bold mold in the cold”
 Blastomyces, Histoplasma, Coccidioides, and Sporothrix
- Fungal Reproductive Life Cycles
o Asexual and Sexual spores (majority)
 Anamorph = asexual spores
 Teleomorph = sexual spores
o Rely mostly on anamorphic stage and minimize teleomorphic stage
 Asexual designations used in clinical setting
 Usually isolated from patient’s tissues
 Sexual stage is typically not seen clinically
- Candida albicans: An exception to the rule
o Polymorphic fungus can grow in several forms
 Primarily yeast, Pseudohyphae, and Hyphae
o Pathogenicity:
 Ovoid-shaped budding yeast
 Important in spread of C. albicans
 Parallel-walled true hyphae
 More prevalent for infection
 Pseudohyphae
 Not well understood
 Intermediate form between yeast and hyphae
- Lab Diagnosis of Fungal Disease
o 10-20% KOH wet mount of skin scrapings
 Dissolves skin cells and leaves fungal cells
 Can be stained in same prep with fungal stains
o India ink wet mount of CSF
 Detects capsule of Cryptococcus neoformans
 Not very sensitive (only about 50%)
o Latex agglutination test for cryptococcal antigens more common
 Much more sensitive
 Particularly valuable for CSF from meningitis
o Giemsa or Wright’s stain of blood
 Detection of intracellular Histoplasma capsulatum
o Lactophenol cotton blue
 Quick and easy
 Stains chitin in fungal cell walls (Aspergillus sp.)
o Calcofluor white
 Fluorescent stain
 Binds strongly to structures containing cellulose and chitin
 Cryptococcus neoformans
o GMS (Gomori methenamine silver)
 Method of choice among silver stains for fungi
o Fontana-Masson silver stain
 Specific stain for melanin
 Identification of dematiaceous fungi
o Culture
 Time issues
 Temperature issues
 Growth media
 Tend to be more general purpose than bacterial growth media
 Sabouraud dextrose agar, potato dextrose agar, brain heart infusion
agar with blood
 Antibacterial antibiotics frequently added to media to inhibit
normal flora bacteria and environmental fungi that are possible
contaminants
o Gentamicin, chloramphenicol, cycloheximide
- Fungal Pathogenesis
o Only about 200 outright pathogens
 Mycoses
o Many patients are at increased risk for opportunistic fungal infections:
 Immunocompromised patients
 Transplants
 AIDS
 Cancer chemotherapy
 Patients hospitalized with severe underlying conditions
 Diabetes
 Invasive procedures
 Severe burns
o Few fungi considered primary pathogens
 Do not have necessary virulence factors
 NO endotoxin
 NO bacteria-like exotoxins
o Fungi that qualify as primary pathogens have variety of virulence factors:
 Growth at 37oC
 Thermal dimorphism
 Immune response resistance
 Resistance to phagocytosis/survival inside phagocytes
 Alteration of cell wall composition
 Modulation of the host immune response
o Dimorphism
 Yeast forms larger than conidia or hyphal fragments
 Harder to phagocytize
 Able to persist inside phagocytes
 Some hyphal fragments have cell walls with antiphagocytic properties
o Modification of cell wall composition
 Removal of antigens needed for recognition by macrophages
o Stimulation of the “wrong” kind of immune response
 Cell wall antigens that stimulate the TH2 response instead of TH1
 TH2 is ineffective for immune control of (these particular) fungi
o Usually makes infection worse
- Resistance to antifungal agents
o Develops slowly and generally uncommon
o Resistance is NOT transmissible from cell to cell
 Unlike bacteria
o Some of the same mechanisms are present as in bacteria
 Multidrug efflux pumps
 Target alterations
 Reduced drug target access
- Antifungal Therapy
o Good drug targets:
 Ergosterol and enzymes involved in production of its metabolic precursors
 Glucan-containing cell walls
o Inhibition of protein synthesis
 Sordarins and Azasordarins
o Cell wall inhibitors of:
 Glucan synthesis
 Echinocandins
 Chitin synthesis
 Nikkomycin
o Cell membrane inhibitors of:
 Ergosterol synthesis
 Azoles
 Allylamines
o Direct membrane damage
 Polyenes
o Disruption of:
 Microtubules and inhibition of mitosis
 Griseofulvin
o Inhibition of nucleic acid synthesis
 Flucytosine