Microbio Lec 4 - Host-parasite Relationship

MICROBIOLOGY LECTURE 4 – Host-Parasite Relationship - Dra. Evelina N. Lagamayo Notes from Lecture USTMED ’07 Sec C – AsM

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SYMBIOTIC ASSOCIATION

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commensalisms mutualism parasitism MUTUALISM

When relationships move in this direction, the infectious process begins

COMMENSALISM

When relationships move in this direction, reestablishment of a healthy host occurs

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PARASITISM

Commensalism one species of organism uses the body of a larger species commensal microbes can benefit the host: o by preventing colonization by more pathogenic species (e.g. the intestinal flroa) o by producing metabolites that are used by the host (e.g. the bacteria and protozoa in the ruminant stomach) o can merge into either mutualism or parasitism Mutualism reciprocal benefits on the two organisms involved o Ex. Bacteria and protozoa living in the stomachs of domestic ruminants – play an important role in digestion and utilization of cellulose o In humans: good health and resistance to colonization by pathogens, depends on the integrity of the normal commensal enteric bacteria Parasitism symbiotic relationship that benefits only the parasite o many parasites are harmful to the host o some are not pathogenic (e.g. when the natural host is in good health) o Ex. Rabies virus coexists with many wild animals but can be fatal in humans o E. histolytica cysts in healthy carriers •

Characteristics of Parasitism o Many different groups of organism are parasitic and all animals are parasitized (Ex. Viruses)

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Disadvantages of Parasitism o Several adaptations have evolved to promote prolonged survival in the outside world and maximize successful host contact o Ex. Virus particles, bacterial spores, protozoan cysts and worm eggs

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Evolution of Parasitism 1. Bacterial Parasite evolved through accidental contact:  Ex. E. coli with other free-living bacteria  Blood feeding arthropods as their parasites have access to tissues of animals on which they feed 2. Many bacterial parasites have evolved to live inside the host cells  survival of the microbe depends upon the possession of surface or metabolic properties that prevents digestion and destruction by the host cell  integrated their biology with that of the host cell 3. The pathway of virus evolution is uncertain a. Viruses evolved from cellular ancestors by a process of secondary simplification b. Viruses are primitively non-cellular c. Viruses were fragments of the nuclear material of other organisms and have always been parasitic o modern viruses may have arisen by all 3

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pathways Eukaryotic parasites have evolved through accidental contact Examples:  Free-living amoeba Naegleria, opportunistically invade the human body and cause disease  Several spp. of nematode worms that can live either as parasites or free-living organisms (Strongyloides stercoralis)  Trypanosomes were primarily adapted as parasites of blood feeding flies and secondarily become parasites of mammals Parasite adaptation to overcome host inflammatory and immune responses  The environment in which the parasite lives (the body of the host) is not passive; it is capable of an active response to the process of the parasite  Pressure of infection has been a major influence in evolution, development of inflammatory and immune response of humans

Changes in parasites create new problems for hosts Example: o HIV infections – changes in the virus have permitted extensive infections in humans o Acquisition of drug resistance in bacteria and protozoa

• Evasion Strategies Strategy Example Elicit minimal Herpes simplex virus survives in host cells for response long period in a latent stage; no pathology Evading effects of Mycobacteria – survive unharmed in response granulomatous response designed to localize and destroy infection Depress host’s HIV – destroy T cells response Malaria – depresses immune responsiveness Antigenic change Viruses, spirochetes, trypanosomes all change target antigens so host response is ineffective Rapid replication Viruses, bacteria, protozoa – produce acute infections before recovery and immunity Survival in weakly Genetic heterogeneity in host population responsive means some individuals respond weakly or individuals not at all, allowing organism to reproduce freely • -

Host Adaptations to overcome changes in parasites Example: o Selective pressure exerted by falciparum malaria which is responsible for the persistence of many alleles associated with hemoglobinopathies (sickle-cell hemoglobin)  They persist because they are associated with resistance to infection o Social and behavioral changes can be as important as genetic changes in altering hostparasite relations o 1.

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Social and behavioral changes and ID Living  causes: altered environments (e.g. air conditioning)  results: water used in cooling systems provides suitable growth conditions for Legionella bacteria spread in aerosols Food  cause: changes in food production and food handling practices  results: intensive husbandry under antibiotic protection leads to drug resistant bacteria in animal products deep freeze-storage; fast food production and inadequate cooking allows bacteria and toxins to enter body (eg. Listeria, Salmonella) Medicine  causes: a. routine use of antibiotics in

b.  a.

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Sex  

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Water  a. b. 

cause: altered sexual habits results: promiscuity increases transmission of sexually transmitted diseases (e.g. gonorrhea, genital herpes, AIDS) causes: breakdown of filtration systems overuse of limited water supplies result: transmission of animal infections through contaminants leading to diarrheal and other infections (e.g. Cryptosporidiosis, giardiasis, leptospirosis)

Pets  

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medicine; routine use of immunosuppressive therapy results: emergence of antibiotic resistant bacteria as hazards to hospitalized patients (e.g. multiply resistant S. aureus) opportunistic infections in patients with reduced resistance (e.g. Pseudomonas, Candida & Pneumocystis)

Travel 

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cause: increase in ownership of pets, particularly species results: transmission of animal infections through contaminants (e.g. Chlamydia, Salmonella, Toxoplasma, Toxocara) cause: increased frequency of journeys to tropical and subtropical countries result: exposure to organisms and vectors not found in country of origin (e.g. malaria, viral encephalitis)