MUTUALISM

COMMENSALISM

PARASITISM

When relationships move in this direction, the infectious process begins

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

MICROBIOLOGY LECTURE 4 – Host-Parasite Relationship - Dra. Evelina N. LagamayoNotes from LectureUSTMED ’07 Sec C – AsM

SYMBIOTIC ASSOCIATION commensalisms mutualism 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

parasiteo many parasites are harmful to the hosto 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 humanso E. histolytica cysts in healthy carriers

Characteristics of Parasitismo Many different groups of organism are

parasitic and all animals are parasitized (Ex. Viruses)

Disadvantages of Parasitismo 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

Evolution of Parasitism1. 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 uncertaina. Viruses evolved from cellular

ancestors by a process of secondary

simplificationb. Viruses are primitively non-cellularc. Viruses were fragments of the

nuclear material of other organisms and have always been parasitic

o modern viruses may have arisen by all 3 pathways

4. Eukaryotic parasites have evolved through accidental contact

o 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

5. 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 humanso Acquisition of drug resistance in bacteria

and protozoa

Evasion StrategiesStrategy ExampleElicit minimal response

Herpes simplex virus survives in host cells for long period in a latent stage; no pathology

Evading effects of response

Mycobacteria – survive unharmed in granulomatous response designed to localize and destroy infection

Depress host’s response

HIV – destroy T cellsMalaria – 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 responsive individuals

Genetic heterogeneity in host population means some individuals respond weakly or 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 host-parasite relations

o Social and behavioral changes and ID

1. Living causes: altered environments

(e.g. air conditioning) results: water used in cooling

systems provides suitable growth conditions for Legionella bacteria spread in aerosols

2. 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)

3. Medicine causes: a. routine use of antibiotics in

medicine; b. routine use of

immunosuppressive therapy results: a. emergence of antibiotic resistant

bacteria as hazards to hospitalized patients (e.g. multiply resistant S. aureus)

b. opportunistic infections in patients with reduced resistance (e.g. Pseudomonas, Candida & Pneumocystis)

4. Sex cause: altered sexual habits results: promiscuity increases

transmission of sexually transmitted diseases (e.g. gonorrhea, genital herpes, AIDS)

5. Water causes:a. breakdown of filtration systemsb. overuse of limited water supplies result: transmission of animal

infections through contaminants leading to diarrheal and other infections (e.g. Cryptosporidiosis, giardiasis, leptospirosis)

6. Pets cause: increase in ownership of

pets, particularly species results: transmission of animal

infections through contaminants (e.g. Chlamydia, Salmonella, Toxoplasma, Toxocara)

7. Travel 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)

-fin-

audsmartinez@gmail.comustmedc3@yahoogroups.com