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General Bacteriology

Host-Parasite Relationship

Prof. Dr. Amany Mostafa

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Infection & Disease

Infection: is the process of microbialinvasion of the body by which the organismenters into a relationship with the host.

Disease: is the clinical signs and symptomsresulting from invasion of themicroorganisms, multiplication andproduction of pathological changes insidethe tissues.

InfectiousDisease: is the disease that istransmissible from one patient to another bytransferring the causative organism.

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Koch's Postulates

These have been applied to be sure that a certainorganism is the causative agent of a certain disease.They can be summarized as follows:

The organism should be present in every case of the

disease. The organism must be isolated in pure culture in vitro

from every case. The isolated organism must cause the same disease

when injected in laboratory animals.

The organism must be re-isolated from lesions of suchexperimental animal. Specific antibodies must be detected in serum of

patient.

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Some organisms can not follow Koch'spostulates, e.g.:

- Leprosy bacilli can not be cultivated onartificial media

- Gonococci produce gonorrhea inhuman only

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Source of Infection

Endogenous: from the patient himself.Commensal organisms may become pathogenicunder certain conditions, as when they movefrom their normal site in the body to other sites.

For example, commensal microbes in themouth, if they get into the blood stream (asduring tooth extraction), they may causesubacute bacterial endocarditis in patients withheart valve lesions.

Exogenous: include: Human (patients or carriers).

Animals.

Inanimate (soil).

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Human source

- Patients with typical manifestation of thedisease.

- Carriers.Definition of carrier:

He is apparently healthy individual, carryingthe organism and excreting it to theenvironment. So, infecting other peoplewithout showing any symptoms of thedisease.

Carriers are more dangerous than thepatients because: They are not easily detected

(asymptomatic). They mix freely with others.

Difficult in treatment

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Animal source

Diseasestransmitted fromanimals to humanare known as

"zoonotic diseases e.g. intestinal

tuberculosis, Qfever, brucellosis,and plague.

Inanimate source

As soil and dust.

This source ismainly for spore

formingorganismpresent insoil andcontaminates

wounds e.g. tetanus, gas

gangrene

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Methods of Transmission ofInfection

1- Air-borne infection (droplet infection):

Droplets from the mouth or noseduring coughing or sneezing are

transmitted by air from patients toother individuals.

Respiratory diseases are mainly

transmitted by this route, e.g.influenza, diphtheria, and pulmonarytuberculosis

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2- Ingestion of contaminated food or drinks

(oral transmission): This occurs in food or water or milk-

borne infections. Food and drinks

contaminated by flies, human excreta onthe hands of food handlers.

Mainly infections affecting

gastrointestinal tract (GIT) aretransmitted by this route, e.g. cholera,typhoid fever, and bacillary dysentery.

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3- Blood or blood products:

Through blood transfusion or injectionsby contaminated needles, e.g.hepatitis, AIDS.

4- Sexual transmission: e.g. syphilis,gonorrhea and AIDS.

5- Arthropod-borne infection: through

mosquitos, fleas, ticks e.g. malaria,plague and Q fever.

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Factors Affecting the Host-Parasite Relationship

These factors are either host factorsor microbial factors:

Host factors The major host factor that

determines the outcome of the host-

parasite interaction is host defensesor immunity both natural andacquired.

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Microbial factors

Pathogenicitymeans the ability of themicroorganism to invade the tissue andproduce a disease. It is a qualitativedescription of a species of bacteria.

Virulenceis the degree of pathogenicity. Thisis a quantitative character of a strainbelonging to a pathogenic species.

Virulence Factors of bacteria:

Adherence factors. Invasion factors. Toxin production

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Adherence Factors: Many bacteria depend on the ability to

adhere to mucosal cells as a first step in

causing disease. Without adhesionfactors, many pathogens would be washedaway before they could cause disease.

Adhesion factors are surface structures,and these include:

- Pili (fimbria) which are specializedstructures, for example, the fimbria of E.coli help the attachmentof this organism tothe urinary tract epithelium

- Glycocalyces which are substances

produced by microorganisms, examples,glycocalyx of viridans streptococci (Mprotein) allows the organism to adherestrongly to the endothelium of heartvalves.

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Invasion Factors: Invasiveness is the ability of the

microorganism to enter host'stissues, multiply there and spread toother tissues. It is one of the main

mechanisms by which bacteria cancause disease.

This invasion is helped by:

Enzymesthese are produced by many species ofbacteria and play an important role inthe infectious process, for example:

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Collagenaseand hyaluronidasewhich break

down collagen and hyaluronic acid allowingthe bacteria to spread through the tissues,e.g. Streptococcus pyogenes.

Immunoglobulin A proteasewhich degrades

IgA allowing the organisms to adhere tomucous membrane, e.g. Streptococcuspneumoniae.

Leukocidinwhich can destroy bothpolymorphonuclear leucytes andmacrophages.

Lecithinase that breaks down lecithin of cell

membrane, e.g. Clostridium perfringens.

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Antiphagocytic Factors:

Capsule: some bacteria resist phagocytosis bythe presence of capsule that prevents thephagocytes from adhering to the bacteria., e.g.Streptococcus pneumonia polysaccharidecapsule.

Cell wall proteins of Gram positive cocci, suchas protein A of Staphylococcus aureus and theM protein of Streptococcus pyogenes.

Coagulase Enzyme: Accelerates the formationof a fibrin clot from fibrinogen. This clot canprotect the bacteria from phagocytosis, e.g.Staphylococcus aureus.

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Toxin Production: Toxigenicity means the ability of the

organism to produce toxins.

Toxin production is anothermechanism by which bacteria canproduce disease.

Two types of toxin are described:exotoxins and endotoxins.

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Exotoxin

Hostcell

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ENDOTOXIN

G-ve cell wall

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EndotoxinsExotoxins

Integral part of cell wall of Gramnegative organism.

Secreted by both Gram positive(mainly) and Gram negativeorganisms.

1) Source

Lipopolysacchride.Protein in nature (polypeptide).2) Chemistry

Non diffusible, released on celllysis.

Diffusible, secreted by livingcells.

3) Mechanism ofrelease

Chromosomal genes.Extrachromosomal genes (e.g.

plasmid or bacteriophage).4) Genes control

their synthesis

Low toxicity.Very high toxicity.5) Toxicity

Poorly antigenic.Highly antigenic.6) Antigenicity

No effect, can not be convertedto toxoid.

Convert it into toxoid (antigenicbut non toxigenic).

7) Effect offormalin

Stable at temperature above60C for several hourswithout loss of toxicity.

Unstable to temperature above60C, toxicity destroyedrapidly.

8) Effect of heat

Not specific in action (all givefever and shock).

Every toxin has specific action.9) Specificity

Produce fever in the hostthrough release ofinterleukin-1 frommacrophages.

Do not produce fever in the host.10) Fever

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Examples of exotoxins

exotoxins of Corynebacteriumdiphtheria, Clostridium tetani, and

erythrogenic toxin of Streptococcuspyogenes.

Examples of endotoxins

endotoxins of all Gram negativeorganisms due to the presence ofLipopolysacchride.

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Clinicaluses of exotoxins:

1- Production of vaccine: Treatment of exotoxin with formalin at 37C

for 3 weeks produces toxoid which loses itstoxigenicity but remains antigenic.

When it is given to the children, it willinduce antibody production without anyharmful effect. Example: Diphtheria toxoid,tetanus toxoid.

2- Production of antitoxic serum:

It is prepared by repeated injection of thetoxin or toxoid in cow or horse. Then aftersuitable time, the animal is bleeded, and theserum is separated.

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This antitoxic serum is used either fortreatment or prophylaxis against thedisease (artificial passive immunization

3- Test for susceptibility of the disease:

Examples: In vivo neutralization test.

(Dick test: to test susceptibility for scarletfever).

4- In vitro diagnosis of diseases:

Examples: Anti streptolysin-o-titration(ASOT) for diagnosis of rheumatic fever.

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Metabolic Bacterial Products

Many products are produced as aresult of bacterial metabolism:

Toxins.

Enzymes.

Pigments.

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Bacterial Pigments

These are colored substancesproduced by bacteria.

They have respiratory function orantibacterial activity or evadingmechanism.

Two types of pigments are known:endopigments and exopigments.

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ExopigmentsEndopigments

Not restricted to the

bacterial cell.Restricted to the

bacterial cell.1) Site ofdetection

Diffused to the

surrounding

medium.Do not diffuse.

2)Diffusiblity

Water-soluble.Water-insoluble.3)Solubility

Pseudomonas

exopigments

Staphylococci

endopigments

4)

Example