Infection

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CONTENTS

HOST-PARASITE INTERACTION

 1.SAPROPHYTES:  

     Free-living microbes that live on dead or decaying org.matter . They are generally unable to invade the living body . However,when the host resistance is lowered may cause infection.

2.PARASITE:

    A microorganism which lives on living host and derives nutrition from the host.

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RELATED TERMS

1.COMMENSALS:

    Commensals live in complete harmony with the host without causing any harm to it.

2.PATHOGEN:

    Microorganism capable of producing disease in the      host.

3.OPPURTUNISTIC PATHOGENS:

    Some commensals can produce disease when host resistance is lowered.

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4)PATHOGENICITY:

     It refers to the ability of a class of microbes to produce  disease.

5)VIRULENCE:

      Degree of pathogenicity of a microbe.

—    FACTORS    PREDISPOSING   TO      MICROBIAL  PATHOGENICITY   

Determinants of virulence

1.ADHESION:

—The initial event in the pathogenesis is the attachment of bacteria to body surfaces .The adhesive structures are named as adhesins .Adhesins usually occur as fimbria or fibrillae and pili or as colonisation factors.

2.INVASIVENESS:

    This ability of an organism to spread within the host tissues after establishing infection.Highly invasive pathogens produce spreading or generalised lesions,while less invasive pathogens cause localised lesions.

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3.ANTIPHAGOCYTIC FACTORS:

   Macrophages and polymorphs play role in phagocytosis of microbes. Factors which opposes phagocytosis for survival of bacteria:

   a)CAPSULE: Capsule contributes to the virulence of bacteria by preventing phagocytosis . Non capsulated variants usually exhibit low virulence

  b)STREPTOCOCCAL M PROTIEN : The M protein present on the surface of Gp A streptococci binds both fibrinogen and fibrin  to the bacterial cell wall thus masking the bacterial receptors  from the complement

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    c)CYTOTOXIN: Cytotoxins produced by certain bacteria interfere with chemotaxis or kill the phagocyte.Staph.aureus produces  haemolysins  and leukocidin.

    d)BACTERIAL SURFACE ANTIGENS: Vi antigen of S.typhi and K antigen of E.coli enable these bacteria to resist phagocytosis and lytic activity of complement.

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4.SURVIVAL WITHIN THE PHAGOCYTE:

    Ingestion of a microbe by a phagocyte results in the formation of a phagholysosome by the fusion of phagosome with lysosome.Any organism which interferes with the formation of phagolysosome can survive intracellularly.

  5.BACTERIAL TOXINS:

       2 types.

    a)Endotoxin

    b)Exotoxin

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ENDOTOXINS

— Lipopolysaccharide in nature and form an integral part of the cell wall of gram-ve bacteria.

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— They are released from the bacterial surface by natural lysis of the bacteria or by the disintegration of the cell wall. They are heat stable.

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—The toxicity of endotoxin depends on the lipid component(lipid A).

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EXOTOXINS

—Heat labile protiens which are secreted by certain   species of bacteria.

—They diffuse readily into the surrounding medium.

— They are highly potent even in minute   amounts.Exotoxins can be converted into toxoids by  treatment with formaldehyde.

— Toxoids lack toxicity but retain antigenicity and thus induce protective immunity when used as vaccines.

—Tetanus toxoid is one such example.

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6.ENZYMES:

    Bacteria produce enzymes which directly damage host tissue.

—PROTEASES:

    These enzyme cleave IgA which protects at mucosal surfaces.

—HYALURONIDASE:

    It breaks down hyaluronic acid and helps in the spread of infection.eg: streptococcus

—COAGULASE:

    It cause deposition of fibrin around the bacteria and thus prevents phagocytosis.eg:staphylococcus

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—TYPES OF INFECTIOUS DISEASES

—LOCALISED

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—GENERALISED

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                LOCALISED INFECTION

  Localised infection may be superficial or deep-seated.

—GENERALISED INFECTION

Generalised infection involve the spread of infecting agent from the site of entry by contiguity through tissue spaces or though the blood stream.

1.BACTERAEMIA:     

     It is the circulation of bacteria In the blood.

2.SEPICAEMIA:

     Condition where bacteria circulate and multiply in the      blood, form toxic pdts. and cause high fever.

3.PYAEMIA:

     Condition where the pyogenic bacteria produce septicemia with multiple abscesses in internal organs. Eg:liver,kidney.

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    DEPENDING ON THEIR SPREAD IN THE COMMUNITY,INFECTIOUS DISEASES ARE CLASSIFIED INTO DIFFERENT TYPES:

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—Endemic diseases

—Pandemic diseases

—Epidemic diseases

—Prosodemic diseases

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ENDEMIC DISEASE

—An endemic disease is one that is constantly present in a particular area(Typhoid fever)

EPIDEMIC DISEASE

—An epidemic disease is one which spreads rapidly, involving many persons in an area at the same time(Influenza)id

PANDEMIC DISEASE

—A pandemic is an epidemic that  spread through many areas of the world, involving numerous individuals within a short span of time(influenza, severe acute resp.syndrome(SARS)

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PROSODEMIC DISEASES

—Prosodemic diseases are creeping or smouldering epidemics(Cerebrospinal fever).