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Hostparrel/med/notes/1109/ Host Parasite Relationships Organisms: Two types of organisms: 1. Saprophytes (Non-living host). 2. Parasites (Living host). Parasitic organisms are further divided into three categories. 1. Pathogenic (Produce disease). 2. Commensal (Live on host, Cause no harm, Part of normal flora, Protective, Often beneficial, produce vitamins). 3. Oppurtunist (Low virulence, Normal Flora, Disease due to abnormality in host, Change in natural habitat). Definitions: 1. Pathogenicity: The capacity of bacteria to cause disease. Bacteria differ in their ability to cause disease. 2. Commensals: Those bacteria that normally infect humans after birth and cannot penetrate normal host defenses unless they are impaired. 3. Opportunists: Those organisms that normally co-exist with the host but may cause disease when normal defense is broken e.g., Peumococci, Staphylococci. 4. Virulent organisms: Those that nearly always produce disease when they infect. 5. Virulence: Indicates degree of pathogenicity. Includes two features, ability to infect & severity of disease produced. 6. Carriers / Chronic Infections. Most bacteria are eradicated after treatment (typhoid and diphtheria). Bacteria that produce plague and anthrax are only present during active disease. Others can lead to dormant infections (agents of syphilis and brucellosis) which may relapse later. Important to distinguish the two terms infection & disease. Infection: Process by which microbe enters into relationship with host. Disease: Power of the infecting microbe to cause disturbance of the physiologic functions of the host with different signs and symptoms. Disease occurs when an infecting organism invades an area of the body that is normally sterile or when a microbe that is not part of the normal flora becomes established in a site where it can cause harm to the host. 1 Hostparrel/med/notes/1109/ Bacteria cause infection by 3 basic mechanisms: 1. Invasion of tissues 2. Production of toxins 3. Hypersensitivity reactions Microbes enter the host (& produce infection) via either: Gastrointestinal route (GIT) Respiratory tract (RT) Skin and Mucous Membranes Genitourinary tract Blood Occasionally through broken skin or wounds. Following infection microbes may multiply in: Site of entry i.e. locally Lymph node Blood (transmitted to other parts of the body producing bacteraemia or viraemia). Microbes enter the host (& produce infection) via either: Gastrointestinal route (GIT) Respiratory tract (RT) Skin and Mucous Membranes Genitourinary tract Blood Occasionally through broken skin or wounds. Following infection microbes may multiply in: Site of entry i.e. locally Lymph node Blood (transmitted to other parts of the body producing bacteraemia or viraemia). Pathogenicity of a organism is determined by its virulence (ability to produce disease). 1. Secretion of toxin. Endotoxin Exotoxin 2. Presence of capsule. 3. Secretion of extracellular enzymes. Produced by Gram-positives e.g. hyaluronidase (spreading factor), coagulase (clots plasma) and collagenase (breaks down collagen in muscles). 4. Adherence. Some bacteria have pili or fimbrae that adhere to specific receptors ,e.g. N.gonorrhoea and E.coli. 2 Hostparrel/med/notes/1109/ Properties of Exotoxins and Endotoxins. Exotoxins Endotoxins Bacteria Gram-positive Gram-negatives Diffusability Nature Released in medium Protein Remains with Cell wall LPS Effect of Heat Antigenecity Labile High Stable Weak Toxicity Strong Weak Specificity High Low Toxoid Form Yes No Host Resistance to Infection Non-specific Resistance (Natural /Innate) 1. Skin. Physical barrier. Sebaceous glands produce sweat with lysozyme. 2. Respiratory Tract. Ciliated cells and Mucous produced. Nasal hairs. Reflex of coughing. 3. Gastrointestinal Tract. Normal flora in mouth and stomach. Acid and enzymes produced in stomach. 4. Genital Tract. Stratified epithelium (in vagina). Acid produced by lactobacilli. 5. Conjunctiva. Reflex of blinking. Tears act to clear (washing) and have lysozyme. 6. Phagocytic system. Polymorphs and macrophages. 7. Chemical constituents of blood, e.g. complement. Specific Resistance. Directed against specific microbes. 1. Humoral antibodies. 2. Cell Mediated Immunity. Acquired immunity. 3 Hostparrel/med/notes/1109/ Host Resistance to Infection Other factors. 1. Age. Extremeties of age, more susceptible. 2. Hormonal factor. Increase in corticosteroids suppress antibodies. Therefore more susceptible to disease. 3. Species factor. Some microbes have specific host range, e.g. Mycobacterium avium pathogenic for birds not humans. 4. Genetic factor. Genetic defects such as agammaglobulinemia increase susceptibility to infection. ********** 4