HOST PARASITE RELATIONSHIP

Introduction of a flora

Range of relationships Saprophytes – Free living microbes subsisting on dead or decaying organic matter. Parasites – Microbes establishing and multiplying themselves in hosts. Commensals – Microbes living in complete harmony with host without causing any harm to them. Pathogens - Microbes capable of causing disease in host.

Definitions Microbial pathogenesis—process of causing disease Colonization—presence of microbes at site of body Does not imply tissue damage or disease symptoms Does imply invasion of site and multiplication

Characteristics of Parasitism Encounter: agent meets host Entry: agent enters host Spread: agent spreads Multiplication: agent multiplies Damage: agent, host response, or both Outcome: agent or host wins, or coexist

Encounter In utero Do not normally come in contact with organisms Protection of fetal membranes Do not normally come in contact with organisms from mother Normally only present sporadically Exceptions: sexual diseases, virus causes, rubella

Encounter At moment of birth Comes in contact with organisms present in vaginal canal and on skin Previously, antibodies passed from mother to fetus Defenses are good for a period of time, then they wane

Encounter Challenge between man and microbe wages many times during lifetime Most disappear rapidly Some become part of normal flora Only a few cause disease

Penetration into Deeper Tissues Very few organisms can penetrate unbroken skin (worms are an exception) Some organisms can penetrate epithelial tissue; for example: S. pneumoniae, Treponema pallidum Normally after some injury to tissue (many times caused by a virus) Viruses, by receptors

Carried in by Macrophage Alveolar macrophage trap organisms in lung Normally carry upward on ciliary epithelium Some cases, can carry deeper into tissues Some organisms can live, grow in macrophage: Legionella Bordetella pertussis HIV (via virus-laden macrophage from semen)

Terms used Primary Infection Reinfection Secondary Infection Focal Infection (Focal Sepsis) Generalized Infection Cross Infection Iatrogenic Infection Endogenous Infection Exogenous Infection Inapparent Infection (Subclinical Infection) Clinical Infection Typical Infection Atypical Infection Latent Infection Compromised host

SOURCES OF INFECTION Human Beings Animals Insects Soil & Water Food Patients Carriers Healthy Convalescent Temporary Chronic Contact Paradoxical Animals Zoonosis Reservoir Insects Mechanical Biological Soil & Water Food

Other terms commonly used Endemic Epidemic Pandemic Prosodemic Bacteremia Common occurrence while brushing, chewing etc Septicemia Circulation, multiplication, formation of toxins with high swinging fever Pyemia – Septicemia by pyogenic organisms. Produces multiple abscesses in internal organs.

METHODS OF TRANSMISSION OF INFECTION Contact Congenital Inhalation Ingestion Inoculation Insects Iatrogenic Lab Acquired

Factors predisposing to Microbial Pathogenicity Toxigenicity Exotoxins Endotoxins Plasmids Enzyme production-IgA proteases Bacteriophages Communicability Bacterial appandages Infecting dose MID ID50 MLD LD50 Route of Infection Terms used Pathogenicity (Refers to a species) Virulence (Refers to an isolate) Exaltation Attenuation Factors Adhesion Adhesins Pili Invasiveness

1. Portal of entry Respiratory Tract - most common portal Gastrointestinal - via food, water contaminated fingers Fungi live on skin Parenteral Route via bites, injections, wounds Multiple portals of entry Tuberculosis - respiratory droplets, food & milk, wounds 18

2. Dose High virulence fewer organisms needed Lower virulence many organisms needed LD 50 & ID50 19

3. Tissue penetration Genetic basis E. coli O157:H7 uses unique sites Invasiveness causes structural damage Cell penetration some pathogens live inside the cells 20

4. Adherence Adhesins (ligands) adhere to receptors Ligands associated with fimbria Mannose is the most common receptor Pili attachment - N. gonorrhoeae attach to cervix and urethra M Proteins - S. pyogenes adhere to throat epithelial cells

6. Enzymes Coagulase Staphylococci Forms fibrin clot Streptokinase Streptococci Dissolves fibrin clot Hyaluronidase Pneumococci Digests Hyaluronic Staphylococci Acid Streptococci Leukocidin Staphylococci Disintegrates phagocytes Hemolysin Staphylococci Dissolves red blood cells 22

8. Plasmid – Extra chromosomal genetic material 7. Capsules Reduce phagocytosis Slow drug penetration 8. Plasmid – Extra chromosomal genetic material Antibiotic resistance 9. Lysogenic Bacteriophage – Imparts additional genetic character eg Diphtheria bacilli acquire ability to produce exotoxin. 23

Exo Vs Endotoxins Protein-Polysaccharide-Lipid Complex Protein Heat Stable Part of cell wall. No diffusion Obtained only by cell lysis No Enzymatic action Effect non-specific Non-specific Active in large doses Weakly antigenic Antibody does not neutralize Can not be toxoided Produced only by Gram negative bacteria Protein Heat Labile Secreted by cells. Diffuse out Separable by filtration Enzymatic action Specific effect Specific tissue affinity Active in small qty Highly antigenic Action neutralized by specific antibody Can be toxoided Mainly produced by Gram + bacteria. Few Gram – ve bact

Biological activities of the Endotoxins Pyrogenicity Lethal action  Blood Pressure Activation of complement Intravascular coagulation Leucopoenia / Leukocytosis  Glucose / Glycogen synthesis in Liver Stimulation of B cells Macrophage inhibition Interferon release Induction of prostaglandin synthesis