HOST PARASITE HOST PARASITE RELATIONSHIPRELATIONSHIP

Introduction of a floraIntroduction of a flora

Range of relationshipsRange of relationships Saprophytes Saprophytes – Free living microbes – Free living microbes

subsisting on dead or decaying organic subsisting on dead or decaying organic matter.matter.

ParasitesParasites – Microbes establishing and – Microbes establishing and multiplying themselves in hosts.multiplying themselves in hosts.– CommensalsCommensals – Microbes living in complete – Microbes living in complete

harmony with host without causing any harm to harmony with host without causing any harm to them.them.

– PathogensPathogens - Microbes capable of causing - Microbes capable of causing disease in host.disease in host.

DefinitionsDefinitions

Microbial pathogenesis—process of causing Microbial pathogenesis—process of causing diseasedisease

Colonization—presence of microbes at site Colonization—presence of microbes at site of bodyof body– Does Does notnot imply tissue damage or disease imply tissue damage or disease

symptomssymptoms– DoesDoes imply invasion of site and multiplication imply invasion of site and multiplication

Characteristics of ParasitismCharacteristics of Parasitism

Encounter: agent meets hostEncounter: agent meets host Entry: agent enters hostEntry: agent enters host Spread: agent spreadsSpread: agent spreads Multiplication: agent multipliesMultiplication: agent multiplies Damage: agent, host response, or bothDamage: agent, host response, or both Outcome: agent or host wins, or coexistOutcome: agent or host wins, or coexist

EncounterEncounter

In uteroIn utero– Do not normally come in contact with Do not normally come in contact with

organismsorganisms» Protection of fetal membranesProtection of fetal membranes

» Do not normally come in contact with organisms Do not normally come in contact with organisms from motherfrom mother

Normally only present sporadicallyNormally only present sporadically Exceptions: sexual diseases, virus causes, rubellaExceptions: sexual diseases, virus causes, rubella

EncounterEncounter

At moment of birthAt moment of birth– Comes in contact with organisms present in Comes in contact with organisms present in

vaginal canal and on skinvaginal canal and on skin» Previously, antibodies passed from mother to fetusPreviously, antibodies passed from mother to fetus

» Defenses are good for a period of time, then they Defenses are good for a period of time, then they wanewane

EncounterEncounter

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

Penetration into Deeper TissuesPenetration into Deeper Tissues

Very few organisms can penetrate unbroken Very few organisms can penetrate unbroken skin skin (worms are an exception)(worms are an exception)

Some organisms can penetrate epithelial Some organisms can penetrate epithelial tissue; for example:tissue; for example:– S. pneumoniaeS. pneumoniae, , Treponema pallidumTreponema pallidum

» Normally after some injury to tissue (many times Normally after some injury to tissue (many times caused by a virus)caused by a virus)

– Viruses, by receptorsViruses, by receptors

Carried in by MacrophageCarried in by Macrophage

Alveolar macrophage trap organisms in lungAlveolar macrophage trap organisms in lung– Normally carry upward on ciliary epitheliumNormally carry upward on ciliary epithelium– Some cases, can carry deeper into tissuesSome cases, can carry deeper into tissues

Some organisms can live, grow in macrophage:Some organisms can live, grow in macrophage:– LegionellaLegionella– Bordetella pertussisBordetella pertussis– HIV HIV (via virus-laden macrophage from semen)(via virus-laden macrophage from semen)

Terms usedTerms used

Primary InfectionPrimary Infection ReinfectionReinfection Secondary InfectionSecondary Infection Focal Infection Focal Infection

(Focal Sepsis)(Focal Sepsis) Generalized InfectionGeneralized Infection Cross InfectionCross Infection Iatrogenic InfectionIatrogenic Infection Endogenous InfectionEndogenous Infection

Exogenous InfectionExogenous Infection Inapparent Infection Inapparent Infection

(Subclinical Infection)(Subclinical Infection) Clinical InfectionClinical Infection Typical InfectionTypical Infection Atypical InfectionAtypical Infection Latent InfectionLatent Infection Compromised hostCompromised host

SOURCES OF INFECTIONSOURCES OF INFECTION

Human BeingsHuman Beings– PatientsPatients– CarriersCarriers

» HealthyHealthy

» ConvalescentConvalescent

» TemporaryTemporary

» ChronicChronic

» ContactContact

» ParadoxicalParadoxical

AnimalsAnimals– ZoonosisZoonosis– ReservoirReservoir

InsectsInsects– MechanicalMechanical– BiologicalBiological

Soil & WaterSoil & Water FoodFood

Other terms commonly usedOther terms commonly used

EndemicEndemic EpidemicEpidemic PandemicPandemic ProsodemicProsodemic

BacteremiaBacteremia– Common occurrence while Common occurrence while

brushing, chewing etcbrushing, chewing etc

Septicemia Septicemia – Circulation, multiplication, Circulation, multiplication,

formation of toxins with formation of toxins with high swinging feverhigh swinging fever

PyemiaPyemia – – – Septicemia by pyogenic Septicemia by pyogenic

organisms. Produces organisms. Produces multiple abscesses in multiple abscesses in internal organs.internal organs.

METHODS OF TRANSMISSION METHODS OF TRANSMISSION OF INFECTIONOF INFECTION

ContactContact CongenitalCongenital InhalationInhalation IngestionIngestion InoculationInoculation InsectsInsects Iatrogenic Iatrogenic Lab Acquired Lab Acquired

Factors predisposing to Microbial Factors predisposing to Microbial PathogenicityPathogenicity

Terms usedTerms used– Pathogenicity (Refers Pathogenicity (Refers

to a species)to a species)– Virulence (Refers to an Virulence (Refers to an

isolate)isolate)– ExaltationExaltation– AttenuationAttenuation

FactorsFactors– AdhesionAdhesion

» AdhesinsAdhesins» PiliPili

– InvasivenessInvasiveness

– ToxigenicityToxigenicity» ExotoxinsExotoxins» EndotoxinsEndotoxins

– PlasmidsPlasmids– Enzyme production-Enzyme production-

IgA proteasesIgA proteases– BacteriophagesBacteriophages– CommunicabilityCommunicability– Bacterial appandagesBacterial appandages– Infecting doseInfecting dose

» MID ID50MID ID50» MLD LD50MLD LD50

– Route of InfectionRoute of Infection

1. Portal of entry1. Portal of entry– Respiratory Tract - most common portalRespiratory Tract - most common portal– Gastrointestinal - via food, water contaminated Gastrointestinal - via food, water contaminated

fingersfingers– Fungi live on skinFungi live on skin– Parenteral Route via bites, injections, woundsParenteral Route via bites, injections, wounds– Multiple portals of entryMultiple portals of entry

» Tuberculosis - respiratory droplets, food & milk, Tuberculosis - respiratory droplets, food & milk, woundswounds

2. Dose2. Dose– High virulence fewer organisms neededHigh virulence fewer organisms needed– Lower virulence many organisms neededLower virulence many organisms needed

– LD LD 5050 & ID & ID5050

3. Tissue penetration3. Tissue penetration– Genetic basis Genetic basis E. coli O157:H7E. coli O157:H7 uses unique uses unique

sitessites– Invasiveness causes structural damageInvasiveness causes structural damage– Cell penetration some pathogens live inside the Cell penetration some pathogens live inside the

cellscells

4. Adherence4. Adherence– Adhesins (ligands) adhere to receptorsAdhesins (ligands) adhere to receptors

» Ligands associated with fimbriaLigands associated with fimbria

» Mannose is the most common receptorMannose is the most common receptor

– Pili attachment - Pili attachment - N. gonorrhoeae attach to N. gonorrhoeae attach to cervix and urethracervix and urethra

– M ProteinsM Proteins - S. pyogenes - S. pyogenes adhere to throat adhere to throat epithelial cellsepithelial cells

6. Enzymes6. Enzymes– Coagulase Staphylococci Forms fibrin clotCoagulase Staphylococci Forms fibrin clot

– Streptokinase Streptococci Dissolves fibrin clotStreptokinase Streptococci Dissolves fibrin clot

– Hyaluronidase Pneumococci Digests HyaluronicHyaluronidase Pneumococci Digests Hyaluronic

Staphylococci AcidStaphylococci Acid

Streptococci Streptococci

– Leukocidin Staphylococci DisintegratesLeukocidin Staphylococci Disintegrates

phagocytes phagocytes

– Hemolysin Staphylococci Dissolves red Hemolysin Staphylococci Dissolves red

blood cellsblood cells

7. Capsules7. Capsules– Reduce phagocytosis Reduce phagocytosis

– Slow drug penetrationSlow drug penetration

8. Plasmid – Extra chromosomal genetic material8. Plasmid – Extra chromosomal genetic material- Antibiotic resistanceAntibiotic resistance

9. Lysogenic Bacteriophage –9. Lysogenic Bacteriophage –- Imparts additional genetic character eg Diphtheria Imparts additional genetic character eg Diphtheria

bacilli acquire ability to produce exotoxin.bacilli acquire ability to produce exotoxin.

ExoExo Vs Endotoxins Vs Endotoxins ProteinProtein

Heat LabileHeat Labile Secreted by cells. Diffuse outSecreted by cells. Diffuse out Separable by filtrationSeparable by filtration Enzymatic actionEnzymatic action Specific effectSpecific effect Specific tissue affinitySpecific tissue affinity Active in small qtyActive in small qty Highly antigenicHighly antigenic Action neutralized by specific Action neutralized by specific

antibodyantibody Can be toxoidedCan be toxoided Mainly produced by Gram + Mainly produced by Gram +

bacteria. Few Gram – ve bactbacteria. Few Gram – ve bact

Protein-Polysaccharide-Lipid Protein-Polysaccharide-Lipid ComplexComplex

Heat StableHeat Stable Part of cell wall. No diffusionPart of cell wall. No diffusion Obtained only by cell lysisObtained only by cell lysis No Enzymatic actionNo Enzymatic action Effect non-specificEffect non-specific Non-specificNon-specific Active in large dosesActive in large doses Weakly antigenicWeakly antigenic

Antibody does not neutralizeAntibody does not neutralize

Can not be toxoidedCan not be toxoided Produced only by Gram Produced only by Gram

negative bacterianegative bacteria

Biological activities of the Biological activities of the EndotoxinsEndotoxins

PyrogenicityPyrogenicity Lethal actionLethal action Blood PressureBlood Pressure Activation of Activation of

complementcomplement Intravascular Intravascular

coagulationcoagulation Leucopoenia / Leucopoenia /

LeukocytosisLeukocytosis

Glucose / Glycogen Glucose / Glycogen synthesis in Liversynthesis in Liver

Stimulation of B cellsStimulation of B cells Macrophage inhibitionMacrophage inhibition Interferon releaseInterferon release Induction of Induction of

prostaglandin prostaglandin synthesissynthesis