2. Animal & Microorganisms
relationship
• Saprophytes
– Free living microbes
– Dead & Decaying matter
– Generally incapable of growth on living tissue
– Except: B. subtilis (opportunistic infection)
• Parasites
3. Animal & Microorganisms
relationship
• Parasites
– Microbes establish & multiply in host
– Pathogen: Produce disease
– Commensals:
• Live in harmony with host without causing disease
• Facultative pathogen: host resistance ↓ ↓
4. Definitions
• Disease and Infectious Disease
– Disease
• Any deviation from a condition of good
health and well-being
– Infectious Disease
A disease condition caused by the presence
or growth of infectious microorganisms or
parasites
5. Entry of pathogen:
•Infection:
– Pathogen enter----establish ---multiply---invade
Normal anatomical barrier—c/f---infectious ds
– Lodgement & multiplication of pathogen in or on
tissue of host
– Infection----disease
•Colonization
– Pathogen enter—multiply—not invade/ds/elicit
immune response
6. Classification of Infections
• Acute infection vs. chronic infection
– Acute Infection
• An infection characterized by sudden onset,
rapid progression, and often with severe
symptoms
– Chronic Infection
• An infection characterized by delayed onset
and slow progression
7. • Primary infection vs. secondary infection
– Primary Infection
• An infection that develops in an otherwise
healthy individual
– Secondary Infection
• An infection that develops in an individual
who is already infected with a different
pathogen
8. • Localized infection vs. systemic infection
– Localized Infection
• An infection that is restricted to a specific
location or region within the body of the
host
– Systemic Infection
• An infection that has spread to several
regions or areas in the body of the host
9. • Clinical, subclinical, atypical, Latent infection
– Clinical Infection
• An infection with obvious observable or
detectable symptoms
– Subclinical/ in-apparent/Asymptomatic Infection
• An infection with few or no obvious symptoms
with active infection
– Atypical Infection
• No characteristic/typical manifestation
– Latent infection
• Infection—inactive or dormant latent/hidden form—
↓↓ host resistance---proliferation of pathogen-----
Disease
10. • Opportunistic infection
– An infection caused by microorganisms that
are commonly found in the host’s environment
This term is often used to refer to infections
caused by organisms in the normal flora
11. Definitions
• The suffix “-emia”
– A suffix meaning “presence of an infectious agent”
• Bacteremia = Presence of infectious bacteria
• Viremia = Presence of infectious virus
• Fungemia = Presence of infectious fungus
• Septicemia = Presence of an infectious agent in
the bloodstream
12. Definitions
• The suffix “-itis”
– A suffix meaning “inflammation of”
• Examples:
–Pharyngitis = Inflammation of the pharynx
–Endocarditis = Inflammation of the heart
chambers
–Gastroenteritis = Inflammation of the
gastointestinal tract
13. Definitions
• Epidemiology
– The study of the transmission of disease
• Communicable Disease
– A disease that can be transmitted from one individual to another
• Contagious Disease
– A communicable disease that is easily spread from one individual
to another
• Noncommunicable Disease
– A disease that is not transmitted from one individual to another
14. Definitions
• Endemic Disease
– A disease condition that is normally found in a
certain percentage of a population
• Epidemic Disease
– A disease condition present in a greater than
usual percentage of a specific population
• Pandemic Disease
– An epidemic affecting a large geographical area;
often on a global scale
15. Source ….Reservoir
Source:
Person/Animal/Object from which a microorganism is
transmitted to host
Reservoir:
Natural habitat in which organism multiply, live.
Maintain parasite in nature
Asymptomatic infection
16. Source ….Reservoir
Infection Source of
infection
Reservoir
Tetanus Soil Soil
Hookworm Soil contaminated
with larva
Man
Typhoid Contaminated
water & Food
Cases/carrier
18. Human
• Commonest source
• Cases/Patients
– On basis of symptoms
• Subclinical infection
• Clinical
• Latent
• Carriers
19. Carriers
• Carriers
– Person/animal harbour infectious agent in absence of clinical
symptom & Shed organism from body via contact, air or secretions
– Inadequate Treatment/Immune response ---incomplete elimination
of organism
– Less infectious than Cases BUT More Dangerous
• Types:
– Incubatory carrier: Measels, Mumps, Polio, Hepatitis B, Influenza,
Diphtheria etc
– Healthy carrier: Subclinical infection without suffering from overt
disease converted into carrier eg. Polio, Diphtheria etc.
– Convalescent carrier: Recovered from disease but still continue
to harbour pathogen---shed from body
20. Carriers
• Based on duration:
– Temporary carrier: <6months duration
Healthy, incubatory, convalescent carrier
– Chronic carrier: Indefinite period
Hepatitis B, Typhoid, Gonorrhoea
• Based on Source:
– Contact carrier: acquire pathogen from patient
– Paradoxical carrier: Acquire infection from another carrier
21. Source &/or Reservoir
• Human
• Animal:
Zoonosis: Infection transmitted from animal to human
Bacterial (Plague-Rat)
Viral (Rabies-Dog)
Protozoal (Toxoplasma-Cats)
Helminthic (Hydatid disease-Dogs
Fungal (Zoophilic Dermatophyte infection)
• Insects
• Non living things
22. Source &/or Reservoir
• Human
• Animal
• Insects
– Arthropod borne disease
– Mechanical vector
– Biological vector: multiply/development
– Extrinsic incubation period
– Transovarian/ Transstadial passage
• Non living things
– Soil & water
– Food
25. Mode of transmission
• Vertical
– Transplacental: TORCH
– During passage through birth canal: Gp B streptococcus, HIV,
HBV, HCV
• Laboratory induced & Iatrogenic
26. Mechanisms of Microbial
Pathogenicity
• Pathogenicity and Virulence
– Pathogenicity
• Ability of a microbe to cause disease
• This term is often used to describe or
compare species
– Virulence
• Degree of pathogenicity in a microorganism
• This term is often used to describe or
compare strains within a species
27. Virulence variations
• Spontaneous
• Induced
↓----Attenuation
– Passage in unfavourable conditions
– Repeated subculture artificial media
– Prolonged storage
– Growth in presence of inhibitors/high temperature
↑---Exaltation
– Serial passage in susceptible host
28. Determinants of Virulence
• Route of entry of pathogen
– Cholera: oral ---ds
Subcutaneous- no ds
• Infective Dose/Inoculum
– Low: (10-500 organism)
• Shigella
• Cryptosporidium parvum
• EIEC
• C.jejuni
– High (106
-108
)
• Salmonella
• Vibrio cholera
29. Determinants of Virulence
• Adhesions
– Fimbriae or pilli
– Biofilm
– Non pilus adhesins
• M protein (GAS)
• Lipotechoic acid
• Invasiveness: Ability of a pathogen to spread in host tissue
after establishment
– Staphylococcus/Streptococcal infections
– Antiphagocytic factors
– Capsule
– Cell wall proteins
– Cytotoxins
30. Determinants of Virulence
• Intracellular survival
–Facultative/Obligate intracellular
Inhibition of Phagolysosomal
fusion
Legionella
M. tuberculosis
Resistance to lysosomal
enzymes
Salmonella Typhimurium
Coxiella spp
Leshimania
M. leprae
Adaptation to cytoplasmic
replications
Listeria
Rickettsia
32. Property Endotoxin Exotoxins
Nature LPS Proteins
Source Cell wall GN Secreted by G+
G-ve
Released by Cell lysis Actively secreted to surrounding
medium
Heat stability Highly Stable Destroyed at 600
C
M/A ↑IL-1, TNF Most enz mediated
Effect Broad non specific Specific to particular tissue
Tissue affinity No Yes
Fatal dose Large Small
Antigenicity Poorely immunogenic Highly
Neutralization by
Ab
Ineffective Neutralised
Used for vaccine No Toxoid form used
33. References
• Textbook of Microbiology by Ananthanarayan &
Paniker’s 10th
Edition
• Essentials of Medical Microbiology by A S Sastry &
Sandhya Bhat K, 1st
Edition
