sources of infection

1. Dr. V. S. Vatkar
Associate Professor,
Microbiology department
D Y Patil Medical College,
Kolhapur

2. OBJECTIVES
Classification of infection
Sources of infection
Modes of transmission of infection
Predisposing factors to microbial
pathogenicity (virulence factors)
Types of infectious diseases

3. Micro-organisms: classified as
 Saprophytes:
 free living organisms that live on dead or decaying
organic matter.
 Found in soil & water
 Usually unable to invade body, but when host’s
resistance is low it causes infection (opportunistic
infection).

4. Parasites:
 Lives & multiply in the host, receives nutrition from
host
 Either pathogens or commensals.
Pathogens: capable of producing disease in a
host.
Commensals: live in complete harmony with
host without causing damage to it, behave as
facultative pathogens.

5. INFECTION
Lodgment & multiplication of
parasite in or on the tissue of the
host.
Classified as:
 Primary infection: initial infection with a
parasite in a host.
 Reinfection: subsequent infection with a same
parasite in a same host.
 Secondary infection: body resistance of the host
is lowered by pre-existing inf, a new parasite
sets up an inf.

6.  Focal inf: sepsis or inf at localized sites e.g.
appendix or tonsils
 Cross inf: patient is already suffering from a
disease, a new inf set up from another host or
another external source.
 Nosocomial inf: cross inf occurring in a
hospital.
 Iatrogenic inf: physician induced inf resulting
from investigative, therapeutic or other
procedures.

7.  Subclinical inf: clinical symptoms of a
parasite are not apparent, also called as
inapparent inf.
 Depending on the source of inf ( inside or
outside host’s body) called as
Endogenous or Exogenous inf.
 Latent inf: following inf some parasites
remain latent or hidden in host’s body &
proliferate & produce clinical symptoms
when body resistance is low.
 Atypical inf: inf in which typical or
characteristic symptoms are not seen.

8. Sources of infection
Endogenous sources: normal flora
occasionally produce a disease outside
their habitat. e.g. E.coli causing UTI,
Strep viridance : present in mouth causes
infective endocarditis.
Exogenous sources: man (carriers),
animals, insects, soil & water, food

9. 1) Man: CARRIERS: person who harbors
the pathogenic micro-organisms without suffering
from a disease.
Types of carriers:
Healthy carrier: harbors pathogenic
organisms without suffering from a
disease.
Convalescent carrier: who recover from
a disease and continue to harbor the
pathogen

10. Temporary carrier: lasts less
than six months
Chronic carrier: lasts for several
years or sometimes life long.
Paradoxical carrier: who
acquires inf from another carrier.
Contact carrier: who acquires
inf from the patient.

11. 2) ANIMALS:
zoonotic
diseases: e.g.
Plague,
Brucellosis,
Rabies, Hydatid
cyst,
Toxoplasmosis
etc.
3) INSECTS:
like mosquito,
ticks, fleas,
louse, flies
transmits inf
called Vectors.
Mechanical :
Biological :
pathogen
multiply inside
the vector

12. 4) Soil & water:
some pathogens survive for a long period
in soil: e.g. spores of Cl.tetani, fungi like
Histoplasma capsulatum, Nocardia spp.
Eggs of hookworm & roundworm etc.
Contaminated water : cholera, hepatitis ,
guinea worm infection (Cyclops in water)

13. 5) Food
Contaminated food: presence of
pathogen in food e.g. food poisoning
due to staphylococci, pre-existing
toxins in canned food (meat), other
animal products (salmonellosis).

14. METHODS OF TRANSMISSION
OF INFECTION
 a) CONTACT: direct: STD= HIV, Syphilis.
Indirect: fomites like clothings, pencils, toys etc.
 b) INHALATION: respiratory inf: Influenza,
tuberculosis (microbes shed in environment thr’
secretions form nose or throat during sneezing,
coughing) Large drops of such secretion fall on
ground & dry there, such a droplet is resistant &
remain viable for a long period.

15.  c) INGESTION: ingestion of pathogens
thr’ contaminated food & water, intestinal
inf like cholera, hepatitis, food poisoning,
dysentery.
 d) INOCULATION: directly inoculated
into the tissue of the host. E.g. spores of
tetanus present in deep wound, rabies
virus inoculated subcutaneously by dog
bite, Hepatitis B & HIV : thr’
contaminated syringes, blood transfusion
etc

16. e) CONGENITAL: form mother to
fetus : inf crosses placental barrier
e.g. cong syphilis, rubella, CMV,
toxoplasma etc
f) Iatrogenic & Lab inf: inf
transmitted during administration of
injection, lumbar puncture, exchange
transfusion, dialysis, organ transplant
surgery etc.

17. FACTORS PREDISPOSING TO
MICROBIAL PATHOGENICITY
(VIRULENCE FACTORS)
Ability of micro-organism to produce a
disease is called PATHOGENICITY.
VIRULENCE: applied to the same property
in a strain of micro-organism.
Enhancement of virulence : EXALTATION.
Reduction of a virulence : ATTENUATION :
achieved by passage through unfavorable hosts, repeated
subcultures in artificial media, growth in high temp or in
weak antiseptics.

18. i) ADHESION
 Attachment of the bacteria to the body surface.
Specific reaction between the surface receptors
of host cell and adhesive structure of bacteria,
they are known as Adhesins.
 Structures like fimbria, pili, colonization factors
etc
 Usually protein in nature & antigenic in nature.

19. ii) INVASIVENESS
Ability of pathogen to spread in the host
tissue after establishing infection.
Generalized or localized lesions e.g.
staphylococcal inf
Lack of invasiveness e.g. tetanus toxins

20. iii) TOXIGENESITY
 EXOTOXINS
 Protein
 Heat labile
 Readily separated from
cultures by physical
means like filtration
 Enzymic action
 Specific tissue affinity.
 Diffuse in surrounding
medium
 Very minute dose
required
 Highly Agenic
 Neutralised by sp Ab
 ENDOTOXINS
 Lipopysaccharides
 Heat stable
 Obtained by cell lysis,
does not diffuse in
surrounding medium
 No enzymic action
 Effect : non-specific
 No specific tissue
affinity.
 Active in large doses.
 Weakly Agenic
 Not neutralized with Ab

21. iv) PLASMID:
genes coding for some virulence are
plasmid mediated. E.g. surface Ag
responsible for colonization of
intestinal mucosa by E.coli &
enterotoxins produce by E.coli &
Staph aureus , multiple drug
resistance plasmids responsible for
antibiotic resistance.

22. v) BACTERIOPHAGE:
phage directed virulence of diphtheria
bacilli, genes produce toxins
(corynephage).
vi) COMMUNICABILITY:
ability of parasite to spread from one
host to another. Survival &
distribution of parasite in community,
develops epidemics & pandemics.

23. vii) OTHER BACTERIAL
PRODUCTS
 Some bacterial products other than toxins contribute
to decrease host resistance e.g. staphylococcal enz
- coagulase which prevents phagocytosis.
- Fibrinolysin: promotes the spread of infection by
breakingdown the fibrin barrier in the tissue.
- Hyluronidase : breake down of hyluronic acid
(component of intercellular connective tissue), causes
spread of the disease.
- Leucocidins: damages polymorphonuclear cells.
- Hemolysins: capable of destroying erythrocytes

24. viii) Bacterial appendages
capsule : resist phagocytosis e.g.
pneumococci, H.influenzae, K.pneumoniae etc
Some bacterial surface Antigens
(Ag): e.g. Vi Ag of Salmonella typhi & K
Ag of Escherichia coli : help the bacteria to
resist phagocytosis and lytic activity by
complements

25.  Biofilms : well organized micro - colonies of
bacteria enclosed in self produced extracellular
polymer matrices known as GLYCOCALYX.
 Separated by water channels that removes water &
delivers the nutrients
 Two types of biofilms
 Monomicrobial biofilm
 Polymicobial biofilm.

26. Infective dose:
 Minimum Infective Dose (MID) or
Minimum Lethal Dose (MLD):
- is min no of bacteria to produce clinical evidence of disease.

27. TYPES OF INFECTION
Endemic: ds constantly present in a particular area
e.g. Typhoid fever common in most of the parts of
India.
Epidemic: ds spreads rapidly, involving many
persons in an area at same time. Influenza v epidemic
Pandemic: ds that spread through many areas of
the world, involving large no of people within a short
period e.g.H1N1, cholera etc

28. Bacteremia: bacteria present in blood
Septicemia: bacteria circulate &
multiply in bl & forms toxic products
causes high gr fever.
Pyemia: pyogenic bacteria produce
septicemia with multiple abcesses in
internal organs.

29. THANK YOU