Microbial pathogenicity

DEEPAK CHAUDHARY
SENIOR DEMONSTRATOR
GMC AMBIKAPUR

Infection and Disease
A. Definitions
B. Classification of infections
C. Methods of transmission of infection
D. Factors predisposing to microbial
pathogenicity.
E. Types of infectious diseases.

Depending on the relationship of microbes with
respect to humans, they may be divided into 2
broad groups:
1. Free living:
microbes living in natural habitat such as soil
& water. E.g. P.aeruginosa, B.subtilis,
Clostridium, Actinomyces and Micrococcus.
 Saprophyte : bacteria living on dead and
decaying organic matter. Majority of free living
organisms are saprophytes,

2. Parasitic:
an organism which lives on a living host and
derives nutrition from it, without any benefit to the
host.
 Commensals: which live in complete harmony with
the host without causing any harm to it. They
constitue the normal bacterial flora of the body.
For eg Staph.Epidermidis and E.coli.
 Pathogen : microorganism capable of producing
disease in host.
 Opprtunistic pathogen: pathogen produce disease
when body resistance is lowered.
 Pathogenicity :refers to the ability of a class of
microbe to produce disease.
 Virulence : degree of pathogenicity of a microbe.

Definitions
Infection: the lodgement and multiplication of an
infectious agent in the body.
 Colonization
 Infestation
Classification of infection:
 Primary infection: initial infection with a parasite in a
host
 Reinfection : subsequent infection with the same
parasite in the same host.
 Secondary infection: When body resistance is
lowered by a pre existing disease, a new infection
via new parasite.

Definitions
 Focal infection: when due to localised infection
generalised effects are produced.
 Cross infection: when a paitent already suffering
from a disease acquires new infection from
another host.
 Nosocomial infection: cross infection acquired in
hospitals.
 Subclinical infection: when clinical symptoms an
infection are not apparent.
 Iatrogenic infection: physician induced infection
resulting from drug therapy or investigative
procedures.

Infection:
 Depending on the source of infection:
1. Endogenous: inside
2. Exogenous : outside
 Based on clinical manifestations infections are:
 Asymptpomatic/inapparent- active infection but no
noticeable symptoms.
 Symptomatic: 2 types
 acute – symptoms last for short period
Chronic- symptoms persist for a long period.
 Latent infection - infection that is inactive or dormant
but is capable of reactivating later.
 Atypical infection : usual manifestations of disease are
not present.atypical symptoms.

Infection:
 Portal of entry: through oral, respiratory,
genitourinary, conjuctival or cutaneous
routes.
 Incubation period: the time interval
between the entry of infective agent and
the onset of clinical manifestation of
disease.

Types of infectious diseases
 A. Localised infection
An infection that is limited to a specific body part
and has local symptoms.
There is Acute inflammatory reaction at the site of
invasion to arrest infection. Eg ulcers.
 B. Generalized / systemic infection
An infection that has entered the bloodstream and
has general systemic symptoms such as fever,chil
ls, and low bloodpressure. Eg typhoid fever.

The suffix “-emia”
 –A suffix meaning “presence of an infectious
agent”
 Bacteremia = circulation of bacteria in blood
 Septicemia = bacteria circulate and multiply
in the blood, form toxic products and cause
high fever.
 Pyaemia = when pyogenic bacteria produce
septicemia with multiple absceses in internal
organs.

The suffix “-itis”
 –A suffix meaning “inflammation of”
 Examples:
 Pharyngitis = Inflammation of the pharynx
 Endocarditis = Inflammation of the heart
chambers
 Gastroenteritis = Inflammation of the
gastrointestinal tract

Infection:
Epidemiological pattern of infection:
 Endemic: when a disease is constantly is present
in particular locality. Eg typhoid fever in parts of
india.
 Epidemic: when an infectious disease spreads
rapidly and involves many persons in an area at
the same time. E.g influenza causing annual
winter epidemics in cold countries.
 Pandemic: when an epidemic spreads through
many areas of the world affecting a large number
of people within a short period of time. Eg
cholera, influenza and plague.
 Sporadic: infections that occur at irregular
intervals or only in few places, scattered or
isolated.

Transmission of infection
1. There are 3 links in the chain of transmission
of communicable diseases.
 RESERVOIR
 MODE OF TRANSMISSION
 MECHANISM OF MICROBIAL
PATHOGENICITY.

I. Reservoir :
IT refers to any human being, animal, plant,
soil or inanimate matter in which parasite
lives, multiplies and depends for its survival.
Sources of infection:
1. Endogenous sources:
organisms of normal flora that are usually
non pathogenic but occasionally behave as
pathogens outside their habitat.

I. Reservoir :
2. Exogenous sources:
a. Human cases and carriers
Commonest source of infection is man.
i. carrier: person who harbours the pathogenic
microorganism without suffering from its ill
effects.
ii. Healthy carrier: harbours the pathogen but never
suffered from the disease caused by particular
pathogen.
iii. Covalescent carrier: one who has recovered
from the disease but continues to harbour the
pathogen in his body.

I. Reservoir :
iv. Temporary carrier: this carrier state lasts for
about 6 months.
v. Chronic carrier: harbours the pathogen for
several years and sometimes for the rest of
one’s life.
vi. Paradoxical carrier: A person who acquires
the pathogen from another carrier.
vii. Contact carrier: one who acquires the
pathogen from a paitent.

I. Reservoir :
b. Animals
Infectious diseases transmitted from animals to
man are called zoonoses.
Zoonotic diseases may be:
Bacterial – bovine tuberculosis
Viral- rabies from dogs
Protozoal - leishmaniasis
Helminthic- hyadatid disease from dogs
Fungal- dermatophytes from dogs

I. Reservoir :
c. Insects
Disases transmitted by insects are called arthropod
borne diseases.
Insects transmitting diseases are called vectors.
Mechanical vectors: carry the organisms on their
wings, legs and body. Eg transmission of
typhoid bacilli to man through food by domestic
fly.
Biological vectors: the pathogen multiplies in the
body of vector. Eg female anopheles mosquito
in malarial parasite.
Extrinsic incubation period: after the entry of
pathogen in the vector the time required for the
vector to become infective.

I. Reservoir :
d. Soil and water:
 Some pathogens survive in the soil for long
periods. Eg spores of tetanus and gas
gangrene bacilli, fungus causing
histoplasmosis and parasites such as
roundworm and hookworm.
 Contamination of water with vibrio cholera
and hepatitis virus act as the source of
infection.
e. food- contaminated food act as the source of
infection in the case of food poisoning,
gastroenteritis, diarrhoea and dysentry.

II. MODES OF TRANSMISSION
i. Contact
a. Direct contact – directly through physical
contact. STD such as syphilis, gonorrhoea,
herpes simplex type 2 and AIDS.
B. Indirect contact- indirectly acquired through
fomites.
ii. Airborne transmission:
 droplets of respiratory infection are spread by
inhalation.
 Droplet nuclei (1-10 micron diameter) remain
airborne as aerosols and act as source of
infection.

iii. Ingestion
 Intestinal infections like cholera, dysentry,
food poisoning etc are acquired by ingestion
of food or drink contaminted by pathogens.
 Occurs mostly through carriers engaged in
food handling or contaminated water supply.
iv. Inoculation
 Direct inoculation in the tissues eg rabies virus
inoculation through dog bite or spores of
Cl.tetani leading to tetanus.

v. Transplacental
 Transmission of pathogen from mother to
foetus via placenta. Eg rubella virus,
toxoplasma, CMV.
 Also known as vertical transmission.
vi. Iatrogenic infection
 Infections like AIDS and Hepatitis B
transmitted during lab and surgical procedures
such as lumbar puncture, blood transfusion,
dialysis and surgery.

III. MECHANISM OF PATHOGENICITY
 Exaltation- enhancement of virulence of a strain.
 Attenuation- reduction of virulence of a strain.
 Determinants of virulence
i. Adhesion
 Specific reaction between surface receptors on the epithelial
cells and adhesins on bacteria
 Adhesins usually are pilli and fimbriae.
 It prevents the microbes from being flushed away in mucus
secretions, urine and gut peristalsis.
 Assured delivery of toxin in high conc. Directly to host cells.
 Helps in penetration of host cells.

ii.Invasiveness
Ability of organism to spread within the host tissue after
establishing infection.
Highly invasive pathogens produce generalised lesions.
Eg streptococcal infections.
Less invasive cause localised lesions.eg staphylococcal
abscess.
iii. Antiphagocytic factors
a. Capsule- it enhances virulence by preventing
phagocytosis.
b. Streptococcal M protein- M protein present on the surface
of grp A streptococci binds fibrinogen and fibrin to
bacterial cell wall masking bacterial receptors from

c. Cytotoxin- interfere with chemotaxis or kill the
phagocyte.
d. Bacterial surface antigen- Vi antigen of s.typhi
and K antigen of E.coli enable bacteria to resist
phagocytosis.

iv. Bacterial toxins
a. Exotoxins
A type of bacterial toxin with the following properties:
 Mainly produced by gram-positive bacteria and some
gram negative bacteria.
 Is secreted by the bacteria
 The action of the exotoxin does not necessarily
require the presence of the bacteria in the host
 Most exotoxins are heat labile peptide or protein
 Can be converted into toxoid after treatment with
formaldehyde.
 They are highly antigenic and stimulate formation of
antitoxin.
 Highly specific for a particular tissue. Eg tetanus toxin
for CNS.

Classes of exotoxins: Neurotoxic, cytotoxic, or
enterotoxic exotoxins
 Neurotoxins: Interfere with proper synaptic
transmissions in neurons. Eg botulinum toxin.
 Cytotoxins: Inhibit specific cellular activities, such
as protein synthesis.
 Enterotoxins: Interfere with water reabsorption in
the large intestine; irritate the lining of the
gastrointestinal tract. Eg TSST

B. Endotoxin
A type of bacterial toxin having the following
properties:
 They are lipopolysaccharide in nature
 Produced only by gram-negative bacteria
 Endotoxins are a component of the gram-
negative cell wall
 The action of endotoxin requires the presence of
the bacteria in the host. The endotoxin is
released from the cell wall as the cells die and
disintegrate.
 Alter the metabolic and bactericidal properties of
neutrophils.
 Causes fever, leucopenia, thrombocytopenia, fall
in blood pressure, circulatory collapse and DIC.

EXOTOXIN Vs Endotoxins
 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
 Protein-Polysaccharide-Lipid
Complex
 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

6. Enzymes
Coagulase
Forms fibrin clot around bacteria and prevents
phagocytosis. Eg staphylococci
Streptokinase
Dissolves fibrin clot. Eg streptococci.
Hyaluronidase
breaks down hyaluronic acid. Eg streptococcus.
Collagenase
breaks down collagen in connective tissue. Eg
Cl.perferigens

Microbial pathogenicity

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