Infection by Dr. Rakesh Prasad Sah

1. INFECTION
By Dr. Rakesh Prasad Sah
Assistant Professor, Microbiology

2. INFECTION
INTRODUCTION
DEFINITION
CLASSIFICATION
SOURCE OF INFECTION
METHOD OF TRANSMISSION OF INFECTION
TYPES OF INFECTIOUS DISEASES

3. INFECTION
Introduction :
-Infection and immunity involve interaction between the animal
body (host) and the infecting microorganism.
-Microorganisms can be classified as saprophytes and
parasites.
1. Saprophytes : These are free - living microbes that live on
dead or decaying organic matter. They are found in soil and
water.
2. Parasites : In a microorganism which live on a living host
and derives nutrition from the host, without any benefit to the
host.

4. Parasites
Pathogens
Primary
Pathogen
Opportunistic
Pathogens
Commensals
Live in complete harmony with
the host without causing any
harm to it.

5. DEFINITIONS
 Infection : The lodgement and multiplication of a
parasite in the body is known as infection.
 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

6. CLASSIFICATION
Infection can be classified in various ways.
1) Primary infection : Initial infection with a parasite in a
host is termed as primary infection. e.g. Shigella
dysenteriae.
2) Reinfections : Subsequent infections by the same
parasite in the host are termed reinfections.
3) Secondary infection : When the body resistance is
lowered by a preexisting infectious disease, a new
parasite sets up an infection this is termed Secondary
infection. E.g. Bacterial pneumonia following viral lung
infection.

7. 4) Cross infection : when a patient already suffering
from a disease acquires a new infection from another
host or another external source it is know as cross
infection.
5) Nosocomial infection : Cross infection occurring in
hospitals are called nosocomial infections.
6) Subclinical infection : It is one where clinical affect
are not apparent.
7) Iatrogenic infection : As physician induced
infection resulting from drug therapy or investigative
procedures.

8. 8) Atypical infection : Is one in which the typical or
characteristic clinical manifestations of the particular
infectious disease are not present.
9) Latent infection : Following infection some parasites
may remain in a latent or hidden from host tissues and
may they proliferate and produce clinical disease when
the host resistance is lowered. It is know as latent
infection.

9. SOURCE OF INFECTION
 2 types
1. Endogenous
2. Exogenous

10. Exogenous sources
1. Humans cases & carrier: The commonest source of
infection in humans are humans themselves. The
parasite may originate from a patient or a carrier.
 Carrier : A carrier is a person who harbours the
pathogenic microorganism without suffering from its ill
effects. Several types of carrier are known.
 Healthy carrier : One who harbours the pathogen but
has never suffered from the disease caused by the
particular pathogens.

11.  Convalescent carrier : One who has recovered from
the disease but continues to harbour the pathogen in his
body.
 Temporary carrier : It depends on the duration of
carrier. It last for less than six months.
 Chronic carrier : When carrier state last for several
years and sometimes even for the rest of one’s life.
 Paradoxial carrier : Who acquires the pathogen from
another carrier.
 Contact carrier : A person who acquires the pathogen
from a patient.

12. Carriers are very important source of infection to spread the
disease in a community
2. Animals : Many pathogens are capable of causing
infection & animal. Animal act as a source of human
infection. Infectious diseases transmitted from animals to
man are called zoonosis.
Zoonotic diseases may be:
Bacterial : Bovine tuberculosis, leptospira
Viral : Rabies from dog.
Protozoal : Leishmaniasis
Helminthic : Hydatid disease from dogs.
Fungal : Dermatophytes from cats and dogs.

13. 3) Insects : The disease caused by insects are called
arthropod borne disease.
- Blood sucking insects such as mosquitoes ticks, mites, flies,
fleas and lice may transmit a number of pathogen to man.
- Insects transmitting these pathogens are known as vectors.
They are two types –mechanical and biological vector.
i) Mechanical – e.g. transmission of typhoid bacilli to man
through food by domestic fly.
ii) Biological – e.g. female anopheles mosquito in malarial
parasites.

14. 4) Soil and water : Soil may be serve as source of
parasiting infection like round worm & hook worm.
- Spores of tetanus bacilli remain viable in soil for a long
time.
- Fungi like histoplasma capsulatum also survive in soil and
causes human infections.
Water – vibrio cholera , infective hepatitis virus.
5) Food : contaminated food may be source of infection
Presence of pathogen in food may be due to external
contamination e.g. food poisonings by staphylococcus,
diarrhoea, dysentery.

16. METHODS OF TRANSMISSION OF INFECTION
1)Contact : infection may be acquired by contact which may be
direct or indirect
Direct contact : syphilis and gonorrhea
Indirect contact : May be through the agency of fomites
which are spread through objects such as clothing , pencil
or toys .which is contaminated by a pathogen from one person
to another person e.g. diphtheria, trachoma through face
towel.
2) Inhalation : respiratory infection such as influenza and
tuberculosis a transmitted by inhalation of pathogen.
- Secretion from the nose and throat during speaking, sneezing
or cough.

17. 3) Ingestion : intestinal infection are generally acquired
by the ingestion of food or drink contaminated by
pathogen. Infection transmitted by ingestion may be
water born (cholera), food born (food poisoning)or hand
born( dysentery).
4) Inoculation : Pathogen may be inoculated directly into
the tissue of host.
e.g. tetanus spores - implanted in deep wounds
rabies virus - by dog bites
arbovirus - infected by insect vector
HIV , Hep.B may be transmitted through infected blood or
syringes.
5) Insects (vectors) : Insects may act as mechanical or
biological vector of infectious diseases

18. 6) Congenital / Transplacental : Some pathogens are
able to cross the placental barrier and infect the fetus
in utero.
-intrauterine infection with the rubella virus specially during
pregnancy, may interfere with organogenesis and lead to
congenital malformation such infection are known as
teratogenic infection.
7) Laboratory infection/iatrogenic: Infection may be
transmitted during procedures like injection, lumbar
puncture, catheterization etc if proper care is not taken.

19. FACTORS PREDISPOSING TO MICROBIAL PATHOGENICITY
Related Terms
 Portal of Entry : infectious agents enters the body by one of the
following routes : oral, respiratory, genitourinary, cutaneous.
 Incubation Period : time interval between the entry of infective agent
and the onset of clinical features.
 Pathogenicity : ability of a class of microbes to produce disease.
 Virulence : degree of pathogenicity of a microbe. OR ability of a
strain of a microorganism to cause disease.
 Exaltation : Enhancement of virulence of strain is known as
exaltation.
 Attenuation: Reduction of virulence of a strain is known as
attenuation.

20. CHARACTERS OF PATHOGENS
 Bacteria should be able to enter the body.
 Organism should be able to multiply in the tissue.
 They must be capable to resist the host defense.
 They should be able to damage the tissue.

21. MOLECULAR DETERMINANTS OF PATHOGENICITY
Production and
delivery of
various factors
Attachment to
host tissues
Replication
and evasion of
immunity
Damage to host
tissues

22. MICROBIAL MECHANISMS OF PATHOGENICITY:
HOW MICROORGANISMS CAUSE DISEASE

24. DETERMINATS OF VIRULENCE
 Adhesion
 Invasiveness
 Antiphagocytic factors
 Survival within the phagocyte
 Bacterial toxins
 Enzymes
 Siderophore and iron aqcuisition
 Genetic Factors
 Infecting dose
 Route of infection
 Communicability

25. ADHESION
 Attachment between surface receptors of host cell &
adhesive str.
 Occurs only when receptor is free.
 Part of bacteria attaches with host cells or by producing
some substance is called Adhesins.
 Advantage of adherence
 Prevents the microbe being flushed
 Close adherence of the bacterium assures that the toxin will be
deliver ed in high concentrations directly to the host cells
 If invasive in nature, adherence helps in penetrating host cells.
 Occurs generally thru Pilli & Fimbriae.

26. Adherence factor Description
Filamentous hemagglutinin Causes adherence to erythrocytes
Fimbriae Help attach to solid bacteria to solid
surfaces
Glycocalyx or capsule Inhibits phagocytosis and aids in
adherence
Pili Bind bacteria together for transfer of
genetic material
Slime Tenacious bacterial film that is less
compact than a capsule
Teichoic and lipoteichoic
acid
Cell wall components in gram
positive bacteria that aid in adhesion

28. INVASIVENESS
 Ability of an organism to spread within the host tissues
after establishing infection.
 Highly invasive pathogens produce, spreading or
generalised lesions (e.g. streptococcal infection).
 Less invasive pathogens cause localised lesions (e.g.
staphylococcal abscess).

29. ANITIPHAGOCYTIC FACTORS
 Macrophages and polymorhs
play a role in phagocytosis of
microbes.
 These factors include:
 Capsule

30.  Cytotoxin interfere with chemotaxis or kill the
phagocyte.
 Bacterial surface antigens  Vi antigen of S. typhi and
K antigen of E. coli enable these bacteria to resist
phagocytosis and lytic activity of complement.

31. SURVIVAL WITHIN THE PHAGOCYTE
 Survive intracellularly by interfering with formation of
phagolysosome (Phagosome + Lysosome).
Interference woth the oxidative burst
 Staph aureus  Superoxide H2O2 H2O + (O)
 Listeria monocytogens  superoxide dismutase  neutralises
oxygen radicals.
Prevention of fusion and degranulation
 Chlamydia
 B. pertussis  adenylate cyclase  inhibitor of degranulation.
 Resistance to lysosomal enzymes  M. tuberculosis, M.
lepeaemurium
Escape from phagosomes  Rickettsiae
Catalase

32. BACTERIAL TOXINS
Exotoxin Endotoxin
Proteinous in nature Lipopolysaccharides in nature
Produce manily by Gram positive but Gram negative
also.
E.g. Pseudomonas auereginosa, Shigella
Produce only by Gram
negative
Heat labile because of proteinous in nature Heat stable
Diffuse into surroundings No diffusion
Actively secreted by cell Part of cell wall
Enzymatic in action Non-Enzymatic/mediated by
interleukin (IL-1) & TNF
Readily separate from culture by physical means of
filtration.
Obtain only by lysis
Specific tissue affinity of act on particular tissue or site
like
On Neuron – Neurotoxin & On Intestine - Enterotoxin

34. BACTERIAL TOXINS
Exotoxin Endotoxin
Antibody production occurs No antibody production
Neutralized by antibody Ineffective
Toxoid – It is not toxic. Cannot be toxoided
Active in minute dose (High potency) Active in high dose (Low
potency)
More lethal – It can be measured by LD50 or MLD
(Minimum Lethal Dose)
 LD50 – The dose which kills the 50% individual in a
study.
 MLD – Minimum concentration which can destroy
life
Less lethal
Genes are located in the plasmid or Bacteriophage. Genes are located in the
chromosomes
No Fever Fever

35. MODIFICATION OF TOXIN TO TOXOID
chemical
modification
Toxoid
toxin moiety antigenic determinants
Toxin

36. MODE OF
ACTION OF
EXOTOXIN

37. MODE OF ACTION OF ENDOTOXIN

38. ENZYMES
 IgA proteases  cleave IgA which protects mucosal surfaces.
 Fibrionolysin (Kinase)  enhances spread of bacteria by
dissolving fibrin clots e.g. streptococcus and staphylococcus
 Hyaluronidase  breaks down hyaluroni acid and helps in
spread of infection. E.g. Streptococcus
 Coagulase deposition of fibrin around the bacteria and thus
prevent phogocytosis e.g. Staphylococcus
 Collagenase  breaks down collagen in connective tissue and
contributes in spread of infection. E.g. Clostridium perfingens.

39.  Siderophore production
 Genetic factors

40. INFECTING DOSE
 Adequate no. of bacteria is required for successful
infection.
 MID : Minimum number of bacteria required to produce
clinical evidence of infection in a susceptible animal
uder std. conditions.
 MLD : death in animal.
 ID50
 LD50

41. ROUTE OF INFECTION

42. TYPES OF INFECTIOUS DISEASES
 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

43.  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 bacteria in the
bloodstream, multiply and form toxic products
in blood.

44. iii) Pyaemia : It is condition where pyogenic bacteria produce
septicemia with multiple abscess in internal organs e.g. –
kidney, spleen, liver.
 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

45. Depending on their spread in the community infectious
diseases may be classified into different types :
Endemic diseases : Are those which are constantly presence in
a particular area e.g. - Typhoid fever is endemic in most parts
of India.
Epidemic diseases : is one that spreads rapidly involving many
persons in an area at the same time
e.g.- meningococcal meningitis
Pandemic : Is an epidemic that spreads through many areas of
the world involving very large no of people with in short period.
e.g. cholera, influenza, plagues.