The document discusses various types of microorganisms, categorizing them into saprophytes, parasites, commensals, pathogens, and opportunistic pathogens. It outlines the concept of infection and different forms including primary, secondary, and nosocomial infections, as well as sources and modes of transmission of infections. Additionally, it identifies factors that contribute to microbial pathogenicity such as adhesion, invasiveness, and the production of toxins.

1. INFECTION
C.SWAMINATHAN
Assistant Professor of Microbiology
Vivekanandha College of Arts and Science for Women (Autonomous)
Tiruchengode - 637 205
Tamilnadu, South India

2. TYPES OF MICROORGANISMS
1) Saprophytes
 Free living microorganisms that live on dead or decaying organic
matter
2) Parasites
 Microorganisms that live on a living host and derive nutrition from
the host without any benefit to the host and causes harm to the
infected host
3) Commensals
 Microorganisms that live on a living host without causing any injury to
the host

3. 4) Pathogens
 A microorganism capable of causing disease in immunocompetent
people
5) Opportunistic pathogens
 A microorganism that is capable of causing disease only in
immunocompromised people

4. INFECTION
“Lodgement and multiplication of a parasite in or on the tissues of a
host constitutes infection”
1) Primary Infection
 Initial infection with a parasite in a host is termed primary infection
2) Reinfection
 Subsequent infection with the same organism in the same host
3) Secondary infection
 Infection with a new organism in a host whose body resistance is
already lowered by a pre-existing infectious disease

5. 4) Cross infection
 Infection with a new organism from another host or another external
source in a patient who is already suffering from a disease
5) Nosocomial infection
 Cross infections acquired in hospitals are hospital acquired /
nosocomial infections
6) Iatrogenic infection
 Physician induced infections resulting from investigative or therapeutic
procedures

6. Based on clinical effects
1) Inapparent infection / subclinical infection
 Clinical effects are not apparent
2) Atypical infection
 Typical or characteristic clinical manifestations of the particular
infectious disease are not present
3) Latent infection
 Some organisms may remain in a latent or hidden stage in host and
subsequently they multiply to produce clinical disease when host
resistance is lowered

7. SOURCES OF INFECTION
ENDOGENOUS SOURCE
 Source of infection is the normal bacterial flora
 Normal flora are usually not pathogenic but in certain situation
become pathogenic and cause infections
• e.g. E.coli present as normal flora of the intestine may cause UTI
EXOGENOUS SOURCE
 Source of infections is from outside the host’s body
 Exogenous source include humans, animals, insects, food and water

8. 1) Humans
A) Patient
B) Carrier
 Carrier is a person who harbours pathogenic microorganisms without
showing any signs and symptoms of disease
a) Healthy carrier
 Healthy carrier is one who harbours the pathogen without ever
suffering from the disease caused by the pathogen
b) Convalescent carrier
 A convalescent carrier is one who has recovered from the disease and
continues to harbour the pathogen in his body

9. c) Temporary carrier
 Temporary carrier is the host who harbours the microorganisms up to
six months after recovering from the disease caused by the same
pathogen
d) Chronic carrier
 Chronic carrier is the host who harbours the microorganisms for many
years after recovering from the clinical disease caused by the same
pathogen
2) Animals
 Cattles – Brucellosis, TB, Anthrax
 Goat – Brucellosis, Sheep – Anthrax
 Dog – Rabies, Rat - Leptospirosis

10. 3) Insects
 Insects that transmit infections are called vectors
 The disease transmitted by insects are called arthropod borne
diseases
Mechanical vector
 Flies – transmit dysentery and typhoid fever
Biological vector
 Female anopheles mosquito – transmit malaria
4) Food
 Food items contaminated with pathogens act as source of infection
and cause diarrhoea, dysentery, food poisoning and gastroenteritis

11. 5) Water
 Water contaminated with pathogens act as source of infection and
cause
Leptospirosis
Cholera
Dysentery
Hepatitis A

12. MODE OF TRANSMISSION1) Contact
Direct contact
 Occurs through act of touching, kissing, sex, etc
 e.g. Common cold, Staphylococcus infection
 e.g. STD – Gonorrhea, Syphilis, AIDS
Indirect contact
 Occurs through inanimate objects
 e.g. Influenza, TB
2) Inoculation
 Infection can be transmitted by inoculation of microorganisms directly in to
tissues of the host
 e.g. Tetanus, Rabies

13. 3) Ingestion
 Ingestion of food and water contaminated with microorganisms can
transmit infections
 e.g. Cholera, Typhoid, Food poisoning, Hepatitis A, Poliomyelitis
4) Inhalation
 Measles, Influenza, Whooping cough, TB
5) Insects
 Rat flea – transmit plague
 Mosquito – transmit malaria
 Lice – transmit relapsing fever

14. FACTORS PREDISPOSING TO MICROBIAL
PATHOGENICITY
1) Adhesion
 Attachment of bacteria to body surfaces
 Adhesions – Pili, Glycocalyx
2) Invasiveness
 Ability of a pathogen to spread in the host tissues after establishing
infection
 e.g. Streptococcal septicaemia following wound infection

15. 3) Toxigenicity
Exotoxins
 Proteins
 Heat stable
 Highly antigenic
 1 mg tetanus toxin – kill more than 1 million guinea pigs
 3 kg botulinum toxin – can kill all the inhabitants of the world
Endotoxins
 Lipopolysaccharide
 Heat stable
 Weakly antigenic

16. 4) Plasmids
 Plasmid borne virulence factors are
 Enterotoxin production by E.coli and S.aureus
5) Bacteriophages
 In diphtheria bacilli, the gene for toxin production is present in tox+
corynephages
6) Communicability
 Ability of a microorganism to spread from one host to another
7) Other Bacterial Products
 Coagulase – prevents phagocytosis by forming a fibrin barrier around
the bacteria

17.  Fibrinolysin – promotes the spread of infection by breaking down fibrin
barrier in tissues
 Hyaluronidase – split hyaluronic acid and facilitate the spread of
infection along tissue spaces
 Leucocidins – damage leukocytes
 Hemolysin – destroys RBCs
8) Bacterial appendages
 Capule of K.pneumonia – prevents phagocytosis
 Vi Ag of S.typhi and K Ag of E.coli help the bacteria to withstand
phagocytosis

18. 9) Biofilm
 Aggregate of interactive bacteria attached to a solid surface or to
each other and encased in an exopolysaccharide matrix.