This document discusses various types of symbiotic relationships between organisms, including parasitism, commensalism, and mutualism. It provides examples of each type of relationship. It also discusses the definitions and characteristics of hosts, parasites, normal flora, pathogens, toxins, and infections. The key types of symbiosis are defined as commensalism, mutualism, and parasitism. Commensalism benefits one organism without affecting the other. Mutualism benefits both organisms, while parasitism benefits the parasite at the expense of the host.

2.  The term host & parasite relationship is also known as
SYMBIOSIS means the coexistence of two organisms , in
which one extracts the nutrition from other.
 PARASITOLOGY is a branch of microbiology, refers to the
scientific study of parasitism.
 A HOST is any organism that allows another
organism/bacteria to live in or on it.
 The PARASITE is the organism living on or in the host.

3. WHAT IS SYMBIOSIS?
Symbiosis refers to the “living together” of organisms and includes
commensalism, mutualism, and parasitism.
TYPES OF SYMBIOSIS RELATIONSHIPS:
 Commensalism: The relationship in which parasite driving
nourishment from a host without causing any harms or benefit to
the host.
EXAMPLE:
Orchids: Some orchids grow on trees and that does not harm the
tree.
 MUTUALISM: Association between organisms of two different
species in which both organisms get benefits.

4. TYPES OF MUTUALISM:
 Obligate mutualism
 Facultative mutualism
 In obligate mutualism, the survival of one or both
of the organisms involved is dependent upon the
relationship.
 And in facultative mutualism, both organisms
benefit from but are not dependent upon their
relationship for survival.

5. EXAMPLE OF MUTUATISM:

6. PARASITISM:
Parasitism is type of symbiotic relationship, or long-term
relationship between two species, where one member, the
parasite, gain benefits that come at the expense of the host
member. The word parasite comes from the Latin form of the
Greek word (parasitos), “meaning one eats at the table of
another”.
 TYPES OF PARASITES.
 1. Ectoparasites
 2. Endoparasites

7. ECTOPARASITISM and ENDOPARASITISM:
 Ectoparasites are parasites that live on the
outside of the host’s body, such as lice and ticks.
Endoparasites are parasites that present inside
the host’s body, such as nematodes and
hookworms.

8. EXAMPLE OF PARASITISM;

9. Normal Flora
 Normal flora are the microorganisms that live on another
living organism (human or animal) or inanimate object
without causing disease.
 DISTRIBUTION OF NORMAL MICROFLORA ON THE SKIN:
The microflora of the skin consists primarily of gram positive
bacteria. For example staphylococcus epidermilis (gram
positive cocci) and propionibacterium acne (gram positive
bacilli).

10. NORMAL MICROFLORA OF THE ORAL CAVITY:
 The normal microflora of the oral cavity (mouth)
consists of microorganisms that possess ability to resist
mechanical removal and firmly adhere to surface like
gums and teeth.

11. NORMAL MICROFLORA OF THE
GASTROINSTESTINAL TRACT:
 Human gastrointestinal tract, the site of food
digestion, consists of the stomach, small
intestine, and large intestine. The composition of
the gastrointestinal flora in humans varies
considerably and is somewhat depend on diet.

12.  HUMAN GASTROINTESTINAL TRACT SHOWING
NORMAL MICROBIAL FLORA FOUND IN DIFFERENT
PARTS OF IT:

13. VIRULENCE AND ITS DETERMINANTS
 The term virulence (Latin virulentia, from
virus, poison) refers to the degree or intensity
of pathogenicity.
 Pathogenic bacteria are able to produce disease because
they possess certain biochemical or genetic traits that
makes them pathogenic or virulent.
 The total of the characteristics that allow a given
bacterium to produce disease are the
pathogen's determinants of virulence

14. WHAT IS BACTERIAL PATHOGENESIS
 Pathogenicity is the ability of a microorganism to cause disease
in another organism and this process is known as bacterial
pathogenesis.
 Potential pathogen that lives in a commensal or parasitic
relationship without producing disease.
Some pathogens do not associate with their host except in the case
of disease. These bacteria may be thought of as obligate
pathogens.
 Some Bacteria cause disease in a immuno -compromised host
which typically would not occur in a healthy (noncompromised)
host are acting as opportunistic pathogens.

15. ENTRY OF PATHOGEN INTO HOST:
 Most pathogens must gain access to the host tissues,
penetrate or evade host defenses and damages the host
tissue. Pathogens can enter to human body or other host
through several avenues called portals of entry.
PORTALS OF ENTRY:
 MUCOUS MEMBRANES (moist mucosa)
 SKIN (karatinized cutaneous membrane),
 PARENTERAL ROUTE:

16. WHAT IS AN INFECTION?
When a parasite is growing and multiplying
within or on a host, the host is said to have
infection.
Organisms that induce infectious disease are
referred as pathogenicity and the organism is called
as pathogen.

17. How does the Host Defenses itself??
 The host defenses may be of such a degree that infection
can be prevented entirely. Or, if infection does occur, the
defenses may stop the process before disease is
apparent. the host defense mechanisms are divided
into two groups:
1. Innate defenses. Defenses common to all healthy animals
Since they are inherent to the host.
2. Inducible Defenses. Defense mechanisms that must be
induced or turned on by host exposure to a pathogen
The inducible defenses are synonymous with acquired or
adaptive immunity and involve the immunological
responses to a pathogen causing an infection.

18. WHAT IS TOXINS?
 Toxins are usually poisonous substance produced by
microbes which tend to cause widespread
damage/disease in host. They may be necessary for
virulence.
TYPES OF TOXINS:
 EXOTOXINS:
Exotoxins are soluble, heat-labile, proteins (a few enzymes)
that usually are released into the surroundings as the
bacterial pathogens grows. In generally exotoxins are
produced by gram-positive bacteria, although some gram-
negative bacteria also make exotoxins.

19. Fig: Exotoxin secretion

20. There are three types of exotoxins:
AB TOXINS:
It is composed of two parts, where A
is the enzymes that disrupts some
cell activity, while B-binds surface
receptors to bring A into the host
cell, e.g., botolinum and tetanus
toxin.

21. Action of exotoxin

22. MEMBRANE DISRUPTING TOXINS:
It causes lysis of the host cell by disrupting the
integrity of the plasma membrane; there are two
subtypes of membrane-disrupting exotoxins. The
first is protein that binds to the cholesterol portion
of the host plasma membrane, inserts itself into the
membrane, and form a channel (pore). This causes
the cytoplasmic contents to leak out. Also because
the osmolarity of the cytoplasm is higher than the
extercellular fluid, this causes a sudden influx of
water into the cell, causing it to swell andrupture.

23. The two specific examples of
membrane-disrupting exotoxin:
Example1: Leukocidines; make protein channels
in phagocytic leukocytes.
 Example2: haemolysins; make protein channels
in RBCs (haemolysis: Streptococcus pyogenes).

24. Fig: A channel-forming (pore-forming) type
of exotoxin inserts itself into the normal
host cell membrane and makes an open
channel (pore). Formation of multiple pores
causes cytoplasmic contents to leave the cell
and water to move in, leading to cellular
lysis and death of the host.

25. SUPER ANTIGENS:
 They are the bacterial proteins that cause proliferation of
T-cells and release of cytokines excessive cytokines can cause
fever, nausea, vomiting, diarrhoea, shock and death (septic
shock), e.g., toxic shock syndrome (staphylococcus), e.g.,
enterotoxins: staphylococcal food poisoning.
ENTEROTOXIN:
An enterotoxin is a protein exotoxin released by
microorganism that targets intestines. Enterotoxins are
produced in or released into intestines.

26.  Enterotoxins are pore-forming toxins. Hence, they create
pores in the epithelial cells of the intestine wall. When the
enterotoxins increase the permeability to chloride ions in the
intestinal mucosal cells, it causes secretory diarrhea.
Staphylococcus aureus and E. coli are two bacterial species
which can create such conditions by enterotoxins.
In general, enterotoxins are produced by gram negative
bacteria can also produce enterotoxins. For example, Vebrio
cholera is a well-known enterotoxin producer and is a gram-
negative bacterim.

27. ENDOTOXINS:
 They are the part of the outer membrane portion of
the cell wall of gram negative bacteria, e.g.,
Lipopolysaccharide (LPS) which is released when
dead cells lyse in blood, causes macrophages to
release high levels of cytokines resulting is chills,
fever, weakness, aches, small blood clots, tissue
necrosis, shock and death.

28. Endotoxins

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