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Lecture -1 pathogenic bacteria
Dr . Oruba Kuttof al- Bermani
Host Microbe- relationships
Microorganisms display a variety of complex relationships with other
microorganisms and with larger forms of life that serve as hosts for them.
A host is any organism that harbors another organism.
Symbiosis
Symbiosis is an association between two (or more) species. Meaning
“living together,” the term symbiosis encompasses a spectrum of
relationships. These include mutualism, commensalism, and parasitism.
mutualism
in which both members of the association living together benefit from the
relationshiplarge numbers of Escherichia coli live in the large intestine of
humans. These bac teria release useful products such as vitamin K, which
we use to make certain blood-clotting factors. Although the relationship
is not obligatory, E. coli does make a modest contribution toward
satisfying our need for vitamin K. The bacteria, in turn, get a favorable
environment in which to live and obtain nutrients 2`Many of the
bacteria on human skin are mutualistic. However, most of these
organisms are commensals, which indirectly benefit us by competing
with harmful organisms for nutrients and preventing those organisms
from finding a site to attach to and invade tissue
parasitism,
in which one organism, the parasite, benefits from the relationship,
whereas the other organism, the host, is harmed by itSome parasites
obtain com fortable living arrangements by causing only modest harm to
their host. Other parasites kill their hosts, thereby rendering themselves
homeless . The most successful parasites are those that maintain their
own life processes without severely damaging their hosts
commensalism
in which two species live together in a relationship such that one benefits
and the other one neither benefits nor is harmed. For example, many
microorganisms live on our skin surfaces and utilize metabolic products
secreted from pores in the skin. Because those products are released
whether or not they are used by microorganisms, the microorganisms
benefit, and ordinarily we are nei ther benefited nor harmed. The line
between commensalism and mutualism is not always clear. By taking up
space and utilizing nutrients, microbes that show mutualistic or
commensalistic behavior may prevent colonization of the skin by other,
potentially harmful, disease-causing microbes—a phenomenon known as
microbial competition. Hence these symbiotic relationships confer an
indirect benefit on the host. There is also a fine line between parasitism
and commensalism. In healthy hosts, many microbes of the large intestine
form harmless associations, simply feeding off digested food materials.
But a ‘harmless’ microbe could act as a parasite if it gains access to a part
of the body where it would not normally exist. The situation in which
both species harm each other without either benefiting is called
antagonism.
Contamination, Infection, and Disease
Contamination, infection, and disease can be viewed as a sequence of
conditions in which the severity of the effects microorganisms have on
their hosts increases. Contamination means that the microorganisms are
present. The surfaces of skin and mucous membranes can be
contaminated with a wide variety of microorganisms. Infection refers to
the multiplication of any parasitic organism within or on the host’s body.
If an infection disrupts the normal functioning of the host, disease occurs.
Disease is a disturbance in the state of health wherein the body cannot
carry out all its normal functions . Both infection and disease result from
interactions between parasites and their hosts. Sometimes an infection
produces no observable effect on the host even though organisms have
invaded tissues. More often an infection produces
observable
disturbances in the host’s state of health; that is, disease occurs. When an
infection causes disease, the effects of the disease range from mild to
severe
Virulence refers to the intensity of the disease produced by pathogens,
and it varies among different microbial species. For example, Bacillus
cereus causes mild gastroenteritis, whereas the rabies virus causes
neurological damage that is nearly always fatal. Virulence also varies
among members of the same species of pathogen. For example,
organisms freshly discharged from an infected individual tend to be more
virulent than those from a carrier, who characteristically shows no signs
of disease. The virulence of a pathogen can increase by animal passage,
the rapid transfer of the pathogen through animals of a species susceptible
to infection by that pathogen. As one animal becomes diseased,
organisms released from that animal are passed to a healthy animal,
which then also gets sick. If this sequence is repeated two or three times,
each newly infected animal suffers a more serious case of the disease than
the one before it. Presumably the mi crobe becomes better able to damage
the host with each animal passage The virulence of a pathogen can be
decreased by attenuation, by repeated subculturing on laboratory media
Koch postulates.
The first direct demonstration of the role of bacteria in causing disease
came from the study of anthrax by the German physician Robert Koch
Four criteria that were established by Robert Koch to identify the
causative agent of a particular disease, these include:
1. the microorganism or other pathogen must be present in all cases of
the disease
2. the pathogen can be isolated from the diseased host and grown in
pure culture
3. the pathogen from the pure culture must cause the disease when
inoculated into a healthy, susceptible laboratory animal
4. the pathogen must be reisolated from the new host and shown to be
the same as the originally inoculated pathogen
many organism that do not meet the criteria of posulates have been shown
to cuase the diseases For examples, Mycobacterium leprae, the causative
agent of leprosy, cannot be isolated in pure culture. additionally to
Neisseria gonorrhoeae there is no animal model of infection even though
the bacteria can readily be cultured in vitro
Figure-1:- Experimental Koch postulates
The Disease Process
How Microbes Cause Disease
Microorganisms act in certain ways that allow them to cause disease.
These actions include gaining way access to the host, adhering to and
colonizing cell surfaces, invad ing tissues, and producing toxins and other
harmful meta bolic products. However, host defense mechanisms tend to
thwart the actions of microorganisms. The occur rence of a disease
depends on whether the pathogen or the host wins the battle; if it is a
draw, a chronic disease may result.
How bacteria cause disease
Bacterial pathogens often have special structures or phys iological
characteristics that improve the chances of suc cessful host invasion and
infection.
Virulence factors are structural or physiological char
acteristics that help organisms cause infection and disease. These factors
include structures such as pili for adhesion to cells and tis sues, enzymes
that either help in evading host defenses or protect the organism from
host defenses, and toxins that can directly cause disease.
Direct action of bacteria
Bacteria can enter the body by penetrating the skin or mucous
membranes, by sexual transmission, by being ingested with food, by
being inhaled in aerosols, or by transmission on contaminated object
with an infectious agent. If the bacteria are immediately swept out of the
body in urine or feces or by coughing or sneezing, they cannot initiate an
infection. A critical point in the production of bacterial disease is the
organism’s adherence, or attachment, to a host cell’s surface. The
occurrence of certain infections depends in part on the interaction
between host plasma membranes and bacterial adhesions: are proteins or
glycoproteins found on attachment pili (fimbriae) and capsules . Most
adhesins that have been identified permit the pathogen to adhere only to
receptors on membranes of certain cells or tissues the capsules and
attachment pili are also antiphagocytic structures.
Attachment to a host cell surface is not enough to cause an infection. The
microbes must also be able to colonize the cell surface
. Colonization refers to the growth of microorganisms on epithelial
surfaces, such as skin or mucous membranes or other host tissues. For
colonization to occur after adherence, the pathogens must survive and
reproduce despite host defense mechanisms.
example, pathogenic bacteria on the surface of skin must withstand
environmental conditions and bacteriostatic skin secretions. The degree
of invasiveness of a pathogen—its ability to invade and grow in host
tissues—is related to the virulence factors the pathogen possesses and
determines the severity of disease ,most
pathogen have additional
virulence factors like enzymes that enable the pathogen to invade tissues.
Among bacteria that release enzymes is Streptococci produce
hyaluronidase, or spreading factor. This enzyme digests hyaluronic acid,
a gluelike substance that helps hold the cells of certain tissues together
.Digestion of hyaluronic acid allows streptococci to pass between
epithelial
cells
and
invade
deeper
tissues.
Other
example
is
Staphylococcus aureus that produces coagulase to aid in infection
Coagulase is a two-edged sword: It keeps organisms from spreading but
also helps wall them off from immune de fenses that might otherwise
destroy them the bacterial enzyme streptokinase dissolves blood clots.
Pathogens trapped in blood clots free themselves to spread to other
tissues by secreting these virulence factors
A
B
Enzymatic virulence factors help bacteria invade tissues and evade
host defenses. (a) Hyaluronidase dissolves the “cement” that holds
together the cells that line the intestinal tract. Bacteria that produce
hyaluronidase can then invade deeper cells within the intestinal tissues.
(b) Coagulase triggers blood plasma clotting, allowing bacteria protection
from immune defenses. Streptokinase dissolves blood clots. Bacteria
trapped within a clot can free themselves and spread the infection by
producing streptokinase.
BACTERIAL TOXINS.
A toxin is any substance that is poisonous to other organisms. Some
bacteria produce toxins, which are synthesized inside bacterial cells and
are classified according to how they are released. Exotoxins are soluble
substances secreted into host tissues. Endotoxins are part of the cell wall
and are released into host tissues—sometimes in large quantities—from
Gram negative bacteria, often when the bacteria die or destroyed
endotoxins
are produced by certain Gram-negative bacteria. All endotoxins consist of
lipopolysaccharide (LPS) complexes They are relatively stable molecules
that do not display affinities
for particular tissues. Bacterial endotoxins have nonspecific effects such
as fever They also cause tissue damage in diseases such as typhoid fever
and epi demic meningitis
Exotoxins
are more powerful toxins produced by sev eral Gram-positive and a few
Gram-negative bacteria. Most are polypeptides, which are denatured by
heat, ultra violet light, and chemicals such as formaldehyde. Species of
Clostridium, Bacillus, Staphylococcus, Streptococcus, and several other
bacteria produce exotoxins
Some exotoxins are enzymes. Hemolysins were first discovered in
cultures of bacteria grown on blood agar plates. The action of these
exotoxins is to lyse (rupture) red blood cells
Virulence factors called leukocidins are exotoxins produced by many
bacteria, including the streptococci and staphylococci. These toxins
damage or destroy certain kinds of white blood cells called neutrophils
and macrophages
Table-1- types of bacterial toxinsb14.5 Properties of Toxins
property
Organisms producing
Location in cell
exotoxins
Almost all Grampositive; some Gramnegative
Extracellular, excreted
into medium
Chemical nature
Mostly polypeptides
Stability
Unstable;
denatured
above 60°C and by
ultraviolet
light
Among
the
most
powerful toxins known
(some are100 to 1
million times as strong
as strychnine)
Highly specific; some
act as neurotoxins or
cardiac
muscle toxins
Little or no fever
Toxicity
Effect on tissues
Fever production
endotoxins
Almost all
negative
Gram-
Bound within bacterial
cell wall; released
upon
death of bacterium
Lipopolysaccharide
complex
Relatively stable; can
withstand
several
hours
above 60°C
Weak, but can be fatal
in relatively large
doses
Nonspecific; ache-allover systemic effects
or
local site reactions
Rapid
rise
in
Antigenicity
temperature to high
fever
stimulates Weak; recovery from
production disease often does not
produce immunity
Strong;
antibody
and
immunity
Toxoid conversion and By treatment with heat Cannot be converted to
use
or chemicals; toxoid toxoid; cannot be used
used to
to immunize
immunize
against
toxin
Many exotoxins have a special attraction for particular tissues. Like
Neurotoxins that act on tissues of the nervous system to prevent muscle
contraction (botulism) Enterotoxins, such as the toxin that causes cholera,
are exotoxins that act on tissues of the gut