3rd year/Analytical Invest.
Dr.Hawraa A.Ali Ad-Dahhan
Transmitted Diseases
Lecture 1
Normal microbial flora
Large numbers of microorganisms exist in the air we breathe, on the surfaces we
touch, and on and within our bodies.Microorganisms naturally found on and within the
body are called normal flora—they concentrate in certain body regions, such as the skin,
mouth, and GI tract. The skin harbors more than 10,000 microorganisms/cm2. Scrapings
from the surface of the teeth or gums may show millions of organisms per milligram of
tissue.
The human body and its normal flora coexist in a sort of ecosystem whose
equilibrium is essential to good health.Under normal circumstances, these microorganisms
are nonpathogenic and harmless. In fact, they may aid the body by competing for nutrients
with disease-producing microorganisms or by performing special tasks. For example, the
lumen
of the bowel contains microorganisms that carry out many chemical
functions.Moreover, disruption of the normal ecology of the microbial flora can pose
substantial risks to the host. (See Where normal flora live.) .Relatively few of the many
species of microorganisms that exist become adapted to the unique environments of various
body tissue. Thus, to a certain degree, the flora of a given species—even of specific body
tissue—is predictable.
●What is infection?
Infection is the invasion and multiplication in or on body tissue of microorganisms that
produce signs and symptoms along with an immune response. Such reproduction injures the
host either by causing cellular damage
from microorganism-produced toxins or
intracellular multiplication or by competing with host metabolism. The host’s own immune
response may increase tissue damage, which may be localized (as in infected pressure
ulcers) or systemic. The very young and the very old are most susceptible to infections.
Microorganisms that cause infectious diseases are difficult to overcome for many reasons:
☐ Some bacteria develop a resistance to antibiotics.
☐ Some microorganisms, such as human immunodeficiency virus (HIV), include many
different strains, and a single vaccine can’t provide protection against them all.
◗ Most viruses resist antiviral drugs.
◗ Some microorganisms localize in areas that make treatment difficult, such as the central
nervous system and bone.
◗ New infectious agents, such as HIV and severe acute respiratory syndrome–coronavirus,
occasionally arise.
◗ Opportunistic microorganisms can cause infections in immunocompromised patients.
◗ Much of the world’s ever-growing population has not received immunizations.
◗ Increased air travel by the world’s population can speed a virulent microorganism to a
heavily populated urban area within hours.
◗ Biological warfare and bioterrorism with organisms such as anthrax, plague, and smallpox
are an increasing threat to public health and safety throughout the world.
◗ Invasive procedures and the expanded use of immunosuppressive drugs increase the
risk of infection for many. (See When microorganisms grow resistant).
Also, certain factors that normally contribute to improved health, such as good nutrition,
clean living conditions, and advanced medical care, can actually lead to increased risk for
infection. For example, travel can expose people to diseases against which they have little
natural immunity. The increased use of immunosuppressants, as well as surgery and other
invasive procedures, also heighten the risk for infection.
TYPES OF INFECTION
Microorganisms responsible for infectious diseases include bacteria, viruses, fungi
(yeasts and molds), and parasites. Bacteria are single-cell microorganisms with well-defined
cell walls that can grow independently on artificial media without the need for other cells.
Bacteria inhabit the intestines of humans and other animals as normal flora used in the
digestion of food.Also found in soil, bacteria are vital to soil fertility. These microorganisms
break down dead tissue, which allows the tissue to be used by other organisms. Despite the
many types of known bacteria, only a small number are harmful to humans.
In developing countries, where poor sanitation increases the risk of infection,
bacterial diseases commonly cause death and disability. In industrialized countries, bacterial
infections are the most common fatal infectious diseases. Bacteria are classified by shape.
Spherical bacterial cells are called cocci; rod-shaped bacteria, bacilli; and spiral-shaped
bacteria, spirilla. Bacteria are also classified according to their response to staining (grampositive, gram-negative, or acid-fast bacteria); their motility (motile or nonmotile bacteria);
their tendency toward encapsulation (encapsulated or non encapsulated bacteria); and their
capacity to form spores (sporulating or nonsporulating bacteria).
Viruses are subcellular organisms made up only of a ribonucleic acid or a
deoxyribonucleic acid nucleus covered with proteins. They’re the smallest known organisms,
so tiny they’re visible only through an electron microscope.Viruses can’t replicate
independent of host cells. Rather, they invade a host cell and stimulate it to participate in the
formation of additional virus particles. The estimated 400 viruses that infect humans are
classified according to their size, shape (spherical, rod-shaped, or cubic), or means of
transmission (respiratory, fecal, oral, or sexual).
Fungi are single-cell organisms whose nuclei are enveloped by nuclear membranes.
They have rigid cell walls like plant cells but lack chlorophyll, the green matter necessary
for photosynthesis. They also show relatively little cellular specialization. Fungi occur as
yeasts (single-cell, oval-shaped organisms) or molds (organisms with hyphae, or
branching filaments). Depending on the environment, some fungi may occur in both
forms. Fungal diseases in humans are called mycoses. Parasites are unicellular or
multicellular organisms that live in or on their hosts and are dependent on the host for
nourishment. They usually only take the nutrients they need and rarely kill their hosts,
although they can cause harm. Parasites are divided into two types, depending on their
relationshipwith the host:
Endoparasites live inside the host (for example, protozoans, worms, flukes,and amoebae),
while ectoparasites live on the host’s skin (such as fleas, ticks, and lice).
HOW INFECTION OCCURS
Whether or not an infection develops hinges on variables relating to three crucial factors:
◗ An infectious organism (pathogen)
◗ A host (any organism that can support the nutritional and physical growth of another
organism)
◗ A favorable environment As long as one of these factors is missing, infection does not
occur.However, if an imbalance develops—for example, if a patient’s immune system is
suppressed and can’t fight off pathogens—the potential for infection increases. Infection
starts when a microorganism invades body tissue. Once the microorganism breaches the
patient’s immune defenses and enters the body, it multiplies and causes harmful effects. The
severity of the infection depends on such factors as microbial characteristics, the number of
microorganisms present, and the way in which the microorganisms enter the body and spread
THE INFLAMMATORY RESPONSE
The body reacts to microbial invasion by producing an inflammatory response. The five
classic signs and symptoms of inflammation are as follows:
◗ Redness—Caused by dilation of arterioles and increased circulation to the site; a localized
blush caused by filling of previously empty or partially distended capillaries
◗ Pain—Results from stimulation of pain receptors by swollen tissue, local pH changes, and
chemicals excreted during the inflammatory process
◗ Heat—Caused by local vasodilation, fluid leakage into the interstitial spaces, and
increased blood flow to the area
◗ Swelling—Caused by local vasodilation, eakage of fluid into interstitial spaces, and
blockage of lymphatic drainage
◗ Loss of function—Results primarily from pain and edema Other manifestations of the
inflammatory response include fever, malaise, nausea, vomiting, and purulent discharge from
wounds.
Not all infections are apparent or symptomatic.With subclinical, or asymptomatic,
infection, the infectious microorganism is present and an immune system response is
initiated, but the person shows no signs or symptoms of the disease.
ENDOGENOUS AND EXOGENOUS MICROORGANISMS
Microorganisms may be endogenous or exogenous. Endogenous microorganisms are
those found on the skin and in such body substances as saliva, feces, and sputum. They can
cause disease in a susceptible individual. xogenous microorganisms originate from sources
outside the body.Humans and exogenous microorganisms usually live together in
harmony.However, if something disrupts this harmonious relationship, the microorganisms
may cause infection.
INVASION AND COLONIZATION
The presence of microorganisms in or on an individual is called colonization. Colonized
microorganisms grow and multiply but may not invade tissue and thus don’t produce cellular
injury. In cases of colonization, tissue culture results are positive but the patient lacks
evidence of infection. However, some people who are colonized with bacteria do develop
localized signs and symptoms of infection—tenderness, swelling, redness, and pus—because
the bacteria has invaded the tissue, producing cellular injury.A culture of the pus typically
elicits the microorganism. Colonized bacteria may also cause systemic infection, producing
fever, an elevated white blood cell count, and possibly shock.
PATHOGENICITY
Pathogenicity refers to a microorganism’s ability to cause pathogenic changes, or
disease. An example of a highly pathogenic microorganism is the rabies virus, which always
causes clinical disease in the host. In contrast, alpha-hemolytic streptococci have
low pathogenicity; although they commonly colonize humans, they rarely produce clinical
disease. Factors affecting pathogenicity include the microorganism’s mode of action,
virulence, dose, invasiveness, toxigenicity, specificity, viability, and antigenicity.
MODE OF ACTION.
The means by which a microorganism produces disease is called its mode of
action.Viruses, for example, cause infection by invading host cells and interfering with cell
metabolism. Other modes of microbial action include:
◗ Evasion or destruction of host defenses by preventing host phagocytes (scavenger cells)
from engulfing and digesting them (used by Klebsiella pneumoniae)
◗ Secretion of enzymes or toxins, which allows the microorganism to penetrate and spread
through host tissue (used by the measles virus)
◗ Production of toxins that interfere with intercellular responses (used by tetanus bacilli)
◗ Stimulation of a pathologic immune response (used by group A beta-hemolytic
streptococci)
◗ Destruction of T-helper lymphocytes (used by HIV).
VIRULENCE.
Virulence refers to the degree of a microorganism’s pathogenicity.Virulence
can vary with the condition of the body’s defenses.For instance,Mycobacterium
aviumintracellulare, bacteria commonly found in water and soil, can cause severe
pulmonary and systemic disease in patients with acquired immunodeficiency syndrome but
typically do not cause illness in those with a normal immune system.Virulence can be
enhanced by several factors:
◗ Toxins produced by bacteria such as Streptococcus and Clostridium
◗ The ability of microorganisms to elude host defenses (such as Pneumococcus with its
polysaccharide capsule)
◗ Persistence in the environment (spores and cysts)
◗ Genetic variation (influenza)
DOSE.
A microorganism must be present in a sufficient dose to cause human disease. The
ize of the pathogenic dose varies from one microorganism to the next and from person to
person and may be affected by the mode of transmission. The patient’s immune system also
plays an important role in the pathogenic dose requirement. In order to cause diarrhea, the
infective dose of Shigella is approximately 10 microorganisms, whereas the infective dose of
Salmonella required to cause typhoid fever is 1,000 microorganisms. A lower infective dose
does not necessarily imply that the organism causes more severe immediate
disease.
INVASIVENESS.
Invasiveness (sometimes called infectivity) refers to the ability of a microorganism
to invade tissue. Some microorganisms can enter the human body through intact skin; others
can penetrate only through a break in the skin or mucous membranes. Leptospira interrogans
usually enters the body through a minor skin abrasion; Clostridium tetani, through a deep
puncture wound. The invasiveness of some microorganisms is increased by the enzymes they
produce.
TOXIGENICITY.
Toxigenicity, which is related to virulence, refers to a microorganism’s
potential to damage host tissue by producing and releasing toxins. Some bacteria, such as
diphtheria and tetanus, release exotoxins that are quickly disseminated in the blood, causing
systemic and neurologic manifestations.Other bacteria, such as Shigella, release endotoxins
that can cause diarrhea and shock.
SPECIFICITY.
Specificity refers to the attraction of a microorganism to a specific host or range of
hosts. For example, the flavivirus that causes St. Louis encephalitis has a number of hosts,
including birds and humans; whereas rubeola, the virus that causes measles, is carried only
by humans.
VIABILITY.
Viability refers to the ability of a microorganism to survive outside the host.
Microorganisms can live and multiply in a reservoir, which provides what the
microorganisms need to survive. The microorganisms can then be transmitted from the
reservoir to another person.
ANTIGENICITY.
Antigenicity, the degree to which a microorganism can induce a specific immune
response, varies among microorganisms. Those that invade and localize in tissue initially
stimulate a cellular response, whereas those that disseminate more quickly generate an
antibody response.