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HumanMicrobe Interactions
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Normal flora: the hundreds of species/billions of individual microbes that live ... Dental caries = tooth decay = infectious disease caused by microbes. ... – PowerPoint PPT presentation
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Title: HumanMicrobe Interactions
1
Chapter 21
- Human-Microbe Interactions
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Human-Microbe Interactions
- Normal flora the hundreds of species/billions
of individual microbes that live on or in the
human body. - Pathogens microbial parasites (organisms that
live on or in a host organism, causing damage to
the host). - Virulence the quantitative measure of
pathogenicity. - Virulence vs. resistance of the host are
constantly changing. - Infection microorganism is established and
growing in a host (note that the host may or may
not be harmed). - Disease damage or injury to the host that
impairs host function. Infection ? disease.
3
Human-Microbe Interactions (cont.)
- Why would microorganisms want to live within an
animals body? - Is the animal body a uniform microbial
environment? - What areas of the body are normally colonized by
microorganisms? - Infections often begin at sites in the animals
mucous membranes. Where are mucous membranes
located in the body? - What is mucus made of and what is its purpose?
- Why do mucosal surfaces have a larger variety of
organisms associated than nonmuscosal surfaces?
4
Normal Flora of the Skin
- Avg. adult has 2 m2 of skin surface.
- Skin is not favorable to microbial growth since
it is subject to drying. Most microbes are
associated with the sweat glands. - The flora of the skin consists of resident and
transient populations of microbes. Resident
microbes live and multiple on the skin, whereas
transient flora almost always are unable to
multiple and usually die. - Resident flora remains fairly constant, although
it can be affected by several factors. What are
3 of these factors?
5
Normal Flora of the Oral Cavity
- Saliva contains antibacterial enzymes lysozyme
and lactoperoxidase (also present in milk). - Saliva (pervasive, although not the best nutrient
source), food particles, and epithelial debris
provide nutrients for microbes. - Microbes colonize many surfaces in the mouth,
including teeth and gingival crevices. The flora
of the mouth changes with stages of growth, ex.
acquisition of teeth. - A thin organic film forms on the tooth surface,
to which acidic glycoproteins from the saliva
attach, enabling only a few species of
Streptococcus (ex. S. mutans) to adhere. - Dental caries tooth decay infectious disease
caused by microbes. High sugar diets promote
tooth decay because lactic acid bacteria ferment
the sugars to lactic acid ? decalcification of
the enamel ? proteolysis of the matrix of tooth
enamel ? further infection/decay. Fluoride
prevents the decalcification step. - Microorganisms in the mouth can also cause other
infections.
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Normal Flora of the Gastrointestinal Tract
- The stomach is not hospitable to microbes due to
its low pH (pH 2), except Helicobacter pylori,
which causes ulcers.
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Normal Flora of the Respiratory Tract
- Upper respiratory tract nasopharynx, oral
cavity, and throat. - Staphylococci, streptococci, diphtheroid
bacillia, and gram-neg. cocci, as well as
potentially harmful bacteria (ex. Staph. aureus
and Strep. Pneumoniae) are often part of the
normal flora of the nasopharynx of healthy
individuals. - Lower respiratory tract trachea, bronchi, and
lungs. The lower respiratory tract is
essentially sterile. Ciliated epithelium beat
contaminants upward.
8
Normal Flora of the Urogenital Tract
- The bladder itself is usually sterile, but the
epithelial cells lining the urethra are colonized
by facultatively aerobic gram-neg. rods and
cocci, including E. coli, Proteus mirabilis, etc. - Occasionally, these organisms become
opportunistic pathogens. Factors such as local
pH changes allow organisms to multiply and become
pathogenic. - The vagina of the adult female is generally
weakly acidic and contains significant amounts of
the polysaccharide glycogen. Lactobacillus
acidophilus, yeasts such as Candida,
streptococci, and E. coli may also be present.
Before puberty, the vagina is alkaline and does
not produce glycogen. L. acidophilus is absent
at this time. After menopause, the flora
resembles that before puberty.
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Entry of the Pathogen into the Host
- Virulence the relative ability of a pathogen to
cause disease. - Organisms must penetrate the skin, mucous
membranes, or intestinal epithelium, which
normally act as barriers. - Most microbial infections begin at breaks or
wounds in the skin, etc. - Bacteria or viruses adhere specifically to
epithelial cells through protein-protein
interactions on the surfaces of the host and
pathogen cells. These interactions are often
cell and species specific. - Bacterial adherence not dependent on this type of
attachment include structures such as a capsule,
glycocalyx, and slime layer. - Fimbriae and pili are also used for attachment of
the pathogen to the host. - Most pathogens penetrate the epithelium to
initiate pathogenicity invasion, although some
produce toxins that may eliminate normal flora or
cause breaks in the epithelium, allowing access
of the pathogen to the underlying tissues.
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Colonization and Growth
- Colonization multiplication of a pathogen after
it has gained access to the hosts tissues. - The initial inoculum of a pathogen is rarely
sufficient to cause damage, so it must find
appropriate nutrients and environmental cond. in
order to grow which is most imporant?
Vitamins, growth factors, trace elements may be
in short supply. - Pathogens usually remain localized at the site of
entry, but can pass through the lymphatic
vessels, and eventually to the blood, where it
can travel around the body ? bacteremia.
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Virulence
- Measuring virulence calculated as LD50 (lethal
dose50) dose of an agent that kills 50 of the
animals in a test group. - Highly virulent pathogens show little difference
between cells required for LD50 and required
for LD100. This can be as little as a few cells. - When pathogens are kept in lab culture and not
passed through animals, their virulence is often
decreased or lost attenuated. Why does this
occur? - Toxicity the ability of an organism to cause
disease by means of a preformed toxin that
inhibits host cell function or kills host cells,
ex. Clostridium tetani ? tetanus exotoxin. - Invasiveness ability of an organism to grow in
host tissue in such large s that the pathogen
inhibits host function. Virulence factor
allowing invasiveness may not be a toxin, but may
be something such as a capsule, ex. Streptococcus
pneumoniae.
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Virulence Factors
- Virulence factors pathogen-produced
extracellular proteins that aid in the
establishment and maintenance of disease. - Most are enzymes that help the pathogens colonize
and grow, ex. enzymes that help the pathogen to
spread within the tissues. - Fibrin clots form at the site of microbial
invasion to limit infection, but some organisms
can dissolve these clots. Other organisms
promote the formation of clots in order to
protect themselves from what?
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Virulence Factors
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Exotoxins
- Exotoxins proteins released extracellularly as
the organism grows. - 3 classes
- 1. Cytolytic toxins attacking cell
constituents causing lysis, ex. hemolysins. - 2. A-B toxins consists of 2 subunits - one
binds to cell surface receptor and the other is
transferred into the cell to damage the cell, ex.
diphtheria toxin, botulism toxin, and tetanus
toxin. - 3. SuperAg toxins stimulate large s of
immune response cells.
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Enterotoxins
- Enterotoxins exotoxins that act on the small
intestine, generally causing massive secretion of
fluid into the intestinal lumen, leading to
vomiting and diarrhea. - Ex. cholera toxin, E. coli O157H7 toxin.
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Endotoxins
- Endotoxins generally cell-bound toxins that are
released in large amounts only when cells are
lysed. - e.x. LPS of gram-neg. bacteria.
- Fever is an almost universal symptom because host
cells are stimulated to release endogenous
pyrogens, affecting the temp.-controlling center
of the brain.
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Nonspecific Innate Resistance to Infection
- Natural host resistance varies.
- Age Newborns havent acquire adequate normal
flora, the elderly suffer from declining ability
to make an effective immune response to
respiratory pathogens, for ex. - Stress such as fatigue, exertion, poor diet,
dehydration, drastic climatic changes, increase
the incidence and severity of infectious
diseases. Hormones produced under stress
influence the immune system, for ex. by
suppressing it. - Diet can alter the composition of the normal
flora, ex. either lack of a nutrient can put the
host at a disadvantage or deprive a pathogen of
an essential nutrient.
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Nonspecific Innate Resistance to Infection
(cont.)
- Physical and Chemical Defenses include
structural integrity of tissue surfaces (skin,
mucosum), cilia, secretions (ex. fatty acids and
lactic acid produced by sebaceous glands),
stomach acid, lysozyme. - Most pathogens must first adhere and colonize at
the site of exposure and this interaction is very
specific for the type of cells/tissue. Pathogens
might not survive interactions in other locations
in the body besides the target tissue. - Compromised host host in which one or more
resistance mechanisms are inactive and in which
the probability of infection is increased, ex.
hospital patients (cancer, heart disease,
transplants, AIDS, surgery, routine lab tests),
people outside the hospital (smokers, drinkers,
IV drug users, sleep-deprived, malnourished), or
people with certain genetic diseases.
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Inflammation
- Inflammation nonspecific reaction to noxious
stimuli such as toxins and pathogens. - Inflammatory response redness, swelling, pain,
and heat. - Cytokines produced by leukocytes mediate
inflammation. - Septic shock systemic inflammation results in
severe swelling (edema) and uncontrollable fever.
Inflammation spreads throughout the body via the
circulatory or lymphatic systems.
20
Fever
- Normal body temp. 37C.
- Fever abnormal increase in body temp., usually
caused by infection. - Pyrogenic fever-inducing. Fevers 40C (104F)
benefit the pathogen because host tissues are
further damaged. - Types of fever
- 1. Continuous body temp. is remains elevated
over at 24-hr. period, but does not vary by more
than 1C. - 2. Remittant temp. is abnormal over a 24-hr.
period, but varies by more than 1C. - 3. Intermittant temp. is normal for part of the
day and elevated for part of the day. Most
infectious diseases elicit some intermittant
fever.
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