Download Bacterial Interactions with Hosts

Bacterial Interactions with Hosts
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A. Terminology
B. Hosts
C. The Skin
D. Oral cavity
E. Intestinal Tract
F. Respiratory Tract
G. Genito-urinary Tract
H. Infection
I. Action of pathogens
J. Host defences
A. Terminology
• Most bacteria are free-living
• minority have chosen the body of an animal as their
ecological niche
• parasites - organisms that live on or in (at the expense of) a
host
• normal flora - bacteria that have little/no harmful effect on
host
• pathogens - bacteria that harm or damage the host
– degree of damage is a measure of the pathogenicity or virulence of
a species. More virulent the species = greater damage
• infection - growth of bacteria on a host
– both normal flora and pathogens infect hosts, only
pathogens damage host resulting in an infectious
disease
• infection non synonymous with disease
EXPOSURE
ADHERENCE
INVASION
COLONIZATION & GROWTH
TOXICITY
INVASIVE GROWTH
TISSUE DAMAGE
DISEASE
B. Hosts
• Human bodies provide
– rich source of organic nutrients and growth factors
– constant physical environment (pH, temp, osmotic
pressure)
• Different regions/organs differ chemically and
physically
– provide selective environments where certain species
are favoured over others
• Bacteria found in all body regions that have
contact with outside world
– skin
– oral cavity
– respiratory tract
– intestinal tract
– genito-urinary tract
not normally in organs or bloodstream
• Most of the sites (not skin) infected by
bacteria are lined by mucous membranes
– here that infection begins
– may invade deeper tissues
Bacterial Interactions With Mucous Membranes
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B
A. Loose association
B. Adhesion
C. Invasion
C
C. The Skin
• Not a favourable environment
– dryness
– acidity
• Most bacteria associated with sweat glands and
hair follicles
• Most normal flora are gram positive bacteria
– thick cell wall adapts them better to dry conditions
D. Oral cavity
• Complex bacterial habitat
• Saliva
– not a good culture medium
– contains antibacterial substances
– food particles
• Dental plaque
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film on surface of teeth
an environment that allows bacteria to flourish
attachment of salivary proteins
plaque often depleted of oxygen
• anaerobic microenvironment allows anaerobic bacteria to grow
• Many bacteria convert carbohydrates to lactic
acid
– attacks teeth, tooth decay (dental caries)
• Presence of carbohydrates predisposes the host
to dental caries
E. Intestinal Tract
• Stomach
– pH 2
– barrier against entry of bacteria
– acid-tolerant bacteria can live here
– bacterial counts low but walls often colonised
• Small intestine
– fairly acidic
– resembles stomach in its normal flora
– as distance from stomach increases, so does
numbers of bacteria
– lower part of SI, bacteria found on walls an in
contents
• Large intestine
– huge numbers of bacteria
– oxygen is consumed, creates environment for
anaerobic bacteria
– 1010-1011 anaerobic bacteria/g of intestinal
contents
– bacteria make up 1/3 weight of faeces
• Downward movement of intestinal contents
– many bacteria lost in faeces
– bacteria multiply to replace losses
• Normal flora provides protection against
pathogens
– competition for nutrients and growing space
– antibiotic treatment may destroy normal flora
• opportunistic pathogens
F. Respiratory Tract
• Bacteria enter upper respiratory tract during
breathing
– most trapped and expelled
• Lower respiratory tract sterile
G. Genito-urinary Tract
• Urinary tract normally sterile
– exception: lower part of urethra where mucous
membranes colonised by bacteria
• vagina contains glycogen (polysaccharide)
fermenting bacteria
H. Infection
• Most infections begin on the mucous
membranes
– occasionally skin is breached
• wounds
• insect bites
• adherence to host cells
– specific interactions between macromolecules
on bacterial and host cells, explains:
– Tissue specificity
• a species will preferentially infect a site
because it adheres strongly to those host cells
– Host specificity
• particular species infect humans because they
adhere specifically to human cells
• adhesin - macromolecule on bacterium
involved in specific adhesion
• receptor - host cell macromolecule to which
adhesin specifically binds
– production of receptor not intentional by host
• bacteria must grow and multiply
– nutritional and physical factors (temp, salt, pH)
• ability to adhere and grow on host tissues
shared by normal flora and pathogens
I. Action of pathogens
• Pathogens inflict damage on host
– toxins
– invasion of normally sterile sites
• toxin-producing bacteria produce chemicals that
damage host tissue
– eg. Cholera
• bacteria adhere to intestinal mucosal surfaces
• produce toxin, acts on gut wall resulting in diarrhoea
• Other bacteria invade host tissues, produce
enzymes to break down tissue
• normal flora and pathogens need to adhere
and grow on host tissues
– pathogens also damage host
• toxin production
• invasion of tissues
J. Host defences
• Immune system
– cell-mediated immunity
– antibodies
• Anatomical barriers to infection
ANATOMICAL BARRIERS TO INFECTION