Bacterial Interactions with Hosts A Terminology B Hosts
Bacterial Interactions with Hosts
• • • 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 (p. H, 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 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 A 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 – 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 – p. H 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, p. H) • 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
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