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 TISSUE DAMAGE DISEASE INVASIVE GROWTH

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 ABC A. Loose association B. Adhesion C. Invasion

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 –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 – 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