Week 1 - Host Pathogen Interactions PDF

1️⃣ Week 1 - Host Pathogen Interactions Microbial Diversity - May be as many as 10^12 microbial species on earth 10^13 human cells : 10^14 microbes Microbiota - the total microbial population of a given site or habitat (eg....

1️⃣ Week 1 - Host Pathogen Interactions Microbial Diversity - May be as many as 10^12 microbial species on earth 10^13 human cells : 10^14 microbes Microbiota - the total microbial population of a given site or habitat (eg. the gastrointestinal microbiota) Microbiome - the genetic content of the microbiota Influenced by: pH, amount of water present, nutrients available, amount of oxygen available The human microbiota Makes significant contributions to human health and disease: Bidirectional communication with the gut-brain axis between microbiota, enteric nervous system and CNS to modulate metabolism and GIT homeostasis, appetite, stress and anxiety, memory communication between the gut and the brain Week 1 - Host Pathogen Interactions 1  The GBA also stimulates the host immune response to maintain normal development and maturation of immune function Increased range of metabolic processing Physical barrier for protection against exogenous pathogens* by competing for nutrients, occupying space and producing antimicrobial substances * microbes from the external environment that may cause disease if it’s able to compete with this microbiota Eg. Some bacteria on our skin produce fatty acid that can discourage other species from invading - Lactobacilli found in the vagina, maintaining quite an acidic environment which suppresses the growth of other organisms Symbiosis Mutualism Two independent organisms living together to their mutual benefit E.g., bacteria in the colon Benefit to the human: the bacteria metabolise undigested carbohydrates and provide additional nutrients Protect the colon from colonisation and infection from potential pathogens Stimulation of the immune system Breakdown of waste products Benefit to the bacterium: Assured food supply and sheltered environment Week 1 - Host Pathogen Interactions 2  Commensalism One participant benefits and the other is unharmed E.g., Oral microbiota The microbiota benefit from the food source (particularly sugars) There is no advantage or disadvantage to the human (in good oral health) Normal microbiota are often described as residents/commensals where they exist without disease, but this relationship can rapidly shift with environmental changes e.g. change in diet Parasitism One organism benefits at the expense of the other The host provides the parasite with their physicochemical environment, food, respiratory and other metabolic needs. In health = commensals and mutalists Dysbiosis can lead to disease and some microbiota become parasites Week 1 - Host Pathogen Interactions 3  Microbiota: location Skin Mechanical barrier to infection Quite inhospitable to a lot of microbes Because of low moisture + high salt concentration + slightly acidic Mouth Complex habitat - can have biofilms of microbial communities in the mouth, on the teeth, down the back of the throat 500-600 different bacteria including anaerobes* * An anaerobic organism or anaerobe is any organism that does not require molecular oxygen for growth. It may react negatively or even die if free oxygen is present. Archaea in gingival crevices - anaerobes Entry point for other bacteria to colonise URT Pathogens what want to colonise the respiratory tract entring upper RT Saliva and food particles are potential sources of microbial nutrients → constant food source Saliva contains antibacterial substances (such as lysozome) Respiratory Tract Upper respiratory tract (URT) Nose Nasal cavity and nasopharynx Throat: oropharynx and larynx Many species as part of normal microbiota Similar to organisms in oral cavity Lower respiratory tract Week 1 - Host Pathogen Interactions 4  Trachea, primary bronchi, 2ndary … until alveoli for gas exchange Nose Normal microbiota found just inside the nostrils at the nares Nasal mucus contains lysozyme Most common bacteria found in the nose are Staphylococcus spp. Staphylococcus epidermidis and Staphylococcus aureus (a potential pathogen) predominate (in 10-40% adult noses) Small numbers of Streptococcus pneumoniae, Neisseria meningitidis, and Haemophilus influenzae can also be found in the nasopharynx (upper throat). Lower Respiratory Tract Microorganisms removed by: continuous stream of mucus produced by goblet cells onto ciliated epithelial cells in the mucus gland cilia beat to move the mucus upwards mouth (moving mucus carpet) mucocilary escalator - from LRT to URT to mouth → called the “muco-ciliary escalator” one way traffic for microbial matter to get out of the LRT phagocytic action of alveolar macrophages mop up LRT organism Traditionally been considered a sterile site due to the number of defence mechanisms that help eliminate microbes from the environment Week 1 - Host Pathogen Interactions 5  When gas exchange is happening Stomach Hostile environment for bacteria - not many microbes that are able to resident within the stomach acidic pH (2.0) “barrier” to entry to intestinal tract Microbial load - ~103 per ml of gastric fluid (may be less than 10 viable bacterial cells per ml) gastric mucosa Week 1 - Host Pathogen Interactions 6  where the microbiota are able to colonise and inhibit the lactobacilli that are quite capable of living in acidic environment in some Streptococcus species acid-tolerant Lactobacillus spp. and Streptococcus spp. Helicobacter pylori found in half the human population can lead to gastric ulcers Stomach and small intestines As distance from stomach increases pH increases bacterial/microbial numbers increase secretion decreases Small intestine duodenum colonised to low levels (