Commensalism and pathogenesis

Define commensals

an association between 2 organisms in which one benefits and the other derives neither benefit nor harm - the normal microbial flora of humans 

Name the advantages of Gut bacteria: gastrointestinal flora (3)

• Maintains structure and function of gut 

• Involved in education of our immune system 

• Role in obesity  

Name the advantages of skin microbial flora (3)

• Inhibits pathogen growth 

• Prevents invasion of more harmful bacteria 

• Educates immune system 

As microbe and host co-exist, their interactions can trigger harmful responses, name them (2)

• Bacteria may release toxins that are toxic to the host  

• The host response to the pathogen may result in immunodeficiency  

Name the non sterile sites (3)

• Skin 

• Mucosal surfaces of upper respiratory tract, alimentary canal 

• Female genial tract 

Name the sterile sites (7)

• Blood 

• CSF 

• Bones and joints 

• Sub-cutaneous tissue 

• Muscle 

• Intra-ocular tissue - ears 

• Bladder/ renal tract 

What can alter normal flora? (3)

• Hospitalised patients may become colonised with different Organisms 

  • Patients who access healthcare frequently become colonised with MRSA 

• Gut flora altered by exposure to antibiotics  

• In the right conditions commensals may behave as pathogens 

Name the Non-bacterial commensals (3)

• Viruses: many viruses colonise our skin, respiratory tract, GTI without causing disease 

• Fungi: can colonise colon and vaginal tract 

• Mites: can colonise human skin, hair follicles 

Describe pathogenic adherence (4)

• Call wall determinants adhesins 

• Must adhere or colonise before causing disease 

• Not usually damaging 

• Site specific 

Describe pathogenic invasion (2)

• Many pathogens need help to invade 

• Usually penetrating injury like splinter or needle 

Describe pathogenic survival (3)

• Resist or avoid immunological attack 

• Pathogenic bacteria can have a capsule 

  • Important example: streptococcus pneumoniae 

• Survive in host cells to escape immune system  

Describe pathogenic multiplication (3ish)

• Production of enzymes: proteases, coagulates 

• Breakdown tissue 

• Toxins - most important is endotoxins 

  • Potent inflammatory stimulus 

  • Senses infection 

  • Overwhelming infection can lead to overwhelming host response such as inflammation and clotting which can eventually lead to cardiovascular collapse or endotoxin shock 

  • Gram negative bacteria have lipopolysaccharide in their outer cell membrane 

Describe pathogenic disemination (5)

• Can occur through two routes - lymphatics or blood 

• Dissemination to local lymph nodes is a normal process since LNs normal processes of immune response can occur 

• Viruses  

  • Hep B - blood borne 

  • HIV - sexually transmitted 

  • Dengue - vector 

• Parasites 

  • Malaria - vector 

Positive predictive value 

• PPV = TP/(TP + FP) 

• Probability that the patient has the diseases when restricted to those testing positive 

Negative predictive value 

• NPV = TN/(TN + FN) 

• Probability that the patient does not have the disease when restricted to all negative test results 

Environmental factors influencing susceptibility to infections (5)

• Climate 

• Sanitation e.g. cholera and floods 

• Population density 

• Herd immunity 

• Vectors 

Give two reasons why some get infected but others don’t 

• Increased genetic susceptibility 

  • Sickle cell gene protects from malaria 

• Inadequate immune response 

State the Infections where genetic factors increase susceptibility (4)

• Tuberculosis 

• Leprosy 

• AIDS 

• Meningococcal infection 

Give effects of immunocompromised host (3)

• Burns increase susceptibility to infection both as a result of the break in the skin defences and also by a more general effect on immunity 

• Splenectomy causes defective clearance of encapsulated bacteria and leads to life threatening infection 

  • E.g. streptococcus pneumoniae 

  • Meningococci = Neisseria meningitidis 

  • Haemophilus influenza 

• Role of vaccines and prophylactic antibiotics 

Explain opportunistic infections 

• Caused by organisms of low virulence 

• Cytomegalovirus can cause retinitis in AIDS patients and infections of other sites in transplant recipients 

Explain Inappropriate responses 

• An inappropriate or excessive host response can cause severe tissue damage 

• In meningococcal sepsis, endotoxin from the bacteria causes excessive activation of coagulation system