Normal microflora vs pathogens

Normal microflora vs pathogens

1. TYPES OF MICROBIAL INTERACTIONS

   • Pathogens
     • Can cause disease in individuals with normal host defences
   • Commensals
     • The normal microflora found normally on those parts of the body that are exposed to the external environment
     • COMMENSALS AS OPPORTUNISTIC PATHOGENS
       • Transfer to normally sterile areas
       • E.G. infection of GT
       • E.G. oral streptococci to heart valves
       • Damage to epithelium
       • E.G. staphylococci and pseudomonas burns or wounds
       • E.G. bacterial pneumonia following influenza
       • Immune supression
       • E.G. gut flora septicaemia following radiation
       • E.G. many pathogens following HIV or immunosuppressive drugs
       • Antibiotic treatment
       • E.G. clostridium difficile
   • Opportunistic pathogens
     • Can cause infection if host defences are impaired
     • Can often be derived from normal microflora

2. COMMENSALS OR NORMAL MICROFLORA

   • Organisms that live on or within the body of healthy individuals
   • May be of benefit

3. TYPES OF SYMBIOSIS

   • Close interactions between organisms
   • At least one organism gains
   • Microorganisms may be harmful to host
   • COMMENSALISM
     • Neither harm nor benefit the host
   • MUTUALISM
     • Both organisms benefit
     • Microbes get a place to live and nutrients
     • Microbes may be beneficial to host
   • PARASITISM
     • Host is hormed
     • Parasite gains

4. NORMAL FLORA

   • Microbes normally found on healthy individuals
   • Usually harmless
   • Microbiota/microbiome
   • Very abundant
   • More microbes than host cells in body
   • Much smaller in size
   • Transient (impermanent ) microflora
   • DO NOT INJURE HOST BECAUSE
     • Survive because they are harmless
     • If they become less fit = kill or be killed
     • Fail to gain from damaging host
   • MICROFLORA PROTECTS US
     • Inhibits pathogens
     • Aids digestive processes
     • Colonization resistance
     • Competition for space and nutrients with pathogens
     • Release of bacteriocins and other antibacterials
     • Substances to prevent pathogen growth
     • Stimulation of the immune system
     • Continued antigenic stimulation from commensals
     • Cross-reaching protective immunity against pathogens
     • DETRIMENTAL EFFECTS
       • Dental caries (toothy decay)
       • Acne and other skin disorders
       • Body odour
       • Ulcers
       • Conversion of compounds to toxic forms (cancer)
   • FORMATION OF NORMAL MICROFLORA
     • Foetus is sterile
     • When born they acquire microflora from mothers vagina (environmental acquisition)
     • Contact with others
     • LARGE INTESTINE
       • Anaerobic environment abundant microbial anaerobic populations
       • Greatest numbers of normal microflora
     • Colonization
       • Colonization of the large bowel occurs in stages
       • During a vaginal birth lactobactilli are the first colonizers
       • E.coli soon follows
       • New born does not start to acquire a complex intestinal fora until weaned
     • Where do organisms come from?
       • Oral exposure to faecal microbes
       • Facultative anaerobes colonise first and strict anaerobes later
       • Normal flora, mouth
       • Normal flora, skin
   • CHANGES IN NORMAL MICROFLORA
     • New organisms may be acquired
     • Sterile in utero; neonate colonized from mother during birth
     • Colonization of gut and URT in hospitalised patients (nosocomial infections)
     • Cross infection with Colostridium difficile, Methicillin resistant Staphylococcus aureus (MRSA) and Vancomycin resistant Enterococci (VRE)
     • With changes in physiology and development
     • Gut flora changes after breast feeding and weaning
     • Female genital tract and lactobacilli
     • When antibiotics select for a 'resistant flora'
     • Candida overgrowth in mouth and vagina (thrush)
     • Clostridium difficile (antibiotic-associated colitis

5. About 10^12 bacterial cells/gram in gut

   • Only 10^13 in human cells
   • >1kg of bacteria in adult gut
   • >1000 species of bacteria and archaea (many uncultured)
   • Composition of normal flora varies from individual to individual

6. MICROBIAL PATHOGENESIS

   • Disease
   • Pathogen
   • Virulence (severity)
   • Infection
   • Non communicable
   • Microbial intoxications

7. KOCHS 4 POSTULATES

   • IDENTIFICATION OF INFECTIOUS DISEASE
     • 1. The microorganism (pathogen) must be present in all cases of the disease
     • 2. The pathogen can be isolated from the diseased host and grown in pure culture
     • 3. The pathogen from the pure culture must cause the same disease when inoculated into a healthy, susceptible laboratory animal
     • The pathogen must be re-isolated from the new host and shown to be the same as the originally inoculated pathogen
   • MOLECULAR DETECTION: ADAPTING KOCHS 4 POSTULATES
     • 1. The biochemical or genetic property should always be associated with pathogenic strains
     • 2. Specific inactivation of the gene that specifies a virulence factor should cause a drop in virulence
     • 3. Reversion of the mutated gene or allelic replacement by recombination or complementation should restore virulence
     • 4. Difficulties of defining single 'virulence factors'

8. VERTICAL DISEASE TRANSMISSIONS

   • Persistance of the agent
   • Transmitted from the parent to offspring
   • Neonatal infection
   • E.G. gonorrhoea, HIV and Zika
   • Infection in utero
   • E.G. rubella and listeria

9. HORIZONTAL DISEASE TRANSMISSIONS

   • Inhalation of droplets (aerosols)
   • E.G. influenza
   • Faecal-oral
   • E.G. cholera
   • Vector-bone
   • E.G. malaria
   • Sex or blood exchange
   • E.G. HIV