Med Micro L1

• Microbes that engage in mutual or commensal associations with humans

  Normal resident microbiota
• Microbes colonizing the human body

  • Restricted to outer surfaces without penetrating into tissues even when whole body is colonized
• Factors that make the human body a suitable environment for microbes

  • Microenvironment and niches
  • pH
  • Temperature
  • Nutrients
  • Moisture
  • Oxygen tension from one area to another
• Pathogens


  Microbes that penetrate into tissues and multiply
• Transients


  Microbes that cling to skin surface but do not originally grow there, acquired through contact
• Transition zones


  Where the skin joins mucous membrane of nose, mouth and external genitalia, where microbial colonies are most abundant
• Resident population


  • Live and multiply in deeper layers of epidermis and in glands and follicles
• Layers of the skin

  • Epidermis (outer layer of dead cells continually being replaced and sloughed off)
  • Dermis (lies above the subcutaneous layer of tissue)
  • Subcutaneous layer (tissue containing fats and connective tissues)
• Pathogenicity


  A broad concept that describes an organism's potential to cause infection or disease
• Types of pathogens

  • Primary pathogens (capable of causing disease in healthy people with normal immune defenses)
  • Opportunistic pathogens (cause diseases when the host's defenses are compromised or when they become established in a part of body that is not natural to them)
• Factors that weaken host defenses

  • Old age and extreme youth
  • Genetic/Acquired defects in immunity
  • Surgery and organ transplants
  • Organic disease: cancer, liver malfunction, diabetes
  • Chemotherapy
  • Physical and mental stress
  • Other infections
• Virulence


  The degree of pathogenicity of a microorganism
• Virulence factors


  • Any characteristic or structure of a microbe that contributes to infection or disease state
• Portals of entry for microbes

  • Gastrointestinal Tract
  • Respiratory Tract
  • Urinary Tract
  • Genital Tract
  • Human skin (pores of skin)
• Microbes and their associated diseases

  • Influenza A (Pneumonia and respiratory failure)
  • Cryptosporidium parvum (Cryptosporidiosis)
  • Gardnerella vaginalis (Bacterial vaginosis)
  • Escherichia coli (Urinary tract infection)
  • Trichophyton rubrum (Tinea pedis/Athlete's foot)
• Physical barrier


  • Epithelial cells on body surfaces preventing entry of pathogens, mucous membranes
• Chemical barrier


  • Low pH in stomach, mucus secreted by mucous membrane
• Biological barrier (Normal flora)


  • Lowering pH so that other bacteria can't grow, Competing with invaders for space and nutrients, Producing compounds (bacteriotoxins) which kill other bacteria
• Tools available for microbes to overcome barriers

  • Attachment (Adhesin, Exopolysaccharide, Fimbriae, Flagella, Suction disc)
  • Invasion (Extracellular enzymes, Exotoxins)
  • Escape or defeat host defense (Exopolysaccharide, Proteases)
• Damage caused by microbes

  • Direct damage (Exotoxins, Extracellular enzymes)
  • Indirect damage (Endotoxins/Lipopolysaccharide, Lipoteichoic acid)
• Traces generated by microbes and host system

  • From microbes (Parts of microbes, Toxins, Metabolites)
  • From host (Antibodies, Cytokines/Interleukins)
• Evidences that can be collected and examined

  • Blood specimen
  • Urine specimen
  • Nasal swab
  • Throat swab
• Significance of evidence

  Direct evidences of the microbes have higher significance than indirect evidences from the host
• Methods for identification of microbes

  • Isolation by culture
  • Microscopy
  • Biochemical tests
  • Molecular tests
  • Immunological tests
• Normal flora can interfere in diagnosis by causing contamination or suppressing the growth of pathogens
• Direct microscopy of specimen can be used to identify certain microbes
• Biochemical tests


  • Catalase
  • Coagulase
  • Oxidase
• Molecular tests


  • Polymerase Chain Reaction (PCR)
• Immunological tests


  • Western blot
  • ELISA
• Counter measure strategies

  • Anti-adhesion therapies
  • Disrupting pathogen surface receptor biosynthesis
  • Inhibition of host receptor binding
  • Use of host receptor analogues
  • Anti-adhesion antibodies and vaccines
  • Antimicrobial therapies
  • Use of antitoxins (antibodies against toxin)
  • Personal and hand hygiene, healthy lifestyle, safe sex, preventing indiscriminate use of antimicrobials
• Steps in pathogenesis of infection

  • Entry
  • Establishment
  • Evade host defence
  • Damage the host
  • Exit from the host
• Direct traces (microbe or antigen itself)
• Indirect traces (antibody produced from host immune system)