microbial symbiosis and normal flora

• Microbial Symbiosis


  Co-existence of two microorganisms
• Types of Microbial Symbiosis

  • Mutualistic relationship
  • Commensalistic relationship
  • Pathogenic/Parasitic relationship
• Normal Flora


  Microorganisms that are frequently found in/on the bodies of healthy persons
• Other names for Normal Flora

  • Normal microbiota
  • Normal microflora
  • Physiological microbiota
• Babies are not exposed to bacteria in the womb as this could be deadly for the fetus
• Most babies make their way out through the birth canal which is full of bacteria
• Types of Normal Flora

  • Resident Normal Flora
  • Transient Normal Flora
• Resident Normal Flora


  Microorganisms that colonize an area for months or years and promptly re-establish themselves if disturbed
• Transient Normal Flora


  Non-pathogenic or potentially pathogenic microorganisms that inhabit the skin or mucous membranes for hours or days, mostly derived from the environment
• Beneficial Role of Normal Flora

  • Immunostimulation
  • Protection from External Invaders
  • Nutrition
  • Inhibitory Action
  • Stimulate Development of Certain Tissues
• Harmful Effects of Normal Flora

  • May be a Source of Opportunistic Infections
  • May act as Pathogens
  • Interferes with Therapy
  • Use of Broad Spectrum Antibiotics
  • May cause Diagnostic Confusion
• Normal Flora of the Skin


  • Wide variety of microorganisms found in the most superficial layers of the epidermis and upper parts of hair follicles
  • Propionibacterium acnes colonizes the sebaceous glands
  • Micrococcus, Staphylococcus spp., and diphtheroids are common
  • Intact skin is an effective barrier to microbial invasion
• Factors that help eliminate non-resident microorganisms from the skin

  Low pH, fatty acids in sebaceous secretions, and presence of lysozymes
• Normal Flora of the Oral Cavity

  • Streptococcus (viridans spp.)
  • Coagulase-negative Staphylococcus
  • Peptostreptococcus
  • Other anaerobes
• S. mutans


  Causes dental caries
• Normal Flora of the Conjunctiva

  • Present in low numbers due to high moisture, blinking mechanically removing bacteria, and lachrymal secretions including lysozyme
• Pathogens that infect the conjunctiva

  • Neisseria gonorrhoeae
  • Chlamydia trachomatis
• Gonococcal Conjunctivitis


  Caused by N. gonorrhoeae, also known as gonococcal ophthalmia neonatorum, most likely due to maternal transmission during birth
• Chlamydial Conjunctivitis


  Caused by C. trachomatis, spread by direct or indirect contact, repeated infections lead to scarring of the conjunctiva and blindness
• Modes of Protection in the Respiratory Tract

  • Ciliary action of epithelial cells and mucus movement
  • Phagocytic action of macrophages
  • Production of lysozyme in mucus
• Major Secretions of the Respiratory Tract

  • Upper: Nasopharynx, oral cavity, throat (colonized by a wide variety of microorganisms including pathogens)
  • Lower: Trachea, lungs (normally sterile)
• Most commonly found organisms in the GI tract

  • Bacteroides
  • Clostridium
  • Eubacterium
  • Streptococci
  • Enterococcus
  • Enterobacteriaceae
• More than 90-95% of the microbial population in the GI tract is comprised of anaerobes
• The entire intestinal tract is sterile at birth, but bacteria enter with the first feed</b>
• In breast-fed infants, Bifidobacteria account for more than 90% of the total intestinal bacteria
• Helicobacter pylori


  An acid-tolerant neutrophile due to urease activity, but urease activity also limits its survival to an acidic environment, survives in the submucosal layer
• Probiotics


  Oral administration of living organisms to promote health, mechanism is competition with other bacteria and stimulation of non-specific immunity, examples are Yakult, Yogurt, Fermented foods
• Prebiotics


  Non-digestible food that stimulates growth or activity of GI microbiota, especially Bifidobacteria and Lactobacillus, typically a carbohydrate like soluble fiber
• Normal Flora of the Urogenital Tract

  • Vagina is colonized with Lactobacillus, anaerobic gram-negative bacilli, and gram-positive cocci
  • Outermost segment of the urethra is colonized by skin organisms
  • Lactobacillus in the vagina lower the pH to around 4-5, which is inhibitory for other microorganisms
• Opportunistic Pathogen


  An infectious microorganism that is normally a commensal or does not harm its host but can cause disease when the host's resistance is low
• Opportunistic Infection


  An infection caused by pathogens, particularly opportunistic pathogens
• Examples of Opportunistic Pathogens

  • Staphylococcus aureus
  • Streptococcus mutans
  • Enterococcus faecalis
  • Streptococcus pneumoniae
  • Pseudomonas aeruginosa
  • Escherichia coli
• Opportunistic Conditions


  • When the immune system isn't working properly
  • When the balance of normal microbes is disrupted (dysbiosis), e.g. due to broad spectrum antibiotics
  • Breach of skin/mucosal barrier, bacteria at one site may be commensal but pathogenic at another site
• Gram-negative bacilli as Opportunistic Pathogens


  • Nosocomial pathogens, cause infections particularly in patients with severe burns, cancer patients, and individuals with AIDS
  • Help in food digestion
  • Most frequent causative agent of urinary tract infection (UTI)
• Classification of Gram-negative Bacilli


  • P. aeruginosa
  • E. coli
  • ETEC
  • EPEC
  • EIEC
  • EHEC
  • EAEC
• H. ducreyi


  Causes chancroid, characterized by painful necrotizing genital ulcers, common causative agent of community- or hospital-acquired bacterial skin infections among patients with HIV infection
• S. aureus


  Expresses number of virulence factors which help to establish infection
• Encephalitis is inflammation of the brain caused by herpes virus
• Meningococcemia is when bacteria enter the bloodstream and multiply, damaging the walls of blood vessels causing bleeding in skin and organs
• Gonorrhoea is symptomatic to men and could lead to blindness of a newly born child