Week 9

Microorganisms and Disease: Normal Flora

• Humans have 30 trillion cells and 40 trillion microorganisms.
• Normal flora (or microbiota) is a mixture of pathogenic and harmless organisms.
• It includes resident and transient microflora.
• Normal flora generally does not cause disease.
• Opportunistic infections can arise if the immune system is compromised (elderly, immune deficiencies).
• Examples include Streptococcus pneumoniae and Staph aureus.
• Microorganisms can be beneficial; they produce vitamins B and K, stimulate the immune system, and assist in recognizing foreign bodies.
• Microorganisms prevent pathogens from colonizing tissues.
• Examples of helpful bacteria include Bifidobacterium (probiotic), Escherichia coli, Bacterioides, and Lactobacillus.
• These are found in the large intestine.

Opportunistic Bacteria

• Humans can be infected by microorganisms (MOs) that take advantage of weakened immune systems, wounds, or altered microbiota.
• They do not normally cause disease but may do so in certain situations.
• They may cause infection in locations where they are not usually found.
• Lack of competition from commensals allows them to proliferate.
• Examples are Pseudomonas aeruginosa, Salmonella, and Streptococcus/Staphylococcus species.

Location of Normal Flora

• Normal flora can be found in various locations, including:
  • Human skin (e.g., armpits, groin, soles of feet, hair follicles, sweat glands)
  • Respiratory tract (nose, nostrils, buccal/oral area)
  • Digestive system (mouth, teeth, throat, stomach, duodenum, jejunum, ileum, colon, proximal ileum, distal ileum)
  • Urogenital tract (kidneys, ureters, urethra, bladder).

Factors Affecting Normal Flora

• The location of normal flora depends on several factors including:
  • Moisture level
  • pH level
  • Temperature
  • Salinity
  • Presence of waste products
  • Presence of microbes.

The Integument

• Human skin has a variety of textures and thicknesses.
• Areas that are moist support more microbial growth than dry areas.
• Differences in pH, hair follicles, and sweat glands impact microbial growth.
• Skin often has anaerobic microbes.

The Respiratory Tract (Upper and Lower)

• The upper respiratory tract (URT) is moist, promoting microorganism growth.
• Most URT infections are viral.
• Some infections are bacterial, fungal, or helminthic.
• Defenses like cilia and mucus help remove pathogens.
• Lower respiratory tract (LRT) is normally free from pathogens.
• Damage allows pathogens to enter, smoking, air pollution, and gases also contribute.

Nose and Nostrils

• The nose and nostrils are dense with bacteria.
• Little is known about the beneficial function of many of these microbes.
• Staphylococcus aureus and Streptococcus pneumoniae are common inhabitants.

Buccal Area (Mouth)

• A multitude of species (100–200) can be found in the mouth.

• 100,000 bacteria per tooth surface.

• A large majority of organisms are streptococci.
• Streptococcus mutans is a major contributor to tooth decay.
• Moist environment, pH, and temperature support bacterial growth.

Digestive Tract

• The digestive tract flora is influenced by various factors including:
  • Age
  • Diet
  • Medications
  • Lifestyle
  • Fast food vs. home-cooked foods
  • Cultural aspects.
• The stomach is a first line of defense.
• Gastric acid and enzymes inhibit bacterial growth.
• Some bacteria adapt to the low pH. (example Helicobacter pylori)
• The colon (large intestine) has a diverse microbial population (~600 species), often anaerobic.
• They break down waste materials.
• Approximately half of fecal mass is comprised of bacteria.

The Urogenital System

• Kidneys, ureters, and the bladder are generally sterile.
• The openings of the urethra and vagina have normal flora; microorganisms such as bacteria, viruses, and fungi.
• Constant flushing prevents buildup of pathogens.
• Urinary tract infections (UTIs) may arise from changes in normal flora or blockage.