Normal Flora

Chapter 13: Human and Microbe Interactions

This chapter focuses on the interactions between humans and microorganisms, discussing both beneficial bacteria and pathogens. It includes an overview of normal flora and later topics will delve into pathogenic bacteria and their implications for human health.

Overview of Normal Flora

  • Normal Flora: Refers to the collection of bacteria naturally found in and on the human body.

  • Terminology:

    • Normal Microbiota: Synonymous with normal flora.

    • Indigenous Microbiota / Microflora: Another term for normal flora.

    • Commensals: Bacteria that exist without causing harm to the host.

  • Quantitative Insight: It is noted that human bodies contain approximately 10 times more bacterial cells than human cells.

Definitions and Characteristics of Bacteria

  • Pathogen: A microbe that causes infection or disease.

  • Transient Microbiota:

    • Definition: Temporary microbes influenced by daily activities; usually eliminated through hygiene practices (e.g., handwashing).

    • Example: A person who travels on public transit (e.g., subway) will have different transient bacteria compared to someone who works outdoors (e.g., farmer).

  • Resident Microbiota:

    • Definition: More established microbes that persist over time.

    • Influencing Factors: Varied depending on environment and diet; for example, gut microbes differ greatly between populations (e.g., United States vs. Europe/China).

    • Dietary Impact:

    • A diet high in processed foods leads to unhealthy gut microbes.

    • A diet rich in fermented foods promotes healthier microbiota.

  • Genetic Factors: Studies suggest children often inherit microbiota similar to their parents.

Microbial Distribution in the Body

  • Common Sites of Normal Flora: The regions of the body typically hosting a significant number of bacteria include:

    • Skin: Microbes flourish primarily in the epidermis (outer layers), and cannot penetrate deeper layers.

    • Mucous Membranes: Found in areas exposed to the outside world, such as:

    • Respiratory tract

    • Gastrointestinal tract

    • Urethral opening

    • External genitalia

    • Ears and eyes

Benefits of Normal Flora

  • Microbial Antagonism: The ability of normal flora to prevent the growth of pathogens.

    • Mechanisms:

    • Secretion of antimicrobials that destroy pathogens.

    • Covering binding sites to prevent pathogen attachment.

    • Nutrient competition that starves pathogens.

  • Role in Immunity: Normal flora significantly contribute to the body's immune system. If antibiotics kill beneficial bacteria, pathogens may exploit this opportunity, resulting in infections.

Risks Associated with Normal Flora

  • Endogenous Infections: Occur when normal flora enter sterile body sites; can lead to infections.

  • Example:

    • E. Coli: A normal inhabitant of the large intestine that can cause urinary tract infections if introduced to the urinary tract from feces.

Sites that Should Remain Sterile

  • Sterile Areas:

    • Internal organs (liver, kidneys, bladder, lungs, brain, spinal cord, heart, muscles, bones, ovaries/testes, glands, sinuses, middle and inner ear, and internal eye)

    • Blood and cerebral spinal fluid must always be free of microbes to prevent infections, such as septicemia.

Origin of Normal Flora

  • Initial Colonization: Babies acquire their first microbes during vaginal birth from the mother.

  • C-Section Birth Effects: Babies born via C-section do not receive the same microbial exposure and may have health issues such as asthma or allergies associated with this difference.

  • Hygiene Hypothesis: Suggests that excessive cleanliness may prevent proper exposure to beneficial microbes, leading to allergies and asthma in later life.

    • Controversy: Seeding babies born via C-section with vaginal flora has shown mixed results and potential risk of pathogen introduction.

  • Breastfeeding Benefits: Breastfed babies typically have stronger immune systems compared to bottle-fed infants.

Examples of Normal Flora by Body Part

  • Mouth: Predominantly Streptococcus species; forms biofilms on teeth and is significant in oral health.

  • Large Intestine: Hosting the most diverse microbes, including

    • Anaerobes: Bacteria that thrive without oxygen (e.g., Clostridium; some strains like C. Diff can be pathogenic when normal flora are disrupted. When disrupted it could cause a deadly dirrhea that’s extremely hard to treat.).

    • Additional genera involved in digestion and vitamin production include Bacteroides, Bifidobacterium, and Fusobacterium. Studies correlate gut bacteria with obesity or leanness.

  • Respiratory Tract:

    • Common Flora: Includes Staphylococcus (particularly Staph aureus), Neisseria, and Haemophilus.

    • Important to maintain these bacteria in the respiratory tract to avoid respiratory infections such as meningitis when they invade sterile sites.

  • Genitourinary Tract:

    • In females, Lactobacillus helps maintain an acidic environment that inhibits yeast and fungal overgrowth. Antibiotics can disrupt this balance and lead to yeast infections.

Disruption of Normal Flora

  • Impact of Antibiotics: The unwarranted use of antibiotics can severely disrupt normal flora.

  • Fluctuations: Changes in diet, travel, or stress can affect the diversity of normal flora.

  • Probiotics:

    • Consuming probiotics (e.g., in yogurt and fermented foods) can help replenish healthy bacteria.