Microbial Flora and the Human Body

Study Notes

The human body harbors approximately 10^14 bacteria, collectively called "Normal Flora".
This bacterial population is constantly replenishing itself, even after disturbances such as washing or antibiotic use.

Resident flora refers to the microorganisms usually found on non-sterile parts of the body like the skin, nose, mouth, throat, and large intestine.
Resident flora typically protects against disease-causing organisms.
Under certain conditions, resident flora can cause disease.
Factors determining flora composition: pH, temperature, oxygen, water, nutrition, antibiotic use, sanitary conditions, air pollution, and hygienic habits.

Benefits:
- Competition with pathogens for resources
- Production of nutrients for the host
- First line of defense against pathogens
- Aid in digestion by breaking down food
- Role in toxin degradation
- Immune system maturation
Harmful effects:
- May cause dental caries, abscesses, or other infectious diseases
- May act as commensals: inhabiting the host without causing noticeable harm or benefit.

Mutual or commensal relationship: Host and microbe both benefit or the microbe benefits without harming the host.
Pathogenic relationship: The host experiences negative consequences ranging from slight to fatal.
Host factors: Provides a niche and nutrition for the microbe, and microbial communities may aid in digestion or synthesize nutrients.

Antibiotic overuse: Kills resident bacteria, allowing other resident flora or fungi to grow unchecked.
Injury: Allows resident flora to enter sterile areas and cause infection.
Weakened immune system: Individuals with compromised immune systems are susceptible to infections.

Different areas of the body possess distinct microenvironments and microbial inhabitants.

Characteristics: Relatively dry, slightly acidic, and nutrients are primarily derived from dead cells.
Dominant microorganisms:
- Staphylococcus aureus (common in dermatologic conditions)
- Micrococci
- Staphylococcus epidermidis (major inhabitant; in some areas, it constitutes more than 90% of the resident aerobic flora)
- Corynebacterium diphtheroids (anaerobic diphtheroids are prevalent in areas rich in sebaceous glands)
- Streptococci (alpha-hemolytic streptococci are primarily found in the mouth)
- Gram-negative bacilli (small proportion; seen in moist areas)
- Desiccation (dryness) hinders the growth of gram-negative bacteria on intact skin.

Similar to skin flora
Additional inhabitants: Fungi (Aspergillus, Penicillium) and bacilli trapped under the nail from dust particles.

Dominant organisms:
- Staphylococcus aureus (major cause of surgical wound and systemic infection)
- MRSA (Methicillin Resistant Staphylococcus aureus) - posing a serious health threat due to antibiotic resistance

Upper Respiratory Tract: Primarily bacterial flora found in the oral cavity.

Duodenum & Ileum: Sparse flora due to rapid peristalsis and bile presence.
Jejunum & Ileum: Increased bacterial populations, with streptococci, lactobacilli, Bacteroides, Clostridium, and bifidobacteria dominating.
Large Intestine: Highest bacterial concentrations, including over 400 identified species, mostly anaerobes. These bacteria participate in bile acid conversion, vitamin K and ammonia production.

Anterior urethra: S.epidermidis, enterococci, and diphtheroids; E.coli, Proteus, and Neisseria are occasionally found.

Sparse flora
Lysozyme in tears controls bacteria by interfering with cell wall formation.
Dominant genera: Haemophilus, Streptococci, and Staphylococcus.

Opportunistic Infections: Normal flora can act as opportunistic pathogens in susceptible hosts, often caused by factors like rheumatic heart disease, immunosuppression, radiation or chemotherapy, and perforated mucous membranes

Bacteria perform physiological, nutritional, and protective functions in the human body.
Maintaining a balanced normal flora is crucial.
Normal flora disruptions can lead to unpredictable consequences, often caused by factors like antibiotics, tissue damage, medical procedures, diet changes, and new pathogens.