13.3 Microbial Ecology and Human Microbiome

Microbial ecology explores how microbes interact with each other and their environment. This topic dives into symbiotic relationships, biofilms, and quorum sensing. It also covers the human microbiome, highlighting the importance of our resident microbes in health and disease.

Understanding microbial interactions is crucial for grasping how bacteria cause infections and develop antibiotic resistance. This knowledge connects to broader themes in microbiology, showing how tiny organisms can have big impacts on human health and ecosystems.

Microbial Interactions

Symbiotic Relationships

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• Symbiosis describes close and long-term interactions between different biological species
  • Can be mutualistic (both species benefit), commensalistic (one species benefits while the other is unaffected), or parasitic (one species benefits at the expense of the other)
• Biofilms are communities of microorganisms that attach to each other and adhere to surfaces, often held together by an extracellular matrix of polymeric substances (slime)
  • Protect bacteria from antibiotics and host immune responses (dental plaque)
  • Can form on medical devices like catheters and implants, leading to persistent infections
• Quorum sensing allows bacteria to communicate and coordinate their behavior based on population density
  • Bacteria secrete signaling molecules called autoinducers that accumulate as the population grows
  • At a certain threshold concentration, autoinducers trigger changes in gene expression and behavior (bioluminescence in Vibrio fischeri)

Opportunistic Infections

• Opportunistic infections occur when normally harmless microbes cause disease in individuals with compromised immune systems or altered microbiota
  • Often involve bacteria that are part of the normal flora but overgrow and invade tissues when defenses are weakened
• Opportunistic pathogens include Pseudomonas aeruginosa (pneumonia in cystic fibrosis patients) and Candida albicans (thrush in infants and immunocompromised individuals)

Human Microbiome

Microbiota and Health

• The microbiome refers to the collective genomes of the microorganisms living in and on the human body
  • Includes bacteria, archaea, fungi, and viruses
• The gut flora, or intestinal microbiota, plays a crucial role in digestion, nutrient absorption, and immune system development
  • Helps break down complex carbohydrates, synthesize vitamins, and prevent colonization by pathogens (Bacteroides and Firmicutes)
• Probiotics are live microorganisms that confer health benefits when consumed in adequate amounts
  • Can help restore the balance of gut microbiota after antibiotic treatment and alleviate symptoms of inflammatory bowel disease (Lactobacillus and Bifidobacterium)

Microbiota Dysbiosis

• Disruption of the normal microbiota, known as dysbiosis, has been linked to various health issues
  • Antibiotic use, dietary changes, and stress can alter the composition and diversity of the microbiota
• Dysbiosis may contribute to the development of obesity, diabetes, allergies, and autoimmune disorders
  • Reduced diversity and abundance of beneficial bacteria like Akkermansia muciniphila have been observed in individuals with these conditions

Microbial Pathogenesis

Pathogenic Mechanisms

• Pathogens are microorganisms that cause disease by invading host tissues, multiplying, and producing toxins or other virulence factors
  • Can enter the body through various routes, such as inhalation, ingestion, or skin contact (Streptococcus pyogenes)
• Pathogens employ various strategies to evade host defenses and cause damage
  • Adhesion to host cells, invasion of tissues, production of toxins, and modulation of immune responses (Staphylococcus aureus)

Antibiotic Resistance

• Antibiotics are substances that kill or inhibit the growth of bacteria
  • Act by targeting essential bacterial processes like cell wall synthesis, protein synthesis, or DNA replication (penicillin, tetracycline)
• Antibiotic resistance occurs when bacteria evolve mechanisms to survive exposure to antibiotics
  • Can arise through mutations or acquisition of resistance genes from other bacteria via horizontal gene transfer
• Misuse and overuse of antibiotics have accelerated the spread of antibiotic-resistant bacteria, making infections more difficult to treat (methicillin-resistant Staphylococcus aureus - MRSA)