Bacteria: Biofilms, Microbiome, Pathogenicity, Toxins, Nutrition, Photosynthesis, Decomposers, and the Nitrogen Cycle

Biofilms

• What they are: a matrix of bacteria embedded in a gel-like extracellular polymeric substance that adheres to surfaces; forms a slimy layer that is difficult for antibiotics to penetrate; involves cell-to-cell signaling (quorum sensing) with signaling molecules used to coordinate behavior among the same or different species.
• How they form: bacteria attach to a surface, multiply, and start producing the matrix; this matrix traps more bacteria and facilitates communication, leading to a mature biofilm.
• Real-world examples and contexts: buildup inside water bottles if not washed (classic biofilm formation); contact lenses can develop biofilms if not cleaned properly; hospital settings see biofilms on catheters and joints replaced or implanted devices.
• Why they matter clinically: biofilms are tough to eradicate because they create a protective environment that antibiotics struggle to penetrate; can lead to persistent infections and device-related problems.

Microbiome and host health

• Microbial definition: all bacteria in and on organisms (the microbiome).
• Species diversity in humans: in environments like the mouth and gut, there can be around
  $ext{≈ }500$ different bacterial species (often more in reality).
• Factors that influence the microbiome: diet, stress, medications, and other lifestyle factors.
• Disruption consequences: disruption of the normal microbiome can contribute to autoimmune disorders and other health issues.
• Antibiotics and microbiome disruption: antibiotic treatment often disrupts the microbiome; doctors may recommend probiotics or yogurt to help repopulate beneficial bacteria.
• Early life microbiome formation:
• Normal vaginal delivery + breastfeeding tends to establish a healthy microbiome in the newborn.
• Cesarean section (C-section) delivery and formula feeding can lead to a different, less optimal microbiome composition.
• This difference is associated with higher incidences of certain autoimmune disorders in children.
• Vitamin production by gut bacteria: gut microbes contribute to production of vitamins, including
  $ext{B}_{12}$ and $ext{K}$.
• Nutrient absorption and gut localization: most nutrient absorption occurs in the small intestine, with some absorption in the large intestine.
• Where bacteria reside in the gut: bacteria attach to the mucosal wall of the large intestine, forming a layer that resembles a microbial organ or specialized tissue; this layer aids in uptake and metabolic interactions.
• Role in animals: many animals rely on gut microbes to digest complex components (e.g., grasses) that they could not digest on their own; without these microbes, digestion would be inefficient or impossible and could lead to illness.
• Relationship to disease and health practices: understanding the microbiome informs strategies like diet planning, probiotic use, and antibiotic stewardship to protect host health.

Determinants of bacterial pathogenicity: invasiveness vs toxigenicity

• Two key determinants:
• Invasiveness: how well a bacterium can invade and spread within the host (its growth and dissemination potential).
• Toxigenicity: the ability to produce toxins; potency varies by toxin and organism.
• Conceptual examples:
• A bacterium with low invasiveness but high toxigenicity can cause severe disease in a localized region (toxin-mediated damage without widespread invasion).
• Anthrax example: often described as high invasiveness with relatively lower toxin activity in some contexts; the spread of the organism can cause systemic disease; the toxin component contributes to pathogenicity but dissemination is a major factor.
• Public health relevance: understanding these factors informs risk assessment and vaccination strategies.

Bacterial toxins: endotoxins vs exotoxins

• Endotoxins
• Source: part of the outer membrane of Gram-negative bacteria; released when the bacteria lyse.
• Mechanism: fragments of the membrane stimulate the host immune system, often causing fever and inflammation.
• Clinical impact: endotoxins can provoke strong immune responses but are rarely directly fatal; Proteobacteria are major producers of endotoxins.
• Exotoxins
• Source: soluble proteins secreted by bacteria into the surrounding environment.
• Mechanism: secreted toxins that can act at distant sites and have diverse molecular targets.
• Clinical impact: exotoxins can be extremely potent and cause severe disease even at low concentrations.

Bacterial nutrition and respiration: aerobic, anaerobic, and versatility

• Respiration basics: cellular respiration uses nutrients to generate ATP, the energy currency of the cell.
• Aerobes: use oxygen for cellular respiration (require oxygen).
• Anaerobes: do not use oxygen for respiration.
• Obligate anaerobes: cannot tolerate oxygen at all.
• Facultative anaerobes: can switch between aerobic respiration (with oxygen) and fermentation (without oxygen); they do not require oxygen but can tolerate it.

Photoautotrophs, photoheterotrophs, and chemoautotrophs/chemoheterotrophs

• Photoautotrophs
• Energy source: light (phototrophy).
• Carbon source: carbon dioxide (autotrophy).
• Pigments and membranes: pigments such as chlorophyll a capture light energy; photosynthetic membranes are present in some bacteria (e.g., cyanobacteria) and may involve additional pigments that capture different wavelengths.
• Water as electron donor: in oxygenic photosynthesis, water is the electron donor and oxygen is released as a byproduct.
• If alternative electron donors are used (e.g., H extsubscript{2}S), oxygen may not be produced.
• Photoheterotrophs
• Energy source: light.
• Carbon source: organic compounds (not CO extsubscript{2}).
• Chemoautotrophs
• Energy source: inorganic chemical reactions (e.g., oxidation of nitrite, sulfide).
• Carbon source: carbon dioxide (CO extsubscript{2}).
• Chemoheterotrophs
• Energy source: organic molecules.
• Carbon source: organic molecules (not CO extsubscript{2}).
• Summary table (concepts):
• Photoautotroph: energy from light; carbon from CO extsubscript{2}.
• Photoheterotroph: energy from light; carbon from organic compounds.
• Chemoautotroph: energy from inorganic compounds; carbon from CO extsubscript{2}.
• Chemoheterotroph: energy from organic compounds; carbon from organic compounds.

Decomposers and the role in ecosystems

• Definition: organisms that break down dead and decaying matter.
• Why decomposers matter: recycle nutrients and return inorganic forms to the soil, helping close the carbon cycle and supporting soil fertility.

The nitrogen cycle: fixation, transformation, and denitrification

• Nitrogen fixation
• Organisms: nitrogen-fixing bacteria (e.g., Rhizobium).
• Reaction: molecular nitrogen (N₂) is converted into ammonia (NH₃), which is subsequently usable by organisms.
• Ammonia to nitrite
• Organisms: Nitrosomonas and related bacteria (nitrosifiers).
• Reaction: NH₃ → NO₂⁻ (nitrite).
• Nitrite to nitrate
• Organisms: Nitrobacter and related bacteria (nitrifiers).
• Reaction: NO₂⁻ → NO₃⁻ (nitrate).
• Denitrification
• Organisms: various denitrifiers (e.g., Bacillus species).
• Reaction: NO₃⁻ → N₂ (gas) or other gaseous forms released to the atmosphere, completing the cycle.
• Ecological and agricultural significance: plants typically take up ammonia or nitrate; these transformations make nitrogen available to ecosystems and help regulate soil nitrogen balance.

Key organisms to know

• Rhizobium: nitrogen fixer in root nodules of legumes; supplies ammonia to plants.
• Nitrosomonas and Nitrococcus: ammonia-oxidizing bacteria that form nitrite.
• Nitrobacter: nitrite-oxidizing bacteria that form nitrate.
• Bacillus (denitrifiers): convert nitrate back to nitrogen gas, completing the nitrogen cycle.

Practical and ethical implications

• Antibiotic stewardship: antibiotics disrupt the microbiome; prudent use is important to minimize collateral damage to beneficial bacteria.
• Probiotics and diet: post-antibiotic strategies often include probiotics or yogurt to help restore microbial balance.
• Medical device infections: biofilms on catheters and implants are a major clinical challenge; strategies to prevent biofilms include improved device design and targeted anti-biofilm therapies.
• Early life health: understanding how delivery mode and feeding influence the microbiome can inform neonatal care and long-term health considerations.

Connections to foundational principles

• Microbial ecology: biofilms and microbiomes illustrate community-level interactions and niche construction.
• Metabolism and energetics: respiration types, photosynthesis, and chemotrophy underpin microbial energy generation.
• Host-microbe interactions: microbiomes influence health, disease, nutrition, and immunity.
• Biogeochemical cycles: microbes drive the nitrogen cycle and nutrient cycling in ecosystems.

Quick recap of key equations and forms

• Photosynthesis (general, oxygenic):
  $6\,CO<em>2 + 6\,H</em>2O + \text{light} \rightarrow C<em>6H</em>{12}O<em>6 + 6\,O</em>2$
• Nitrogen cycle species and forms (simplified):
• $N<em>2 \rightarrow NH</em>3 \; (NH_4^+\text{ usable forms})$ (nitrogen fixation)
• $NH<em>3 \rightarrow NO</em>2^-$ (ammonia to nitrite, nitrosomonas)
• $NO<em>2^- \rightarrow NO</em>3^-$ (nitrite to nitrate, nitrobacter)
• $NO<em>3^- \rightarrow N</em>2$ (denitrification, release to atmosphere; example: Bacillus)

Notes on terminology encountered in lectures

• "Microbial" commonly refers to bacteria and other microorganisms associated with a host or environment.
• "Pathogenicity" arises from combinations of invasiveness and toxigenicity, which together determine disease outcomes.
• "Quorum sensing" refers to the signaling process by which bacteria coordinate group behaviors within a biofilm or community.
• A healthy microbiome supports host health, while disruption can contribute to disease; restoration strategies include probiotics and dietary considerations.