Microbiology Study Guide: Bioenergetics, Nitrogen Cycle, and Disinfection

Bioenergetics in Microorganisms

Electron Transport and ATP Synthesis

Microorganisms use electron transport chains (ETC) to generate energy in the form of ATP. The movement of electrons through membrane-bound carriers creates a proton gradient, which drives ATP synthesis.

• Electron Transport Chain (ETC): A series of protein complexes and electron carriers embedded in a membrane that transfer electrons from donors (like NADH) to acceptors (like O2), releasing energy.

• Proton Motive Force (PMF): The energy stored as a proton gradient across a membrane, used to drive ATP synthesis.

• ATP Synthase: An enzyme that uses the PMF to convert ADP and inorganic phosphate (Pi) into ATP.

• Key Equation:

$\text{ADP} + \text{P}_i + \text{H}^+_{\text{outside}} \rightarrow \text{ATP} + \text{H}_2\text{O}$

• Electron Donors: NADH, FADH2, and others provide electrons to the ETC.

• Electron Acceptors: Oxygen (aerobic), nitrate, sulfate, or other molecules (anaerobic).

Example: In Escherichia coli, the ETC can use either oxygen or nitrate as the terminal electron acceptor, depending on environmental conditions.

Types of Phototrophy

Phototrophic microorganisms use light energy to drive electron transport and ATP synthesis. There are several types:

• Cyclic Photophosphorylation: Electrons cycle back to the photosystem, producing ATP but not NADPH.

• Non-cyclic Photophosphorylation: Electrons are transferred to NADP+ to form NADPH, and ATP is also produced.

• Purple Sulfur Bacteria: Use light energy but do not generate enough energy to reduce NADP+ directly; they use reverse electron flow.

• Green Sulfur Bacteria: Use light energy to reduce NADP+ and create a PMF to make ATP.

Additional info: Phototrophy is essential for primary production in many ecosystems, especially aquatic environments.

Electron Carriers

Electron carriers are molecules that transfer electrons during cellular respiration and photosynthesis.

• NAD+ / NADH: Nicotinamide adenine dinucleotide, a key electron carrier in catabolic reactions.

• FAD / FADH2: Flavin adenine dinucleotide, another important carrier.

• Quinones: Lipid-soluble carriers in the ETC.

• Cytochromes: Proteins with heme groups that transfer electrons.

Nitrogen Cycle in Microbiology

Overview of the Nitrogen Cycle

The nitrogen cycle describes the transformations of nitrogen compounds in the environment, largely mediated by microorganisms.

• Nitrogen Fixation: Conversion of atmospheric N2 to ammonia (NH3).

• Nitrification: Oxidation of ammonia to nitrite (NO2-) and then to nitrate (NO3-).

• Denitrification: Reduction of nitrate to nitrogen gas (N2), returning it to the atmosphere.

• Ammonification: Decomposition of organic nitrogen to ammonia.

Process	Microbial Group	Key Enzyme
Nitrogen Fixation	Rhizobium, Azotobacter	Nitrogenase
Nitrification	Nitrosomonas, Nitrobacter	Ammonia monooxygenase, Nitrite oxidoreductase
Denitrification	Pseudomonas, Paracoccus	Nitrate reductase
Ammonification	Decomposers	Various hydrolases

Example: Rhizobium species form symbiotic relationships with legumes, fixing atmospheric nitrogen in root nodules.

Unique Microbial Metabolisms

• Ammonia-oxidizing bacteria: Convert NH3 to NO2-.

• Nitrite-oxidizing bacteria: Convert NO2- to NO3-.

• Denitrifiers: Use nitrate as an electron acceptor in anaerobic respiration, producing N2 gas.

• Diazotrophs: Fix atmospheric nitrogen into ammonia.

Microbial Disinfection and Control

Definitions and Levels of Disinfection

Disinfection refers to the reduction or elimination of pathogenic microorganisms on surfaces or in liquids. There are different levels of disinfection:

• High-level disinfection: Kills all pathogens except high numbers of bacterial spores.

• Intermediate-level disinfection: Kills mycobacteria, most viruses, and bacteria.

• Low-level disinfection: Kills some viruses and bacteria, but not mycobacteria or spores.

Critical, Semi-critical, and Non-critical Items:

• Critical: Items that enter sterile tissue or the vascular system; require sterilization.

• Semi-critical: Items that contact mucous membranes; require high-level disinfection.

• Non-critical: Items that contact intact skin; require low-level disinfection.

Methods of Disinfection

• Physical Methods: Heat (autoclaving, pasteurization), filtration, radiation.

• Chemical Methods: Alcohols, chlorine compounds, phenolics, quaternary ammonium compounds.

Example: Alcohol-based hand sanitizers are effective against many bacteria and viruses but not spores.

Resistance and Special Considerations

• Bacterial Endospores: Highly resistant to disinfection; require sterilization.

• Biofilms: Communities of microorganisms encased in a protective matrix, often more resistant to disinfectants.

• Antibiotic Resistance: Some bacteria can transfer resistance genes via conjugation, making infection control more challenging.

Additional info: Proper selection and use of disinfectants are critical in healthcare and laboratory settings to prevent the spread of infectious diseases.