Microbial Oxygen Relationships

Oxygen Relationships of Microorganisms

• Microorganisms are grouped based on their oxygen requirements or intolerance
• Aerobes grow in the presence of molecular oxygen
• Obligate aerobes need oxygen for growth and aerobic respiration
• Microaerophiles grow only at reduced oxygen concentrations (around 5% less than atmospheric levels)
• Microaerophiles will not grow in air

Facultative Anaerobes

• Facultative anaerobes grow in the presence or absence of air
• Many, like E. coli, switch between aerobic respiration and fermentation depending on oxygen availability
• They use fermentation in the absence of oxygen and aerobic respiration in its presence
• This group includes strictly fermentative bacteria (like streptococci) that are insensitive to oxygen

Other Bacteria and Anaerobes

• Other bacteria are anaerobes and grow only in the absence of air
• Obligate anaerobes carry out fermentative metabolism
• Various bacteria (e.g., sulfate-reducing Desulfovibrio) and archaea (e.g., methanogenic archaea) are included
• Strict anaerobes are killed by brief oxygen exposure

Effects of Acidity and pH

• Acidity or alkalinity affects microbial growth
• Most grow best between pH 6 and 8
• Internal cell pH must remain close to neutral despite external conditions
• Acidophiles grow best at low pH (e.g., Helicobacter pylori)
• Alkaliphiles grow best at high pH (e.g., Vibrio cholera)
• Most pathogenic bacteria are neutrophiles

pH of a Solution and Bacterial Growth

• pH describes hydrogen ion concentration
• Bacterial growth rates depend greatly on pH values and protein structure
• Enzymes are inactive at very high or low pH values
• Bacteria are less tolerant of high temperatures at low pH than at neutral pH
• pH values are controlled in culture media and industrial fermenters to optimize growth

Buffers and pH Control

• Buffers maintain a specific pH range, preventing large changes
• Phosphate buffers are used at neutral pH
• Borate buffers are used at alkaline pH
• Citrate buffers are used at acidic pH
• Most bacteria grow well within a pH range of 6 to 9

pH and Microbial Metabolism

• pH of the medium determines dominant carbohydrate metabolism pathways
• Amino acid fate (carboxylation or deamination) depends on pH

Effects of Pressure (Osmotic and Hydrostatic)

• Growth of all cells is affected by external and internal pressure
• Forces include osmotic and hydrostatic pressure
• Hydrostatic pressure arises from water column weight (deep ocean)
• Osmotic pressure results from water diffusion across the membrane in response to solute concentrations
• Salinity (salt concentration) is often associated with osmotic pressure

Osmotic Pressure and Salinity

• Cell walls make bacteria relatively resistant to osmotic pressure changes
• Hypertonic solutions cause shrinkage (desiccation)
• Hypotonic solutions cause bursting
• Organisms growing in high solute concentrations are osmotolerant
• Osmophiles require high solute concentrations for growth
• Salinity has a substantial effect on osmotic pressure
• Some bacteria have specific responses to salt (NaCl) concentrations, some are halophiles

Effects of Light

• Photosynthetic bacteria need light for ATP production
• They function optimally at specific light intensities and wavelengths
• Some photosynthetic bacteria exhibit phototaxis (movement toward light)
• Certain mechanisms regulate flagellar rotation with changing light intensity
• Bacteria can respond to wavelengths
• Visible and UV light can damage proteins and DNA
• Pigments (like carotenoids) protect from harmful light
• UV light can cause DNA replication issues leading to mutations

Effects of Radiation

• Light is essential for growth in photosynthetic bacteria
• Other bacteria are mostly harmed by light, especially UV radiation
• UV light toxicity is due to absorption in nucleic acids and proteins
• UV light prevents DNA replication and can cause lethal mutations

Effects of Chemicals

• Chemical substances can be nutrients or prevent bacterial growth
• Certain chemicals act as bacteriostatic agents (prevent growth) or bactericidal agents (kill bacteria)
• Some chemicals destroy cell structures (like denaturing proteins with alcohol)

Effects of Antibiotics

• Penicillin inhibits transpeptidase, affecting peptidoglycan cross-linking (prevents cell wall formation in bacteria, not eukaryotes).
• Oxacillin, bacitracin, and vancomycin are cell wall antibiotics, they inhibit cell wall biosynthesis, causing weakened cell walls, cell lysis, and death
• Actinomycin interferes with RNA synthesis by binding to DNA thus preventing transcription..

Explore the diverse oxygen requirements of microorganisms in this quiz. Learn about aerobes, obligate aerobes, microaerophiles, and facultative anaerobes, along with other anaerobes. Understand how these classifications impact their growth and metabolism.