Oxygen requirements shape microbial growth and survival. From obligate aerobes thriving in oxygen-rich environments to anaerobes that can't tolerate it, microbes have diverse needs. Understanding these requirements is crucial for studying microbial ecology and pathogenesis.

Carbon dioxide also plays a role, with some microbes needing higher levels for growth. These gas requirements influence where microbes can live and how they interact with their environment, impacting everything from human health to environmental processes.

Oxygen Requirements for Microbial Growth

Graphs of microbial gas requirements

• Oxygen concentration
  • Aerobic microbes require higher oxygen levels for optimal growth thrive in oxygen-rich environments
  • Anaerobic microbes grow best in environments with little to no oxygen cannot tolerate high oxygen concentrations
  • Facultative anaerobes can grow in both high and low oxygen environments adapt to varying oxygen levels
• Carbon dioxide concentration
  • Capnophiles require higher carbon dioxide levels for growth typically 5-10% CO2 for optimal growth
  • Most microbes tolerate a range of carbon dioxide concentrations but may have specific preferences
• Growth curves depict microbial population changes over time in different conditions
  • Lag phase: microbes adapt to new environment, minimal growth preparing for exponential growth
  • Log phase: exponential growth, optimal conditions for the microbe population doubles at a constant rate
  • Stationary phase: growth rate equals death rate, nutrient depletion or waste accumulation limiting factors balance population
  • Death phase: death rate exceeds growth rate, population declines due to unfavorable conditions or resource exhaustion

Categories of oxygen requirements

• Obligate aerobes
  • Require oxygen for growth and survival cannot survive without oxygen
  • Use oxygen as the terminal electron acceptor in aerobic respiration to generate ATP
  • Utilize the electron transport chain for efficient energy production
• Microaerophiles
  • Require low levels of oxygen for growth (2-10%) thrive in environments with limited oxygen
  • Sensitive to high oxygen concentrations, which can be toxic due to oxidative stress
• Facultative anaerobes
  • Grow in both aerobic and anaerobic environments versatile and adaptable
  • Prefer aerobic conditions but can switch to fermentation or anaerobic respiration when oxygen is limited
• Aerotolerant anaerobes
  • Do not require oxygen for growth but can tolerate its presence oxygen does not harm them
  • Lack the ability to use oxygen in energy-generating processes rely on fermentation or anaerobic respiration
• Obligate anaerobes
  • Cannot tolerate oxygen and are killed by its presence oxygen is toxic to these microbes
  • Lack enzymes to detoxify oxygen radicals which cause cellular damage
  • Can be cultured using an anaerobic jar to create oxygen-free conditions
• Capnophiles
  • Require higher levels of carbon dioxide (5-10%) for optimal growth CO2 is essential for their metabolism
  • Often found in environments with high carbon dioxide, like the human body (respiratory tract, gastrointestinal tract)

Examples of oxygen-dependent microbes

• Obligate aerobes
  • Pseudomonas aeruginosa: opportunistic pathogen, causes infections in immunocompromised patients (pneumonia, sepsis)
  • Mycobacterium tuberculosis: causative agent of tuberculosis infects lungs and other tissues
• Microaerophiles
  • Helicobacter pylori: causes gastric ulcers and is linked to stomach cancer survives in the mucus layer of the stomach
  • Campylobacter jejuni: common cause of bacterial foodborne illness associated with undercooked poultry
• Facultative anaerobes
  • Escherichia coli: normal gut flora, some strains cause intestinal and extraintestinal infections (UTIs, meningitis)
  • Staphylococcus aureus: skin commensal, can cause various infections like pneumonia and sepsis also forms biofilms
• Aerotolerant anaerobes
  • Streptococcus mutans: primary cause of dental caries ferments sugars to produce acid that erodes tooth enamel
  • Lactobacillus acidophilus: probiotic species found in yogurt and supplements helps maintain healthy gut flora
• Obligate anaerobes
  • Clostridium tetani: produces the neurotoxin that causes tetanus spores are highly resistant to environmental conditions
  • Bacteroides fragilis: most common anaerobic pathogen, causes abdominal infections often involved in peritonitis and abscesses
• Capnophiles
  • Neisseria gonorrhoeae: causative agent of gonorrhea sexually transmitted infection
  • Haemophilus influenzae: can cause meningitis, pneumonia, and otitis media common in young children

Oxygen-related microbial processes and tests

• Oxidase test: determines the presence of cytochrome c oxidase in bacteria, important for identifying certain species
• Catalase test: detects the enzyme catalase, which breaks down hydrogen peroxide and is present in many aerobic and facultative anaerobic bacteria
• Reactive oxygen species: harmful byproducts of oxygen metabolism that can damage cellular components
• Redox potential: measure of the tendency of a chemical species to acquire electrons, influencing microbial growth in different environments
• Oxygen toxicity: damaging effects of excessive oxygen on microbial cells, particularly relevant for anaerobic organisms