Chap 7B

Oxygen Requirements in Microbes

Overview of Oxygen Use

Microorganisms exhibit diverse requirements and tolerances for oxygen, which influence their habitats and pathogenicity. Oxygen can be both essential and toxic to cells, depending on their metabolic capabilities.

• Obligate aerobes: Require oxygen for cellular respiration and growth.

• Obligate anaerobes: Cannot tolerate oxygen; oxygen is toxic to them.

• Facultative anaerobes: Can grow with or without oxygen; switch between aerobic respiration and fermentation.

• Microaerophiles: Require low levels of oxygen; high concentrations are toxic.

• Aerotolerant anaerobes: Tolerate oxygen but do not use it for metabolism.

Reactive Oxygen Species (ROS) are toxic byproducts of oxygen metabolism. Microbes have evolved enzymes to detoxify ROS:

• Superoxide dismutase: Converts superoxide ions ($O_2^-$) to hydrogen peroxide ($H_2O_2$).

• Catalase: Converts hydrogen peroxide to water and oxygen.

ROS can damage proteins and DNA, so aerobic microbes rely on antioxidants and detoxifying enzymes.

Table: Oxygen Use and Tolerance Classifications

Type	Oxygen Requirement	Growth Pattern in Tube	Detoxifies ROS?
Obligate Aerobe	Requires O2	Growth at top	Yes
Obligate Anaerobe	Cannot tolerate O2	Growth at bottom	No
Facultative Anaerobe	Grows with or without O2	Growth throughout, more at top	Yes
Microaerophile	Requires low O2	Growth just below surface	Yes (limited)
Aerotolerant Anaerobe	Tolerates O2, does not use	Growth evenly throughout	Yes

Pathogen Oxygen Profiles

Pathogens are adapted to the oxygen levels of their infection sites. For example, most lung pathogens are facultative anaerobes, allowing survival in both oxygen-rich and oxygen-poor environments.

• Lungs: Mycobacterium tuberculosis (obligate aerobe), Streptococcus pneumoniae (facultative anaerobe)

• Skin: Staphylococcus aureus (facultative anaerobe)

• Stomach: Helicobacter pylori (microaerophile)

• Large intestine: Escherichia coli (facultative anaerobe)

Microbial Nutritional Requirements

Essential Nutrients

Microbes require nutrients from their environment to grow, divide, and build cellular structures. About 90% of a cell's dry weight consists of carbon, hydrogen, oxygen (nongaseous), and nitrogen.

• Macronutrients: Needed in large amounts (e.g., carbon, nitrogen, phosphorus).

• Micronutrients: Needed in trace amounts (e.g., iron, copper, zinc).

Other important elements include sulfur, potassium, sodium, calcium, magnesium, and chlorine.

Carbon Sources

Organisms are classified by how they obtain organic carbon:

• Heterotrophs: Require external organic carbon sources (e.g., sugars, proteins).

• Autotrophs: Use carbon fixation to convert inorganic carbon (CO2) into organic carbon.

Most nitrogen and phosphorus are extracted from organic nutrients, but some microbes can fix nitrogen directly from the atmosphere.

Growth Factors

Some microbes cannot synthesize all their required organic precursors (e.g., amino acids, vitamins, nitrogenous bases) and must import them. These are called growth factors. Microbes that require multiple growth factors are termed fastidious.

• Growth factors must be supplied in the culture medium for fastidious organisms.

Microbial Energy and Carbon Classification

Phototrophs vs. Chemotrophs

Microbes are also classified by their energy source:

• Phototrophs: Use light energy.

• Chemotrophs: Obtain energy from chemical compounds.

Type	Energy Source	Carbon Source	Example
Photoautotroph	Light	CO2 (inorganic)	Cyanobacteria in freshwater
Photoheterotroph	Light	Organic	Rhodobacter in ponds
Chemoautotroph	Chemicals	CO2 (inorganic)	Thiobacillus denitrificans in soil
Chemoheterotroph	Chemicals	Organic	Escherichia coli in intestines

Growing, Isolating, and Counting Microbes

Culture Media Formats

Microbes are cultivated using various types of media, classified by physical state, chemical composition, and function.

• Liquid media (broth): Ideal for growing large batches of microbes.

• Solid media: Useful for isolating colonies and observing characteristics.

• Semisolid media: Used for motility testing.

Broth media are made by adding nutrients to purified water and sterilizing. Solid and semisolid media use agar, a polysaccharide, to solidify the medium. Semisolid media contain less agar than solid media.

Tube Media: Slants and Deeps

Solid and semisolid media can be poured into tubes to form slants (cooled at an angle) or deeps (cooled upright). Slants are used for storage and biochemical tests; deeps are used for motility and oxygen requirement tests.

Summary Table: Media Types

Media Type	Physical State	Application
Broth	Liquid	Large-scale growth
Agar Plate	Solid	Colony isolation
Slant	Solid	Storage, biochemical tests
Deep	Semisolid	Motility, oxygen requirement tests

Key Points for Microbial Cultivation

• Media must be sterilized before use to prevent contamination.

• Choice of media depends on the experimental goal (e.g., isolation, identification, motility testing).

• Growth factors and nutrients must be supplied for fastidious organisms.

Example:

To test for motility, a semisolid deep is inoculated with bacteria. Motile bacteria will spread from the line of inoculation, while non-motile bacteria will remain at the stab line.

Additional info: The notes above are expanded with standard definitions and examples for clarity and completeness. Tables are reconstructed and summarized for study purposes.