Oxygen is essential for some microorganisms, harmful to others, and tolerated or used differently by still more. This lesson introduces the main oxygen-requirement categories and shows how growth patterns in thioglycolate tubes reveal them.
The earliest life arose when Earth’s environment had little or no free molecular oxygen (O2). Oxygen levels increased after cyanobacteria released oxygen through photosynthesis and the oceans’ capacity to absorb it was exhausted. This change, called the Great Oxygenation Event, was harmful to many organisms of the time.
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What is an obligate aerobe?
A microorganism that requires oxygen for growth and needs an abundant oxygen supply.
How does a facultative anaerobe differ from an aerotolerant anaerobe?
A facultative anaerobe can grow with or without oxygen and generally grows better with it. An aerotolerant anaerobe tolerates oxygen but does not use it for growth.
What does a growth pattern at the bottom of a thioglycolate tube suggest?
It suggests an obligate anaerobe: oxygen is harmful, so growth occurs in the low-oxygen region at the bottom.
What is a microaerophile?
A microorganism that requires oxygen but grows best at a concentration below that of the atmosphere, often around 1%–10%.
What is a capnophile?
A microorganism that grows best in an environment with increased carbon dioxide and less oxygen than the atmosphere.
Quiz questions
Why did the rise of atmospheric oxygen threaten many early organisms?Answer: Reactive oxygen species formed in oxygen-rich conditions can damage cell components.
Where is oxygen concentration highest in a thioglycolate tube?Answer: Near the upper surface, where oxygen diffuses into the medium.
A culture grows only at the bottom of a thioglycolate tube. Which category best fits it?Answer: An obligate anaerobe, because it grows where oxygen is absent.
A bacterium grows throughout a thioglycolate tube but is densest at the top. What does this pattern indicate?Answer: It is a facultative anaerobe that grows without oxygen but generally grows better with it.
A bacterium grows in a narrow band below the surface of a thioglycolate tube. What is the best interpretation?Answer: It is likely a microaerophile that grows best below atmospheric oxygen concentration.
Based on Microbiology by Nina Parker, Mark Schneegurt, Anh-Hue Thi Tu, Philip Lister, Brian M. Forster (OpenStax), licensed CC BY 4.0. Adapted into a summary, flashcards and quizzes.