Picture it. You come home from a restaurant with leftovers of your favorite food, planning to use it as lunch for tomorrow. Into an airtight container it goes, and you think you're keeping it fresh and safe from bacteria growing on it. Then it gets forgotten in the back of your fridge.
A week later, you find the still sealed container, and when you open it you are met with a diabolical odor coming from your precious leftovers. It's clear bacteria has infiltrated your takeout, but how should this be possible without access to oxygen?
The answer is that different types of bacteria have different needs when it comes to oxygen. In fact, there are five different classifications we can use to describe these organisms based off this.
Obligate aerobes are the strictly on the pro-oxygen side of the spectrum. These bacteria can only survive in the presence of oxygen, and cannot respire without it. This is because in their cellular respiration pathways, oxygen accepts the electrons at the end of the electron transport chain, which generates energy for the cell. Take this away, and it's like taking the wheel off a bike; it's not going to work.
Slightly less extreme are facultative aerobes. They grow best with oxygen, however, when situations get sticky for these microbes, they have the ability to produce energy through other metabolic pathways, like fermentation, at least temporarily. It's sort of like having a scuba tank; they can go on for a little bit, but it doesn't last forever.
Microaerophiles are right smack dab in the middle of the spectrum; they do require oxygen, but less of it than the other aerobes. These bacteria live in conditions where the air contains 1-10% oxygen, as opposed to the 21% usually in the atmosphere. These are typically found in environments like our guts or under the sea, where the oxygen levels are low.
There are also bacteria who are anaerobes, meaning they do not require any oxygen to produce energy and thrive. The unbothered it-girls of the bacterial world are the aerotolerant anaerobes. These divas have a respiration pathway that does not require any oxygen, however the presence of oxygen doesn't affect them one way or another.
The final category of bacteria are the extreme alternatives to obligate aerobes; obligate anaerobes. Oxygen is not only of no use to these bacteria, but is actually toxic to these microbes, and will result in their death. Species like clostridium botulinum, famous for producing the botox that is both in your recalled canned tuna and favorite celebrity's forehead, are examples of obligate anaerobes.
So here's a little food for thought: next time you box up your meal to go, remember not to leave it for too long. No matter what type of environment you put those leftovers in, like a little brother (speaking from experience) they will always find a way into your food.
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