When we think about bacteria, it’s easy to picture them as one large group of invisible germs that make us sick. But in reality, bacteria are incredibly diverse, playing many different roles in our world—and in our bodies. In fact, without that diversity, one of the greatest medical breakthroughs, penicillin, might never have been discovered.
Penicillin, the first antibiotic, was discovered by Alexander Fleming in 1928, and it changed the way we fight infections. But what made penicillin so revolutionary is its ability to target specific bacteria. It's particularly effective against harmful ones like Staphylococcus and Streptococcus—the ones responsible for infections like pneumonia and strep throat, as well as staph infections—while it ignores other, more beneficial gut bacteria. That’s because bacteria are incredibly diverse; they come in different shapes, sizes, and even functions. Some are harmful, while others help us digest food or protect us from disease.
Pictured above is the fungus Penicillium chrysogenum. Today, penicillin is extracted from this fungus. The penicillium fungus that grew on Fleming's petri dish nearly a century ago is called P. rubens.
What makes bacteria so diverse? One key factor is their structure. Some bacteria have thick cell walls, while others have thin cell walls. Penicillin targets the building block of these thick cell walls—peptidoglycan—preventing the formation of a key ingredient in thick-walled bacteria cells. Thin-walled bacteria cells don't need this ingredient, however, so they are much less affected by penicillin. This is why penicillin doesn’t cure every infection—it can only attack bacteria with a certain kind of cell wall. This is also why penicillin doesn't completely obliterate your gut microbiome (most gut bacteria are thin-walled).
The discovery of penicillin opened the door to understanding bacterial diversity even more. Today, scientists study bacteria by looking at their genetic differences, behavior, and even the way they interact with other microbes. This helps them develop more targeted antibiotics for a wider range of infections, while also learning how to combat antibiotic resistance—a growing issue as some bacteria adapt and evolve to survive even our best medicines.
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