Yes. Both. They keep us alive and they also kill us. It’s a complicated relationship.
Bacteria were the first life forms to appear on Earth around 3.5 billion years ago . we can't live without bacteria-they help us digest food
The average adult gut hosts about 1.2 kg of bacteria. While most are beneficial to health, some cause serious infections.
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“More than 100 trillion symbiotic microorganisms live on and within human beings and play an important role in human health and disease. The human microbiota, especially the gut microbiota, has even been considered to be an “essential organ”. There is this interesting review that perfectly answers the question:
https://www.sciencedirect.com/science/article/pii/S2095809917301492
Although many types of bacteria are detrimental to other organisms, many types are essential for life on Earth, including for humans. Examples include nitrogen fixing Azotobacter and the different types in our intestine.
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