Although bacteria are valiant contributors to the health of humans and the planet, they also have a dark side. Certain bacteria have the potential to be pathogenic, meaning they can cause illness and disease.
Throughout human history, some bacteria have (understandably) gotten a bad rap, causing public anxiety and hysteria. Take the plague, for example. The bacteria causing plague, Yersinia pestis, has not only killed more than 100 million people, but it's also suggested to have shaped history, even contributing to the collapse of the Roman Empire [source: Centers for Disease Control and Prevention]. Before the advent of antibiotics, or medications capable of treating bacterial infections, infections were difficult to stop.
Even today, these pathogenic bacteria still weigh heavily on our minds. Bacteria causing a range of illnesses — from anthrax, pneumonia, meningitis, cholera, salmonella and strep throat to E. coli and staph infections — can defy our treatments thanks to antibiotic resistance.
This is especially true for Staphylococcus aureus, the bacterium responsible for staph infections. The "superbug" has posed tremendous problems for hospitals and health care clinics, where patients are more likely to be exposed to it during medical implants and catheter insertion.
In a previous section, we talked about natural selection and how some bacteria have more diverse genes to help deal with what their environment throws at them. If you have an infection, and some of the bacteria in your body are different from others, antibiotics might take care of the majority of the bacterial population. But this also gives the microbes not affected by your antibiotics the room to reproduce and take hold. This is why doctors recommend staying away from antibiotics unless you really need them.
Biological weapons are another frightening aspect of this conversation. Bacteria can be used as weapons in some cases, including being deployed in anthrax scares and embedded in aerosol sprays.
And it's not just humans taking a hit from bacteria. Indeed, bacteria even have an appetite for the sunken ocean liner Titanic, too [source: Kaufman]. The species, named Halomonas titanicae, eats away at the metal of the historic ship.
We've learned how bacteria can be harmful. In the next section, we'll look at how they can help us out.