Some bacteria are responsible for keeping us healthy. These include the bacteria in our intestines that assist digestion and make essential B vitamins, or bacteria that can be made into vaccines to ward off other microbes. Yersinia pestis is a bacterium that causes plague. Although relatively rare in the U.S., plague was responsible for the death of nearly one-third of the European population in the 14th century and is still a problem today in some developing countries. They could cause major agricultural losses or lead to infection of people through contact or ingestion.
Microbes get ugly in at least two ways.
Mutation has the consequence of making vaccines and other treatment regimes less effective, such as those for AIDS, influenza, and hepatitis. Second, microbes can be abused for the purpose of bioterrorism. History shows that biowarfare has existed since the time of the Romans and remains a threat today. The plethora of microorganisms, both good and bad, leaves one in awe of their genetic diversity, of the roles they play in maintaining our ecosystem, and of the elaborate but utilitarian machinery that has evolved over billions of years to allow them to maintain their niche in life. From the metabolic pathways that regulate the utilization of carbon to the biochemical pathways that precisely control the mechanisms of infection, these organisms deserve our respect.