antibiotics and other interventions that put added pressure on the organisms. Bacteria also compete with one another, which affects the pathogen genetics. As mentioned, bacteria must first overcome the innate immune system. This largely means overcoming barrier systems but it also must overcome the inflammatory responses in humans as well as the cells that indiscriminately kill off pathogens without the need for antibody responses. Humans use competition between healthy bacteria on the skin and in the GI tract that prevent pathogens from taking hold. Mucus and the blood-brain barrier keep pathogens out of delicate areas of the body. Some bacteria make enzymes that break down mucus to ensure a greater chance of infection. Other pathogens target areas of the GI tract that have less mucus. There are bacteria that break the cell to cell junctions and bacteria that break up the protective skin barriers. Humans have a way to sequester iron needed by microorganisms. This has led to the evolution of organisms that can scavenge for iron as part of their pathogenicity. There are also bacteria that steal iron from human proteins. The greatest selective pressure that pathogens have placed on humans in gaining the advantage in diseases is the development of antibiotic resistance. It can happen very quickly and can cross to different species of bacteria. Bacteria have evolved to expel antibiotics, avoid attaching to antibiotics, and inactivate antibiotics. A big problem now is the development of multi-drug resistance. Less effective but still a problem is the elimination of rival bacteria by secreting toxic proteins that kill beneficial bacteria.
EVOLUTION OF SENESCENCE As an organism ages, their ability to function, fight off disease, and succeed reproductively goes down. All organisms die at some point and there are several hypotheses as to how this occurs. It was originally believed that aging was a mechanism of making way for the next generation as a necessary part of evolution. This theory has largely been discarded since the beginning of the 20th century. Another theory is that aging is a matter of neglect because of the highly competitive aspect of life on earth. Animals in the wild die from predation, accidents or disease; this lowers the average age at the time of death. It means that there is no evolutionary advantage to being fit as an older organism.
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