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is made up of very thin, stringy material called hyphae, which grows over and within a food source (Figure 3.13). Fungi feed by secreting chemicals that break down the food into small molecules, which they then absorb into the cells of the hyphae. The stringlike form of fungal hyphae maximizes the surface over which feeding takes placeā€”so the vast majority of the "body" of most fungi is microscopic and diffuse. Fungal food sources typically include dead organisms, and the actions of fungi are key to recycling nutrients for plant growth. Fungi are more like animals than plants in that they rely on other organisms for food. In fact, DNA sequence analysis indicates that Fungi and Animalia are more closely related to each other than either kingdom is to the Plantae kingdom (see Figure 3.4).

About one-third of the bacteria-killing antibiotics in widespread use today are derived from fungi. Fungi probably produce antibiotics because their main competitors for food are bacteria. Penicillin, the first commercial antibiotic, is produced by a fungus (Figure 3.14). Its discovery is one of the great examples of good fortune in science.

Before he went on vacation during the summer of 1928, the British bacteriologist Alexander Fleming left a dish containing the bacteria Staphylococcus aureus on his lab bench. While he was away, this culture was contaminated by a spore from a Penicillium fungus that may have come from a different laboratory in the same building. When Fleming returned to his laboratory, he noticed that the growth of S. aureus had been inhibited on the fungus-contaminated culture dish. Fleming had been searching for a method to control bacterial growth, and this chance discovery provided a clue. He inferred that some chemical substance had diffused from the fungus, and he named this antibiotic penicillin, after the fungus itself. The first batches of this bacteria-slaying drug became available during World War II. Many historians believe that penicillin helped the Allies win the war by greatly reducing the number of deaths from infection in wounded soldiers. Since the discovery of penicillin, hundreds of other antibiotics have been isolated from different fungus species.

Some fungi infect living animals, and thus also have potential as sources of drugs. To survive in a living organism, these fungi must be able to escape detection by their host's immune system. Cyclosporin is a drug derived from a fungus that is able to infect live insects by suppressing their host's immune response to ensure their own survival. In an organ-transplant recipient, cy-closporin suppresses the immune response that would be mounted against the foreign organ. Other, related fungi produce substances that interfere with virus reproduction. One of these compounds is used to treat people infected with the virus that causes AIDS.

As with most inconspicuous or hard-to-reach species, the number of unknown fungi is probably much greater than the number that have been identified to date.

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