Why do most natural antibiotics come from soil fungi and bacteria? Why do they produce them?

Answer 1 : 

A simple, though possibly simplistic, explanation is that these organisms use antibiotics to protect their food supply. Soil bacteria and fungi live by digesting and recycling dead plant material such as leaves and seed cases.

Obviously it is impossible for the bacteria to carry away their prized food supply and therefore it is argued that they lace surrounding food with compounds that are toxic to those of other species. While there is some lab-based experimental evidence to support this hypothesis, it has been difficult to prove this theory in the wild. Antibiotics that work just fine in the lab may be absorbed or diluted by different soils and clays to the point of being rendered useless, while organisms that are typically found in the soil may have robust resistance to antibiotics. A second explanation is that antibiotic production is rooted in the plant material that is the food source.

This material is typically carbon-rich and nitrogen-poor. Rather like a human who is given a 2-kilogram portion of chips with every meal he orders, the soil organism has the problem of achieving a balanced diet.

Most common antibiotics are carbon-rich polymers made by enzymes that strongly resemble those that normally make saturated fats.

The building blocks of these polymers are often exactly the same as those used to make saturated fats.

So perhaps we are seeing a form of clever bacterial bulimia: faced with a situation in which the bacteria literally swim in a soup of fat-producing carbon compounds, the bacteria turn these compounds not into fatty lipids but into their structurally close relations, the antibiotics.

These are then excreted and, should they prove to have a useful, coincidental effect, the bacteria thrive.

Answer 2: 

Most antibiotics stem from research programmes that were inspired by the discovery of penicillin. It was simpler to screen for antibiotics made by microbes than for antibiotics made by other organisms.It was easier to produce chemically complex antibiotics by batch culture of the microbe than it was to attempt a synthesis of a molecule made by a plant or animal. So for purely practical reasons microbes were the best route for discovering new antibiotics.

Actually, antibiotics are not that easy to find in microbes. One study screened 400,000 microbial cultures over a 10-year-period and yielded three useful compounds. It can be argued that the ability to produce and retain a rich chemical diversity enhances the chances of an organism producing the very rare compound that gives it enhanced fitness.

This means that individual chemicals with potent antibiotic properties will be made by many organisms but may be only some of these chemicals will give the maker increased fitness because of that antibiotic property.

Source : The Hindu