Re: why do microorganisms produce antibiotics in their natural habitat?

Microorganisms (e. g., fungus, bacteria) have developed antibiotics to 
compete for food sources. Antibiotic production appears to be important to 
the survival of microorganisms through elimination of microbial competition 
for food sources, which are usually very limited in soil. Different types 
of antibiotics are made because these antibiotics have derived from 
accumulation of mutations in their genetic material and have evolved upon 
the microorganism they had to compete with. This evolution has led to 
different kinds of antibiotics with different anti-microbial targets and 
different specificities varying species to species and even from one strain 
to another. Antibiotic production is very common among soil-dwelling 
bacteria and fungi, and many of our most widely used medical antibiotics 
(e.g., streptomycin) are made by soil microorganisms. If they can make 
something that removes or drastically "slows-down" the competition for the 
available nutrients, then they'll have an advantage.