was the first naturally-occurring antibiotic discovered - and the first to be used therapeutically. An antibiotic is any substance produced by a microorganism which can kill or inhibit the growth of a different microorganism. We now call such substances, and any similarly-acting substances which humans design, chemotherapeutic agents. Before we begin to talk specifically about penicillin, it will be useful to kind of place you in the life and times of people in the 1800's. …
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…we can have 100,000 penicillin-resistant disease-causing bacteria! It is for this reason - the potential to select for antibiotic-resistant bacteria, that careful and limited use of antibiotics is necessary. Also, because of the appearance of so many antibiotic-resistant pathogenic bacteria, there is an on-going search for natural substances produced by plants, other recently identified bacteria, and other microorganisms for their ability to kill bacteria while at the same time causing no harm to us.