This is how we determine in the laboratory which antibiotic will kill a particular organism like E. coli.  The picture that you see below is showing a culture plate with bacteria and antibiotic disks.  The bacteria was spread evenly all over the culture plate.  Next, little white disks with different antibiotics in them were dropped on the plate.  Each white disk represents a different antibiotic like ampicillin, tetracycline, gentamicin, and others.  The culture plate was then placed in an incubator for 18-24 hours to allow the bacteria to grow.  If the bacteria is sensitive to a particular antibiotic, it will not grow close to the disk.  If the bacteria is resistant to an antibiotic, it will grow right up to the disk.  In the picture below, the bacteria is sensitive to all the antibiotics.  Some organisms are quite resistant to antibiotics, thus, the need to perform sensitivity studies such as the one you see here.  Without laboratory tests such as this, the physician would be guessing at which antibiotic to use.  Only medical laboratory technologists are licensed to perform and interpret such tests.