haedar's micro lab

Antibiotic susceptibility testing

this is the bacteria Bacillus subtilis, swabbed on two different medias (BHI and MHII) to form a “lawn” of growth. then, an antibiotic dispenser is used to “stamp” the disks onto the surface of the agar, just barely embedding them. the plates are then incubated at 37 degrees celsius for 24 to 48 hours. finally, the zone of inhibition, or lack there of, is noted by measuring the diameter of the clearing. the effects of the antibiotics varies depending on the growth media, length of incubation, dosage and inoculation amount.

Top 2 pictures: Eosin Methylene Blue (EMB) agar

Bottom 2 pictures: Endo agar

both media types select for the growth of gram negative bacteria, and differentiate between lactose fermentors.

however, on EMB, both E. coli and K. pneumoniea show extreme lactose fermentation and therefore a trademark “green metallic sheen”, while on Endo agar, only E. coli has this ability.