In this lesson students will use the Penicillium chrysogenum fungus, which naturally produces the antibiotic penicillin, to investigate the effect of naturally produced antibiotics on bacteria in laboratory cultures. Students co-culture P. chrysogenum with three species of bacteria to observe differences between penicillin-resistant and penicillin-sensitive bacteria. They will normalize fungal spore suspension and bacterial culture concentrations before inoculating cocultures. After bacteria have been exposed to the antibiotic, students will quantify culture density to determine antibiotic effect in liquid culture and on solid media. Students will learn about natural product antibiotics as well as experimental design and application.