Antibiotic susceptibility research. Microbiologist holding up a Petri dish culture plate, demonstrating the results of a modified Hodge test, which is used to identify resistance in bacteria known as Enterobacteriaceae. Bacteria that are resistant to carbapenems (Carbapenem-Resistant Enterobacteriaceae (CRE)), which are considered 'last resort' antibiotics, produce a distinctive clover-leaf shaped growth pattern, as seen here. Photographed at the Centers for Disease Control (CDC), USA.