It is a great day at the lab. You have identified an E. coli F- strain that cannot grow without biotin (bio-). Unfortunately, the next day a rivaling postdoc “accidently” mixed your strain with another strain ‘X’ that cannot grow without arginine (arg-).

a. You decide to separate the bio- and X strain from the mix. You have spreaders, filter paper, Petri plates with agar gels supplemented with either biotin, only arginine, both or none (minimal medium), and an incubator. How would you proceed?

b. During your work under point a. above to recover your precious bio- strain, you discover an F- strain with wild-type genotype (bio+ arg+) that has emerged from the mixing of the two strains. How could you notice the bio+arg+ genotype during the rescue operation?

c. What is the most likely genotype of the strain X?