What is the purpose of using living host cells in culture?

Using living host cells in culture is crucial for growing bacteria that have specific growth requirements that cannot be fulfilled in artificial media. Some bacteria, particularly those categorized as obligate intracellular pathogens, can only replicate within living cells because they rely on the cellular machinery and nutrients provided by the host. This dependency means that standard laboratory media do not provide the necessary environment or conditions for their growth.

In the context of the other choices, the isolation of resistant strains of bacteria and the determination of antibiotic susceptibility can be performed using non-living media; they do not necessarily require living host cells. Similarly, while genetic modifications might involve living cells, they are not limited to the use of living hosts and can also be accomplished using plasmids or other methods in non-host systems. Hence, developing bacterial cultures specifically requires living host cells for those organisms that cannot grow outside of their natural cellular environments.