Are essential genes really essential?

Gene essentiality has emerged as an often-asked question in the wake of bacterial genome sequencing and a renaissance in studies of prokaryotic physiology. Genome-scale efforts at describing essential gene sets have necessarily been carried out under standard and tractable growth conditions in a laboratory setting. In addition to reinforcing our understanding of core bacterial physiology, these studies have also uncovered large numbers of essential genes encoding proteins whose functions remain poorly described. Studies of these and other elements of core physiology have naturally followed and several paradoxes, relating to growth conditions and genetic context, have begun to challenge our understanding of the term "essential gene". Most recently genome-scale genetic interaction studies have revealed remarkable density and redundancy in biological systems with profound implications for dispensability phenotypes associated with single gene mutations. Consequently, the phenotype "essential" should be carefully viewed as contextual.