Challenging the fitness cost: expression of the cryptic E. coli bgl operon lacks a measurable growth disadvantage in exponential phase.

Cryptic genes are phenotypically silent DNA sequences with the potential to code for a function but remain inactive during the normal life span of the organism. However, they can be activated by a single mutational event such as recombination, deletion, insertions or point mutation, resulting in a discernible phenotype. These phenotypes impart a growth advantage to the bacteria under stress and harsh living conditions. The bgl operon of Escherichia coli is involved in the uptake and breakdown of plant-derived aromatic β-glucosides like salicin and arbutin, is one of the well-studied examples of a cryptic genetic system. Previous studies have shown that the activated allele of the operon is beneficial to bacteria under selective conditions such as, survival in long term stationary growth phase, in the presence of predators, and antibiotics. All the observations point towards the possibility of the having a growth disadvantage and therefore a fitness cost associated with the activated operon under nonselective or less stressful conditions. To test the hypothesis the pair-wise competition experiments under nutrient-rich growth conditions using strains of different bgl genotypes were conducted. We report that a strain expressing the bgl operon constitutively, thus bypassing the requirement for a β-glucoside inducer, does not exhibit a measurable growth disadvantage over a strain carrying the wild type (silent or non-activated) bgl locus. These results imply that the classic cost-benefit model of bet-hedging may not be universally applicable and fitness trade-off might be more nuanced, existing only under specific environmental or resource-limited conditions.