Impact of cytosolic crowders on the morphology of bacterial chromosomes

Recent experimental studies on E. coli elaborated the role of macromolecular crowding on chromosome organization. Motivated by such observation, we consider a "feather-boa" type model of chromosome in the presence of non-additive crowders, encapsulated inside cylindrical confinement. Using molecular dynamic simulations, we thoroughly investigated the impact of the size and density variation of such crowders on the spatial organization of a model chromosome in cylindrical cellular confinement. Our results show, in the regime of small crowder size, the depletion effect leads to chromosome localization on the cylindrical surface of the cell, while the crowders occupy the bulk volume. Increasing the crowder size to intermediate values actuates the occurrence of transverse segregation between the crowders and the chromosome leading to a complimentary helical organization of the two. Further size-increment leads to longitudinal segregation causing the complete spatial expulsion of crowders from the central chromosomal region while the segregated crowders further compress the chromosome from the two ends. The increase in crowder size, spatial segregation of crowders and the chromosome, and the augmentation of helical turns in the model chromosome are found to be concomitant. Using crowders of intermediate size, we show how the variation of crowder density recaptures the spatial organization of the chromosome and crowders.