Image based quantification of cell sizes in S. aureus: Effect of environmental factors

Abstract

Variability in cell sizes is seen in isogenic bacterial populations despite of maintenance of cell size homeostasis. Quantification of cell sizes in a bacterial population can help us elucidate mechanisms that might disturb the size distribution in the community. Cell sizes have been quan tified earlier in E.coli and found to be depend ent on growth rate . This dependence results from multi - fork replication in fast growing E. coli . Here we quantif ied cell size variation in a S. aureus population and its dependence on growth rate . We have developed and optimized an algorithm to detect spherical and ellipsoid c ells in label - free DIC images in ImageJ . We assess the population density dependence of c ell size variations in a batch culture and f ind no difference in the noise across different gro wth phases. Perturbing the cell cycle either with Cephalexin ( c ell division inhibitor ) or with Hydroxyurea ( DNA replication inhibitor ) results in an increase in mean cell area of treated cells as compared to non treated cells . However no significant change in the phenotypic noise is observed . This study sets the stage for a quantitative understanding of cell shape variability due to non - genetic factors, across strains in a gram positive coccal bacterium, thus complementing previous wor k on gram negative E. coli . It suggests that the observation of growth rate dependence on cell size variability could be a conserved mechanism.