Regulation of Phenotypic Switching and Heterogeneity in Photorhabdus luminescens Cell Populations

摘要

Phenotypic heterogeneity in bacterial cell populations allows genetically identical organisms to different behavior under similar environmental conditions. The Gram-negative bacterium Photorhabdus luminescens is an excellent organism to study phenotypic heterogeneity since their life cycle involves a symbiotic interaction with soil nematodes as well as a pathogenic association with insect larvae. Phenotypic heterogeneity is highly distinct in P. luminescens. The bacteria exist in two phenotypic forms that differ in various morphologic and phenotypic traits and are therefore distinguished as primary (1 degrees) and secondary (2 degrees) cells. The 1 cells are bioluminescent, pigmented, produce several secondary metabolites and exo-enzymes, and support nematode growth and development. The 2 degrees cells lack all these 1 degrees-specific phenotypes. The entomopathogenic nematodes carry 1 degrees cells in their upper gut and release them into an insects body after slipping inside. During insect infection, up to the half number of 1 degrees cells undergo phenotypic switching and convert to 2 degrees cells. Since the 2 degrees cells are not able to live in nematode symbiosis any more, they cannot re-associate with their symbiosis partners after the infection and remain in the soil. Phenotypic switching in P. luminescens has to be tightly regulated since a high switching frequency would lead to a complete break-down of the nematode-bacteria life cycle. Here, we present the main regulatory mechanisms known to-date that are important for phenotypic switching in P. luminescens cell populations and discuss the biological reason as well as the fate of the 2 degrees cells in the soil. (C) 2019 Elsevier Ltd. All rights reserved.