Heterologous expression of the S-layer-like protein SllB induces the formation of long filaments of E. coli consisting of protein stabilized outer membrane

Escherichia coli is one of the best studied microorganisms and is the mostly used host for genetic engineering. The gram-negative single cells are rod-shaped and filaments are usually not found. Here we describe the reproducible formation of elongated E. coli cells. During heterologous expression of the silent S-layer protein gene sllB from Lysinibacillus sphaericus JG-A12 in E. coli Bl21 (DE3) the cells are arranged as long cell chains and surrounded by a highly stable sheath with a length of >100 µm (defined as filaments) while growing at room temperature. In stationary growth phase, microscopic analyses show the formation of unusual long transparent tube-like structures enclosing separated single cells. The tube-like structures were isolated and analyzed by SDS-PAGE, IR-spectroscopy and different microscopic methods in order to identify their special composition and structure. We show that the tube-like structures are outer membrane-like, enriched in protein content and associated with the recombinant S-layer protein, indicating a disordered cell division. However, bacterial cells show a high viability and stability. To our knowledge, this is the first report of such drastic morphological changes of E. coli, induced by the expression of a foreign protein.