Wall proficient E. coli capable of sustained growth in the absence of the Z-ring division machine

Abstract

The peptidoglycan cell wall is a major protective external sheath in bacteria and a key target for antibiotics(1). Peptidoglycan is present in virtually all bacteria, suggesting that it was probably present in the last bacterial common ancestor(2). Cell wall expansion is orchestrated by cytoskeletal proteins related to actin (MreB) and tubulin (FtsZ)(3). FtsZ is a key essential player in a highly organized division machine that directs an invaginating annulus of cell wall peptidoglycan. The recent discovery that cell-wall-less bacteria (L-forms) can grow and divide independently of FtsZ(4,5), provided a means of generating an ftsZ null mutant of Escherichia coli. Remarkably, we have been able to isolate variants of E. coli that lack FtsZ but are capable of efficient growth in a walled state. Genetic analysis reveals that a combination of mutations is needed for this phenotype. Importantly, the suppressive mutations lead to a major cell shape change, from the normal cylindrical shape to a branched and bulging, ramified shape, which we call 'coli-flower'. The results highlight the versatility of bacterial cells and illustrate possible evolutionary routes leading to the emergence of specialized bacteria, such as pathogenic Chlamydia or aquatic Planctomycetes, that lack FtsZ but retain the cell wall(6-8).

Bacterial physiology: Life minus Z

“…In this issue of Nature Microbiology , Mercier and co-workers 11 spring another surprise. When PG synthesis is restored in their wall-less variants of E.coli or B.subtilis , they readily revert back to life with a sacculus and this is accompanied by the expected changes from fragile L-forms to osmotically stable rods.…”

“…When PG synthesis is restored in their wall-less variants of E.coli or B.subtilis , they readily revert back to life with a sacculus and this is accompanied by the expected changes from fragile L-forms to osmotically stable rods. Inquiring if FtsZ (and, hence, the SR) is required for this de novo generation of the normal rod-shape, they unblocked PG synthesis in wall-less Δ ftsZ mutant cells of E.coli 11 . Interestingly, these cells morphed into non-dividing filaments, as if FtsZ were simply depleted from normal rods (Fig.1c).…”

“…Interestingly, these cells morphed into non-dividing filaments, as if FtsZ were simply depleted from normal rods (Fig.1c). Thus, de novo formation of a cylindrical PG sacculus of normal diameter does not require the ability to form Z-rings 11 . Far more surprisingly, extended incubation of such PG-producing Δ ftsZ cells allowed for secondary mutations to result in a few viable mutants that are both walled and lack FtsZ.…”

“…Instead of filamentous rods, time-lapse imaging show bulbous CFL cells projecting protrusions that develop into branched syncytial structures of multiple bulbous compartments connected with thin bridges of cellular material. Over time, bulbous parts separate from this ramified structure, by fission or maybe breakage of connecting bridges, and go on to form similar branched structures (Fig.1e) 11 . Interestingly, CFL growth minimally requires the absence of FtsZ as well as that of functional PBP1B, one of the PG synthases in the cell.…”

“…Other factors appear to block the transition from rod-shaped or L-form E.coli to CFL growth, but may be less relevant once coli-flowering is established. These include anchoring of the OM to the PG sacculus by Braun's lipoprotein (Lpp), and biosynthetic pathways for colanic acid (CA) or other extracellular polysaccharides (EP) 11 . Strikingly, another group recently showed that functional PBP1B, Lpp, and a CA biosynthesis pathway that must be either intact or completely absent, are actually required for E.coli spheroplasts to reform viable rods after enzymatic removal of their original sacculi 12, 13 .…”