Luria-Bertani broth supports Escherichia coli growth to an optical density at 600 nm (OD600) of 7. Surprisingly, however, steady-state growth ceases at an OD600 of 0.3, when the growth rate slows down and cell mass decreases. Growth stops for lack of a utilizable carbon source. The carbon sources for E. coli in Luria-Bertani broth are catabolizable amino acids, not sugars.

Growing bacterial L forms are reputed to lack peptidoglycan, although cell division is normally inseparable from septal peptidoglycan synthesis. To explore which cell division functions L forms use, we established a protocol for quantitatively converting a culture of a wild-type Escherichia coli K-12 strain overnight to a growing L-form-like state by use of the ␤-lactam cefsulodin, a specific inhibitor of penicillin-binding proteins (PBPs) 1A and 1B. In rich hypertonic medium containing cefsulodin, all cells are spherical and osmosensitive, like classical L forms. Surprisingly, however, mutant studies showed that colony formation requires D-glutamate, diaminopimelate, and MurA activity, all of which are specific to peptidoglycan synthesis. High-performance liquid chromatography analysis confirmed that these L-form-like cells contain peptidoglycan, with 7% of the normal amount. Moreover, the ␤-lactam piperacillin, a specific inhibitor of the cell division protein PBP 3, rapidly blocks the cell division of these L-form-like cells. Similarly, penicillin-induced L-form-like cells, which grow only within the agar layers of rich hypertonic plates, also require D-glutamate, diaminopimelate, and MurA activity. These results strongly suggest that cefsulodin-and penicillin-induced L-form-like cells of E. coli-and possibly all L forms-have residual peptidoglycan synthesis which is essential for their growth, probably being required for cell division.Bacterial L forms are spherical, osmosensitive variants isolated from many different species and reported to have no peptidoglycan cell wall, although they grow and divide indefinitely. In contrast, cell division in normal bacteria is indissociable from cell wall synthesis; a septum, consisting of a double layer of peptidoglycan, is laid down at midcell and then split to form the new poles of the two daughter cells. The division of L forms in the apparent absence of peptidoglycan is thus paradoxical. We wished to explore which cell division functions L forms require, using Escherichia coli K-12 as the model. Septal synthesis is carried out by an assembly of cell division proteins organized in a ring-like membrane-associated complex surrounding the midcell (19, 52). The structural basis of this ring is the FtsZ protein, a bacterial tubulin homologue. In E. coli, some 15 cell division proteins are known to be recruited into the FtsZ ring.The study of L forms goes back to 1935, when Emmy Klieneberger succeeded in establishing a pure culture of the curious mycoplasma-like organism that appeared systematically in cultures of Streptobacillus moniliformis (26, 27). It consisted of spherical, osmosensitive cells that grew on plates of hypertonic complex medium containing serum. Klieneberger called the culture "L1" in honor of the Lister Institute in London, where she had emigrated in 1933 when the German authorities fired her as a Jew. Similar morphological variants, also called L forms, were subsequently isolated from other bacterial species (10, 11).In 1942, L forms were found to be ...