Peptidoglycan synthesis during growth and division: the secrets of bacteria to keep “in shape”

March 10th 2016
Versione stampabile

Venue: Edificio Povo 2, via Sommarive nr. 9, Povo (Tn) - Room B101
 At 2:00 p.m.

  • Orietta Massidda -  Faculty of Medicine & Surgery, Department of Surgical Science, University di Cagliari, Italy

There is Streptococcus pneumoniae is an oval-shaped Gram-positive coccus that, similarly to the rod-shaped models Escherichia coli and Bacillus subtilis, grows by alternating cycles of peripheral and septal peptidoglycan (PG) synthesis. Consistently, the S. pneumoniae chromosome encodes proteins that are part of the cell elongation or cell division machineries, but lacks the gene encoding the actin-like rod-shape determinant, MreB. Moreover, in S. pneumoniae, all these proteins localize at the cell center, raising the question if they exist as two physically separate complexes or rather as a single complex that carries out both processes. As in the model organisms, cell division in S. pneumoniae initiates with the localization of the tubulin-like FtsZ and the actin-like FtsA proteins to midcell and assemble into a ring-like structure, known as the Z-ring. The control of lateral growth over cell division is orchestrated by an Eukaryotic-like Ser/Thr kinase that, through phosphorylation of key cell division proteins, acts as a molecular switch, signaling the cell when it is time to stop growing and start dividing, albeit the precise mechanism remains to be clarified. Recently, using S. pneumoniae ftsA conditional lethal mutants, we demonstrated that inactivation of FtsA results in cell lysis rather than cell filamentation. This suggests that lack of functional of FtsA simultaneously blocks peripheral and septal PG synthesis. In agreement, PG synthesis becomes delocalized when FtsA is depleted. Together, the results support a model in which, in S. pneumoniae, the two PG biosynthetic machineries assembled within a single machinery, or super-complex, located at midcell and coordinated by FtsA, which may functionally replace MreB, likely lost during the transition from rod to ovococcal shape. The analysis of the morphogenetic determinants in S. pneumoniae genome, compared with those present in the genome of other closely related lactic acid bacteria, has previously indicated how cell morphology may have evolved from rod to spherical-ovoid and to spherical shape, through the progressive loss of genes that determine the insertion of the PG along the longitudinal axis of the cell. Besides having an immediate effect on cell morphology, this event may have relieved oval-shaped cocci of the need to maintain two separate PG biosynthetic complexes in favour of a single one. The results discussed here further support this model and suggest that this may be achieved by combining the functions of two specialized actin-like proteins into one. In addition to providing new insight into the process of streptococcal cell growth and division, the characterization of these determinants is of interest for the development of new antibacterial drugs.milieu in vivo in order to test the impact of enhanced cell redox capacity on muscle performance and physiology.