Studies on cell wall recycling and modification in Gram-negative bacteria

Abstract: The bacterial cell wall is made from peptidoglycan (PG), a heteropolymer which forms a bag-like exoskeleton that envelopes the cell. PG is constantly remodelled during growth and division, and in response to environmental stimuli. Decades of study of this process have focused largely on a select few model organisms, leaving its diversity poorly understood. In this thesis, I present studies on different aspects of PG recycling and modification in several Gram-negative models, with a particular focus on the plant pathogen Agrobacterium tumefaciens, a model of the Hyphomicrobiales group of the Alphaproteobacteria which includes several species of medical and environmental interest. It is shown that A. tumefaciens encodes a novel PG transporter, which is vital for cell wall integrity and resistance to β- lactam antibiotics, and widely conserved in the Hyphomicrobiales and Rhodobacterales orders. Growth defects caused by the loss of the transporter are suppressed by mutations in a novel glycopolymer, which is hypothesized to play a role in sequestering metal ions and thereby lowering periplasmic oxidative stress. Next, in collaboration, it is shown that PG recycling in the best studied model, Escherichia coli, is more complicated than previously thought. Rather than depending mostly on the MFS-family transporter AmpG, E. coli uses an ABC transporter, MppA-OppBCDF or AmpG depending on the growth phase and conditions. Finally, two studies on modification of PG by deacetylation are presented. First, A. tumefaciens is shown to encode a novel anhydroMurNAc deacetylase, which specifically deacetylates the PG chain termini. Then, it is shown that the causative agent of Legionnaires’ disease, Legionella pneumophila, depends on deacetylation of its PG during infection for defence against host lysozyme and correct polar placement of its type IV secretion system.