UC Santa Barbara

Klebsiella aerogenes is a nosocomial infectious bacteria that can become multidrug-resistant to drugs saved as the last line of defense, such as colistins and carbapenems, leading to septicemia. It has two contact dependent inhibition (CDI) systems that pro- mote antagonistic toxin transfer among closely related bacteria. These delivery systems are used in a proximity dependent manner. Bacteria that possess the CDI phenotype utilize a large cell surface protein called CdiA, which binds to an outer membrane (OM) receptor to intoxicate neighboring cells. The toxin is the last ∼ 200 residues at the C-terminus of CdiA that enter the target cell and translocate across the outer and in- ner membrane into the cytoplasm. In previous studies, three OM receptors for CdiA in Enterobacteria have been identified: BamA, OmpC/F, Tsx, and the lipopolysaccharide layer. The K. aerogenes CDI receptor has been revealed as a Class II receptor and only utilizes OmpC for binding. In Chapter 2, I discuss the discovery of this receptor and mutations in loop 3 of OmpC which has implications in antibiotic resistance. These mu- tations were then duplicated in another species to confer the effect of the mutation in binding to the barrel wall and its electrostatic effect on the transport of small molecules through this non-selective porin. The mutations affect the transport of antibiotics into the cells rendering them useless. In Chapter 3, I investigate the topology of CdiB and its interaction with CdiA. The topology mapping of CdiB suggests interaction with CdiA, which is further supported with crosslinking data. Upon further investigation, the pre- viii sentation of CdiA on the exterior of the cell is very robust. These results suggest that the mutations in OmpC can affect the uptake of antibiotics in the cell, therefore increasing their effectiveness and ultimately help reduce the 270,000 deaths by septicemia.