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Bacteriophages are viruses which infect bacteria specifically. Over the past decades, phage �� has been extensively studied, especially its interaction with the Escherichia coli LamB (EcLamB) protein receptor. Nonetheless, despite the enormous numbers and near-ubiquity of environmental phages, aside from phage ��, there is a paucity of information on other phages which target EcLamB as a receptor. In this study, to answer the question whether there are other EcLamB-targeting phages in the natural environment, a simple and convenient method was developed and used for isolating environmental phages which target a particular surface structure of a particular bacterium, in this case, the EcLamB outer membrane protein. From the enrichments with the engineered bacterial hosts, a collection of EcLamB-targeting phages (��ZZ phages) were easily isolated. Intriguingly, unlike phage ��, an obligate EcLamB-dependent phage in the Siphoviridae family, the newly isolated ��ZZ phages alternatively recognised EcLamB or E. coli OmpC (EcOmpC) as a receptor when infecting E. coli. Furthermore, ��ZZ phages were suggested to represent new species in the Tequatrovirus genus in the Myoviridae family, based on phage morphology and genomic sequences. Most phages are thought to have a narrow host range due to their exquisite specificity in receptor-recognition. With the ability to optionally recognise two receptors, ��ZZ phages were considered relatively promiscuous. Via the heterologous expression of EcLamB on the bacterial cell surface, the host range of ��ZZ phages was further extended to three different enterobacterial genera. Besides, an interesting selection of evolved phage mutants with broader host range were isolated, and the key mutations involved in their evolution to adapt to new hosts were investigated by genomic analysis. Finally, and importantly, two ��ZZ phages were found to be putative generalised transducers, which could be exploited as tools for DNA manipulations., I am grateful for the financial support provided by the China Scholarship Council (CSC) and the Cambridge Commonwealth, European and International Trust for my PhD study.

The folding mechanisms for β-barrel membrane proteins present unique challenges because acquisition of both secondary and tertiary structure is coupled with insertion into the bilayer. For the porins in Escherichia coli outer membrane, the assembly pathway also includes association into homotrimers. We study the folding pathway for purified LamB protein in detergent and observe extreme hysteresis in unfolding and refolding, as indicated by the shift in intrinsic fluorescence. The strong hysteresis is not seen in unfolding and refolding a mutant LamB protein lacking the disulfide bond, as it unfolds at much lower denaturant concentrations than wild type LamB protein. The disulfide bond is proposed to stabilize the structure of LamB protein by clasping together the two sides of Loop 1 as it lines the inner cavity of the barrel. In addition we find that low pH promotes dissociation of the LamB trimer to folded monomers, which run at about one third the size of the native trimer during SDS PAGE and are much more resistant to trypsin than the unfolded protein. We postulate the loss at low pH of two salt bridges between Loop 2 of the neighboring subunit and the inner wall of the monomer barrel destabilizes the quaternary structure.