Sepsis, a life-threatening organ dysfunction caused by a dysregulated host response to infection, poses a serious challenge in modern healthcare. In 2017, there were 48.9 million cases of sepsis, resulting in 11 million deaths worldwide. Sepsis has a rapid, unpredictable course, and a high associated mortality of almost 1 in 5. Most sepsis infections originate in the urinary tract, and uropathogenic E.coli (UPEC) – a subgroup of extra-intestinal pathogenic E.coli (ExPEC), is the predominant cause of urinary tract infections (UTIs) both in the hospital and the community. UPEC and other extra-intestinal pathogenic E.coli are able to manipulate the immune system of an infected individual, allowing it to spread to the blood and trigger a vigorous host response that causes sepsis. 

UTI-causing bacteria are becoming an increasing threat to public health because of their ability to become resistant to antibiotics. With 6 in 10 women and over 1 in 10 men contracting a UTI in their lifetime, and with each case carrying a risk of progression to sepsis, we need new approaches to prevent these infections. Two important factors in the ability of ExPEC to cause disease and contribute to its virulence are capsule and the O antigen component of lipopolysaccharide (LPS). They shield the bacteria from protective measures by our immune system, such as antibodies and complement protein

and permit disseminated infection. In particular, the O antigen component of ExPEC has been identified as a major determinant for bloodstream survival and sepsis.

Capsule and LPS have an intimate relationship, with capsule depending on normal, fully-formed LPS to associate around the bacteria and provide protection. Previous research by McGarry and Smith, 2022 found that impairing the ability of ExPEC bacteria to form full-length LPS led to significant reduction in capsule association, stripping the bacteria of its ability to survive in the bloodstream. Thus, targeting the relationship between LPS and capsule could be extremely useful clinically, as UPEC would thereby lose its ability to cause bloodstream infection and sepsis. 

My research aims to identify potential therapeutic strategies to target LPS and capsule, which would reduce virulence and prevent bloodstream infection. This approach would also increase the susceptibility of E.coli to antibiotics, as LPS also plays a vital role in preventing access of antibiotics to bacteria. My research aims to exploit the LPS-capsule relationship, through two approaches which have not been explored for this purpose previously.

Firstly, I will expose ExPEC to sub-inhibitory concentrations of Polymyxin (PM) B antibiotics. Polymyxins are antibiotics but are a last resort for bloodstream infections (BSI’s) as they can be nephrotoxic and neurotoxic (in a dose-dependant manner; high doses are needed to eliminate infections). At lower, less toxic concentrations, polymyxins have been shown to cause disturbances in LPS. For my research project, I aim to further understand how Polymyxin B antibiotics affect LPS integrity, as well as the knock-on effect this has on the association of the protective capsule around ExPEC. This would increase susceptibility to other less toxic antibiotics, would enable the immune system to target the bacteria more readily, and would strip the bacteria of the virulent mechanisms it uses to cause sepsis. As polymyxins are already licenced, if my research shows that sub-inhibitory concentrations have significant effects on antibiotic susceptibility, this new approach to the use of polymyxins could reinvent their use in a clinical setting and help us gain an upper hand over sepsis.

Secondly, I aim to explore the use of molecules called O antigen depolymerases (OADPs)–these are proteins that break down the O antigen component of LPS. My research will look at how this impacts capsule association. Since OADPs have been previously shown to be non-harmful to humans, they could also be used as a potential therapeutic treatment. Phage depolymerases have been successfully trialled for use against Klebsiella pneumoniae infections, however their use in E.coli suppression has not been shown before. By using depolymerase therapy, we can target capsule and O antigens to degrade them, allowing other antibiotics to reach the bacterial cell membrane. Depolymerase therapy has many benefits, including preventing superbugs due to improper use and preventing the rise of antibiotic resistant bacteria. The bacteriophages used in this therapy are specific to a certain type of bacteria, meaning they won’t attack all bacteria, therefore protecting the natural microbiome within humans. 

My hypothesis is that exposure of ExPEC bacteria to polymyxins and depolymerases will
cause significant changes to the LPS protective coating surrounding the bacteria,
rendering cells (a) more sensitive to first-line antibiotics and (b) less able to cause sepsis (due to a loss of capsule association).
If my hypothesis is proven correct, these agents could be co-administered at low
concentrations with antibiotics for previously untreatable infections. This could
dramatically improve our ability to clear these infections and, more importantly, prevent the
progression of UTIs to sepsis.