Biogenesis of the outer membrane
Gram-negative bacteria are enveloped by two membranes, which are separated by the periplasmic compartment containing the cell wall. The outer membrane is an asymmetrical bilayer containing phospholipids and LPS in the inner and outer layers, respectively. The integral outer membrane proteins (OMPs) have a β-barrel structure. The components of the outer membrane are synthesized in the cytoplasm and have to cross the inner membrane and the periplasm to be assembled in the outer membrane. We are studying these transport and assembly processes. Using Neisseria meningitidis as a model organism, we have previously identified the principle components of the LPT and BAM machineries required for the transport of LPS and for the assembly of OMPs, respectively. Our goal is to understand the dynamics and the functional mechanisms of these machineries.
Adaptation of the outer membrane
LPS is an endotoxin that is recognized by the innate immune system of the host. Many Gram-negatives try to prevent recognition by enzymatically modifying their LPS. We are studying the structure and function of these enzymes. In addition, we employ such enzymes for the development of new vaccines with low endotoxicity directed against Gram-negative pathogens.
As a part of its defense against invading pathogens, the vertebrate host limits the availability of essential micronutrients, such as iron, a mechanism known as nutritional immunity. How pathogens cope with iron deprivation in the host has been studied for many decades in dozens of pathogens. However, nutritional immunity is not restricted to iron limitation and also involves limitation for other nutrient metals, such as zinc and manganese. We are studying how Gram-negative pathogens are adapting to such conditions. We discovered, for example, that N. meningitidis produces a receptor under zinc limitation that enables this bacterium to use calprotectin, a host defense protein that sequesters zinc and manganese, as a zinc source. Thus, N. meningitidis is able to subvert an important defense mechanism of the human host. We are also studying the vaccine potential of proteins, such as the calprotectin receptor.
Gram-negative bacteria have evolved various mechanisms for the secretion of proteins across the entire cell envelope into the milieu. We are studying these mechanisms in N. meningitidis. We are also studying the functions of the secreted proteins, which have roles in biofilm formation, host-cell adhesion, immune evasion, and interbacterial competition, amongst others.
The outer membrane forms a permeability barrier that prevents access of many antibiotics to their targets. Thus, it is responsible for the intrinsic antibiotic resistance of Gram-negatives. We are studying how adaptations in the outer membrane composition along with other mechanisms lead to the emergence of multidrug resistant bacteria.