Principal Investigator: Michael P. Schmitt, PhD
Office / Division / Lab: OVRR / DBPAP / LRSP
The recent diphtheria epidemics in Eastern Europe and waning immunity to diphtheria among many adults in the US demonstrate the importance of maintaining adequate vaccination and developing better vaccines against diphtheria. Ideally, the next generation of diphtheria vaccines would provide longer lasting immunity against the bacterium that causes this disease, Corynebacterium diphtheriae, eradicate colonization of the bacteria from people who were previously vaccinated, and reduce the adverse events associated with diphtheria vaccination. In order to develop safer and more effective diphtheria vaccines, scientists need additional information as to how C. diphtheriae causes diseases.
About 25% of all licensed vaccines distributed in the US contain diphtheria toxoid (toxins made by bacteria that are weakened so they can be used as the main component in vaccines). Techniques for making this type of diphtheria vaccine are constantly evolving. These changes frequently include modifications in the way the bacteria are cultivated and in the various tests used to measure the stability, purity, or potency of the final vaccine.
This continuous evolution in vaccine production and testing techniques requires FDA to maintain regulatory and review expertise in this field. Our laboratory is meeting this need with an active C. diphtheriae research program that provides us with experience and expertise in the growth characteristics and media requirements of this microorganism. This expertise is available to manufacturers seeking guidance on how to grow these bacteria so they consistently produce safe and effective toxoid for use in vaccines.
Moreover, my laboratory work has provided the FDA with insight into the use of C. diphtheriae plasmids in vaccine production. (Plasmids are DNA molecules in bacteria that are not connected to chromosomes but still transmit important traits, such as the ability to cause disease and resistance to antibiotics.) Another essential part of our research is developing genetic tools that will facilitate the discovery of vaccine candidates and enhance our understanding of how C. diphtheriae causes diseases. This work will contribute to the development of second-generation diphtheria vaccines that contain toxoids with increased ability to trigger immune responses that limit the ability of the bacteria to colonize individuals (i.e., populate the respiratory tract in preparation for causing disease).
Our recent research identified a variety of proteins on the surface of C. diphtheriae, including several proteins that bind to heme molecules (molecules containing an atom of iron; heme is associated with the hemoglobin protein which is an essential component of red blood cells). Such heme-binding proteins could serve as components of future vaccines. Many of these factors are regulated by iron and bacteria produce them simultaneously with diphtheria toxins. Heme and hemoglobin binding proteins in other bacterial pathogens are important for determining how readily bacteria can invade and cause disease. These proteins are currently being evaluated in clinical studies to determine their effectiveness as vaccines.
Corynebacterium diphtheriae is a gram-positive bacterium and the cause of the severe respiratory disease diphtheria. Diphtheria toxin (DT), the primary virulence determinant for this pathogen, has been extensively investigated; however, factors involved in the colonization and survival of this organism on the mucosal surfaces of the human host have not been well characterized. A better understanding of the fundamental mechanisms of pathogenesis of C. diphtheriae will facilitate the development of future vaccines that are directed not only against the toxin, but also to other virulence determinants. A second generation of diphtheria vaccines may assist in the eradication of the carrier-state of the bacterium and provide more effective and longer lasting immunity against C. diphtheriae infection.
The research in my laboratory is focused on 1) characterizing heme-iron transport systems in C. diphtheriae, 2) analyzing a heme-dependent two-component regulatory systems and 3) Identification of surface proteins involved in metal transport or adherence to host cells. C. diphtheriae uses host compounds such as heme and hemoglobin as essential iron sources, and it also requires metals such as zinc (Zn) and manganese (Mn) for survival in the human host. My laboratory has identified and characterized systems associated with the uptake and use of heme-iron as well as Zn and Mn.
Heme uptake in C. diphtheriae involves an ABC-type heme transporter as well as various surface-anchored proteins such as HtaA, ChtA and ChtC. We have shown that HtaA and a related protein, HtaB, are anchored to the cell surface and are capable of binding heme. Deletion analysis of htaA and the hmuTUV genes, which encode an ABC-type heme transporter, indicates that the products of these genes are involved in heme transport in C. diphtheriae, and further suggests that HtaA functions as a cell surface receptor for heme and heme-containing proteins in C. diphtheriae. Our most recent findings have demonstrated that HtaA, as well as ChtA and ChtC, are able to bind the hemoglobin-haptoglobin complex, and utilize the heme-associated iron for growth. Heme that has entered the cytosol of C. diphtheriae is proposed to be degraded by the heme oxygenase enzyme, HmuO, which releases the heme-associated iron.
The ChrSA and HrrSA two-component signal transduction systems in C. diphtheriae regulate expression of the hmuO gene by a heme-dependent manner; recent studies by our group have shown that these systems also controls the expression of additional operons in C. diphtheriae through a similar regulatory mechanism. Our current studies with the ChrSA HrrSA systems have identified regions in these proteins that are essential for function as well as demonstrating how these proteins control signal transduction in vivo.
PLoS One 2019 Aug 27;14(8):e0221711
Identification of zinc and Zur-regulated genes in Corynebacterium diphtheriae.
Peng ED, Schmitt MP
J Bacteriol 2018 Apr;200(7):e00676-17
The Corynebacterium diphtheriae iron-regulated surface protein HbpA is involved in the utilization of the hemoglobin-haptoglobin complex as an iron source.
Lyman LR, Peng ED, Schmitt MP
J Bacteriol 2018 Apr 24;200(10):e00051-18
Iron and zinc regulate expression of a putative ABC metal transporter in Corynebacterium diphtheriae.
Peng ED, Oram DM, Battistel MD, Lyman LR, Freedberg DI, Schmitt MP
J Inorg Biochem 2017 Feb;167:124-33
Characterization of the second conserved domain in the heme uptake protein HtaA from Corynebacterium diphtheriae.
Uluisik RC, Akbas N, Lukat-Rodgers GS, Adrian SA, Allen CE, Schmitt MP, Rodgers KR, Dixon DW
J Biol Inorg Chem 2016 Oct;21(7):875-86
Corynebacterium diphtheriae HmuT: dissecting the roles of conserved residues in heme pocket stabilization.
Draganova EB, Adrian SA, Lukat-Rodgers GS, Keutcha CS, Schmitt MP, Rodgers KR, Dixon DW
J Bacteriol 2016 Aug 25;198(18):2419-30
The ChrSA and HrrSA two-component systems are required for transcriptional regulation of the hemA promoter in Corynebacterium diphtheriae.
Burgos JM, Schmitt MP
Biochemistry 2015 Nov 3;54(43):6598-609
Heme binding by Corynebacterium diphtheriae HmuT: function and heme environment.
Draganova EB, Akbas N, Adrian SA, Lukat-Rodgers GS, Collins DP, Dawson JH, Allen CE, Schmitt MP, Rodgers KR, Dixon DW
J Bacteriol 2015 Feb 1;197(3):553-62
Utilization of host iron sources by Corynebacterium diphtheriae: multiple hemoglobin-binding proteins are essential for the use of iron from the hemoglobin-haptoglobin complex.
Allen CE, Schmitt MP