Directed study opportunities available. Contact Dr. Griffin for more information.
Current Research Focus
Haemophilus influenzae species annually cause serious infection in 300 million people worldwide and kill 400,000 - 700,000 children each year (source: World Health Organization). Gram-negative H. flu (as H. influenzae is commonly referred) falls into two categories: capsulated and unencapsulated. Six serotypes, A-F, have been identified in the encapsulated class. In 1987, a successful conjugate vaccine was developed for H. flu serotype B (Hib). Since then, no other vaccine has been developed for the remaining serogroups nor for the nonencapsulated class of H. influenzae. A better understanding of the mechanism of H. influenzae pathogenesis may lead to a more suitable target for a diverse therapy.
H. flu posses many systems that are known to play a role in its pathogenesis. Some of these include: high molecular weight adhesins (HMW1 and HMW2), high molecular weight hia gene product, Immunoglobulin A protease-like protein, outer membrane proteins (P2 and P5) and hemagglutinating pili (HA) or fimbriae surface appendages. Pili play an important role as an adhesive organelle during the initial infection process of H. influenzae and their structure and composition has been widely studied. The heteropolymer structure is produced by a divergent operon encoding five proteins, HifA-E. HifA is the major structural pilin protein and may also be a tissue-specificity marker. HifB and HifC are the chaperone and usher proteins, respectively. HifD is a minor pilin tip protein while HifE is the putative adhesin protein located at the utmost tip of the pilus appendage. Interestingly, the primary amino acid sequence of HifE homologs diverge up to 60% between H. influenzae serotypes Hib and nonencapsulated H. influenzae biogroup aegyptius (Hae). Additionally, it has been noted that hifEmutants most likely produce only abortive pili and cannot agglutinate human red blood cells (loss of function). Therefore, this adhesin is speculated to play a critical role in the functionality of HA pili host cell tissue recognition. This is my current point of investigation: To determine if HifE is the host tissue-specificity marker of H. influenzae pili.
Most of the body's iron is found intra-cellularly and two-thirds of that iron is found within the hemoglobin molecule. Hemoglobin, a critical transport molecule for tissue-specific oxygen exchange, contains four heme moieties with each containing a single iron atom. Thus, hemoglobin may serve as a powerful source of iron for invading bacteria. Microbial pathogens are now known to possess heme acquisition systems that allow the organisms to utilize heme or heme-bound proteins as sources of iron. These systems have been demonstrated to procure iron from heme, hemoglobin, haptoglobin, or hemopexin and are identified as non-siderophore heme-iron acquisition. As with free iron, free heme is kept in a limited state by being bound by the high-affinity host protein hemopexin. Similarly, hemoglobin is complexed with haptoglobin. Therefore, the versatility of the bacterial heme acquisition systems enables them to circumvent the host defense of bacteriostasis by iron limitation. Some pathogens that have been determined to be able to utilize heme or heme-bound proteins as sources of iron include the neisseriae, yersiniae, pseudomonads, Serratia sp., Corynebacterium diphtheriae, Bacillus fragilis, Vibrio sp.,Haemophilus sp., Plesiomonas, Porphyromonas ginivalis, Helicobacter pylori, and Shigella sp.. As the systems of these various organisms and others are being investigated, general knowledge about the basic heme acquisition and assimilation pathways for both Gram-negative and Gram-positive pathways has been elucidated. We provide data of heme degradation by cytoplasmic heme oxygenase enzymes in distantly related bacteria suggesting that this is also a common mechanism of iron acquisition in bacteria.
Research mentor: Igor Stojiljkovic, M.D., Ph.D. (deceased), Emory School of Medicine, Department of Microbiology
PublicationsIla B. Lansky, Gudrun S. Lukat-Rodgers, Darci Block, Kenton R. Rodgers, Melanie Ratliff, and Angela Wilks. 2006. The cytoplasmic heme-binding protein (PhuS) from the heme uptake system of Pseudomonas aeruginosa is an intracellular heme-trafficking protein to the regioselective heme oxygenase. J. Biol. Chem. 9:13652-62
Ratliff-Griffin, M., Wilks, A., Stojiljkovic, I. 2004. Bacterial Heme Oxygenases. Iron Transport in Bacteria. Book, In press.
Perkins-Balding, D., Griffin-Ratliff, M., and Stojiljkovic, I. 2004. Iron Transport Systems inNeisseria meningitidis. Micro. Mol. Biol. Rev. 68 (1): 154-171.
Ratliff, M., Zhu., W., Deshmukh, R., Wilks, A., and Stojiljkovic, I. 2001. Homologues ofNeisserial heme oxygenase in Gram-negative bacteria: degradation of heme by the product of the pigA gene of Pseudomonas aeruginosa. J. Bacteriol. 183 (21): 6394-6403.