RESEARCH INTERESTS

Aggregatibacter (formerly Actinobcacillus) actinomycetemcomitans (Aa) is a Gram-negative facultative, capnophylic, coccobacillus found in the oral cavities of healthy and periodontally affected individuals. The organism has been implicated as the causative agent of aggressive periodontitis as well as other types of human infections. Aa is also a member of a clinically important group of bacteria, the HACEK group that is responsible for 3-10% of cases of infective endocarditis. For a long time a thorough study of Aa virulence traits was hampered due to the lack of an appropriate genetic system. In recent years, the core of our research has focused on the use of plasmids and other mobile genetic elements to develop molecular biological and genetic tools that will allow for an investigation of the functionality and regulation of putative Aa virulence traits. The use of these genetic tools has been crucial in the initiation of the following three projects.

1. Aa-neutrophil interaction

It has been widely accepted that periodontal pathogens have the ability to evade or inhibit the host immune system. For Aa to be established in the host it must protect itself from the numerous host defenses, starting with resistance to phagocytosis and/or killing by neutrophils. In order to elucidate Aa-neutrophil interactions we have initiated a comprehensive study including Aa strains, which differ in their serotype, their ability to express leukotoxin, and their ability to express surface fimbriae. Bridging the gap between the disciplines of microbiology and cell biology, data obtained have provided new insights as to how Aa may resist killing by neutrophils.

2. Characterization of biofilms formed by Aa

The etiology of periodontal disease is complex and involves bacteria that adhere to and colonize the mostly anaerobic environment in the subgingival pocket to form a biofilm. Fresh clinical isolates of Aa express fimbriae, are self-aggregating and form tenacious biofilms on surfaces, such as glass, plastic and saliva-coated hydroxyapatite. A phenotypically distinct variant no longer expresses these fimbriae and, according to published literature, has lost the ability to self-aggregate. Currently, we are characterizing biofilms formed by Aa under different environmental conditions by use of 2-photon laser scanning confocal microscopy and computational image analysis. Our preliminary data indicate that loss of fimbriae does not prevent biofilm formation, although there are noticeable differences in the structure of biofilms when comparing self-aggregating and non-aggregating variants of Aa. Secondly, Aa can form biofilms under aerobic and anaerobic conditions. Oxygen-related variations in biofilm formation may play a role in the adaptation of Aa to different host environments (periodontal pocket vs. heart tissue).

3. Conjugative transfer in mixed biofilms

Antibiotic resistance has been increasingly described among bacterial species including oral microorganisms. Despite the magnitude of this problem there is still a critical gap in the knowledge base that centers on the frequency and mechanism of spread of resistance genes in microbial biofilms. Resistance genes are often carried on mobile genetic elements capable of interspecies transmission. Our goal is to monitor the potential, the extent, and the dynamics of conjugative gene transfer in biofilms in situ. We are currently attempting to design a simple broad-host-range genetic tool based on a DNA inversion system that we discovered on a native Aa plasmid. The new tool will allow for the non-intrusive visual tracking of mobile genetic elements in multispecies biofilms. The availability of this new genetic tool will facilitate future assessment on the true level of conjugative transfer in situ in any biofilms of clinical significance.

SELECTED PUBLICATIONS

Galli, D.M., and M.A. Menke. Characterization of the transposition mode of insertion element IS1216V in a Gram-negative host. (in preparation)

Gustavsson, A., and D.M. Galli. Oxygen-related variation in biofilm formation of fimbriated and non-fimbriated Aggregatibacter actinomycetemcomitans. (in preparation)

Barry, M., Galli, D.M., Hakim, F.F., Holyfield, L.J., O’Donnell, J.A., Robbins, M.R., and R.S. Wilder. Dental education's role in preparing students for an interprofessional health care team. (in preparation)

Chen, Z., Galli, D. M., Pierce, M.R., and R.L. Gregory. Involvement of quorum sensing in Streptococcus mutans antigen I/II regulation. Infect. Immun., submitted.

Chen, J., Pappas, D., and  D.M. Galli. Functional and mutational analysis of the pVT745-specific oriT. J Bacteriol, submitted, currently being revised.

Galli, D.M., and J. Chen. 2006. Entry exclusion activity on conjugative plasmid pVT745. Plasmid, 55(2):158-163.

Permpanich, P., Kowolik, M., and D.M. Galli.  2006. Resistance of fluorescent-labeled Actinobacillus actinomycetemcomitans strains to phagocytosis and killing by human neutrophils Cell. Microbiol, 8(1):72-84.

Chen, J., LeBlanc, D. J., and D. M. Galli. 2002. DNA inversion on conjugative plasmid pVT745. J. Bacteriol. 184:5926-5934.

Galli, D. M., Kerr, M. S., Fair, A. D., Permpanich, P., and D. J. LeBlanc. 2002. Parameters associated with cloning in Actinobacillus actinomycetemcomitans. Plasmid 47: 138-147.

Galli, D.M., Chen, J., Novak, K.F., and LeBlanc, D.J. 2001. Nucleotide sequence and analysis of conjugative plasmid pVT745. J. Bacteriol. 183:1585-1594.

Galli, D.M., and LeBlanc, D.J. 1997. Identification of a maintenance system on rolling circle replicating plasmid pVT736-1. Mol. Microbiol. 25:649-659.

Galli, D.M., Polan-Curtain, J.L., and LeBlanc, D.J. 1996. Structural and segregational stability of various replicons in Actinobacillus actinomycetemcomitans. Plasmid 36:42-48.

Galli, D.M., and LeBlanc, D.J. 1995. Transcriptional analysis of rolling circle replicating plasmid pVT736-1: Evidence for replication control by antisense RNA. J. Bacteriol. 177:4474-4480.

Galli, D.M. and LeBlanc, D.J. 1994. Characterization of pVT736-1, a rolling circle DNA plasmid from the gram-negative bacterium Actinobacillus actinomycetemcomitans. Plasmid 31:148-157.