Steven Forst
Steven Forst
Molecular Microbiology

Ph.D., New York Univ. Medical Center 1985
M.S., Rutgers Univ. 1976
B.S., Wilkes Univ. 1974

Office: Lapham 458
Phone: 414-229-6373
FAX: 414-229-3926

Research Interests

Our laboratory is interested in understanding the mechanisms by which bacteria adapt to different host environments. We study as a model system Xenorhabdus nematophila, a motile gram-negative bacterium that engages in both mutualistic and pathogenic host interactions. Xenorhabdus forms a species-specific mutualistic association with the insect-invading nematode, Steinernema carpocapsae. The nematode enters insect hosts and X. nematophila is released from an intestinal vesicle into the insect's body cavity (hemocoel) where it functions as a pathogen. Contraction of the esophagus of the nematode stimulates the forward movement of Xenorhabdus out of the vesicle through a connecting structure that allows the bacteria to enter the intestine and ultimately leave the nematode via the anus. In the hemocoel, Xenorhabdus initially colonizes the connective tissue surrounding the anterior midgut of the host insect. The bacteria proliferate in the insect cadaver reaching high cell densities at which point they produce diverse exoenzymes and antibiotics. The bacteria themselves, as well as the macromolecular degradation they stimulate, provide a nutrient base suitable for nematode reproduction. After several cycles of sexual reproduction the nematodes develop into a dauer juvenile stage that possesses the specialized intestinal vesicle that Xenorhabdus colonizes by a monoclonal process.

I. Comparative genomics
We are involved in a collaborative effort ( to annotate and analyze the genomes of X. nematophila, that colonizes a single nematode species, and X. bovienii, that forms mutualistic associations with seven different species of steinernematid nematodes. One of the primary goals of this project is to better understand the genetic and molecular basis of host specificity and co-adaptation.

II. Coordinate regulation of motility, exoenzymes and antibiotics
Xenorhabdus nematophila is an emerging model for both mutualism and pathogenicity in different invertebrate hosts. We are analyzing how the EnvZ-OmpR two component system and the flagella sigma factor, FliA(s 28) coordinately regulate the production of lipase, protease, hemolysin and antibiotic activity. The role of this complex regulatory system in the life cycle of Xenorhabdus is being studied.

III. Cell surface proteins
Fimbriae are cell surface appendages involved in adhesion to biotic and abiotic surfaces. Pathogenic bacteria generally possess several fimbrial operons. In contrast, X. nematophila possesses a single complete fimbrial operon. The regulation and function of the fimbrial operon of X. nematophila is presently being investigated. Outer membrane porins allow passive diffusion of solutes into the cell. In X. nematophila, the OpnP porin is highly expressed during exponential growth. We have identified a second porin, OpnS, that is highly expressed during the transition to stationary phase. The regulation and role of this porin in the life cycle of Xenorhabdus is currently being studied.

IV. Competition and antimicrobial products
X. nematophila elaborates bacteriocins that are bacteriophage-derived products that inhibit the growth of closely related bacteria. Xenorhabdus bacteriocins are believed to provide a competitive advantage for growth in the insect cadaver. The bacteriophage genes responsible for the production of the bacteriocin have been identified in the genome of X. nematophila. The function of bacteriocins in suppressing growth of antagonistic competitors in the insect niche is being investigated. X. nematophila also produces several antibiotic compounds that have activities against a broad range of microorganisms. The genes encoding these antibiotics, their regulation and biological significance is under investigation in our laboratory.

Selected Publications

Role of secondary metabolites in establishment of the mutualistic partnership between Xenorhabdus nematophila and the entomopathogenic nematode Steinernema carpocapsae. 2015. Singh S, Orr D, Divinagracia E, McGraw J, Dorff K, Forst S. Appl Environ Microbiol. 81(2):754-64

Microbial population dynamics in the hemolymph of Manduca sexta infected with Xenorhabdus nematophila and the entomopathogenic nematode, Steinernema carpocapsae. 2014. Singh S, Reese JM, Casanova-Torres AM, Goodrich-Blair H, Forst S. Appl Environ Microbiol. 2014. 80 (14): 4277-4285.

Xenocin export by the flagellar type III pathway in Xenorhabdus nematophila. 2013.Singh P, Park D, Forst S, Banerjee N. J Bacteriol. 195(7):1400-1410.

Characterization of an acid-inducible sulfatase in Salmonella enterica serovar typhimurium. 2013.Das S, Singh S, McClelland M, Forst S, Gyaneshwar P. Appl Environ Microbiol. 79(6): 2092-2095.

Evolution of the Stx2-encoding prophage in persistent bovine Escherichia coli O157:H7 strains. 2013. Park D, Stanton E, Ciezki K, Parrell D, Bozile M, Pike D, Forst SA, Jeong KC, Ivanek R, Döpfer D, Kaspar CW. Appl Environ Microbiol. 79(5): 1563-1572.

Morales-Soto N, Gaudriault S, Ogier JC, Thappeta KR, Forst S. 2012 Comparative analysis of P2-type remnant prophage loci in Xenorhabdus bovienii and Xenorhabdus nematophila required for xenorhabdicin production. FEMS Microbiol Lett. 333:69-76.

Dornfeld CL, Hoelzer M, Forst S. 2012. Proteopedia entry: beta-prime subunit of bacterial RNA polymerase. Biochem Mol Biol Educ. 40:284.

Snyder H, He H, Owen H, Hanna C, Forst S. 2011. Role of Mrx fimbriae of Xenorhabdus nematophila in competitive colonization of the nematode host. Appl Environ Microbiol. 77: 7247-7254.

Morales-Soto, N. and Forst, S. 2011. The xnp1 P2-like tail synthesis gene cluster encodes xenorhabdicin and is required for interspecies competition. J Bacteriol. 193(14):3624-32.

Fodor, A., Fodor, A.M., Forst, S., Hogan, J.S., Klein, M., Lengyel, K., Saringer, G., Stackebrandt, E., Taylor, J., Lehoczky, E. 2010. Comparative analysis of antibacterial activities of Xenorhabdus species on related and non-related bacteria in vivo. J. Microbiol and Antimicrobials. 2 (3), 30-35.

Ogier JC, Calteau A, Forst S., Goodrich-Blair H, Roche D, Rouy Z, Suen G, Zumbihl R, Givaudan A, Tailliez P, Médigue C, Gaudriault S. 2010. Units of plasticity in bacterial genomes: new insight from the comparative genomics of two bacteria interacting with invertebrates, Photorhabdus and Xenorhabdus. BMC Genomics. 11:568-578.

Park,D., Ciezki,K., van der Hoeven,R., Singh,S., Reimer,D., Bode, H. and Forst, S. 2009. Genetic analysis of xenocoumacin antibiotic production in the mutualistic bacterium Xenorhabdus nematophila. Molecular Microbiology 73(5), 938–949.

van der Hoeven, R and Forst, S. 2009. OpnS, an outer membrane porin of Xenorhabdus nematophila, confers a competitive advantage for growth in the insect host. J. Bacteriology 191 (17) 5471–5479.

Morales-Soto, N., Snyder, H. and Forst, S. 2009. Interspecies Competition in a Bacteria–Nematode Mutualism. In Defensive Mutualism in Microbial Symbiosis (ed) White, J.F. and Torres, M.S. pp 117-128.

van der Hoeven, R., Betrabet, G. and Forst, S. 2008. Characterization of the gut bacterial community in Manduca sexta and effect of antibiotics on bacterial diversity and nematode reproduction. FEMS Microbiol Letters, 286: 249-256.

Snyder, H., Stock, P., Kim, S., Flores-Lara, Y. and Forst, S. 2007. New Insights into the Colonization and Release Processes of Xenorhabdus nematophila and the Morphology and Ultrastructure of the Bacterial Receptacle of its Nematode Host, Steinernema carpocapsae. Appl. Environ. Microbiol. 66: 1622-1628

Park, D. and Forst, S. 2006. Co-regulation of motility, exoenzyme and antibiotic production by the EnvZ-OmpR-FlhDC-FliA pathway in Xenorhabdus nematophila. Mol Microbiol. 61:1397-412.

Goetsch, M., Owen, H., Goldman, B. and Forst S. 2006. Analysis of the PixA inclusion body protein of Xenorhabdus nematophila. J Bacteriol.188: 2706-10

Forst, S. and Goodner, B. 2006. Comparative bacterial genomics and its use in undergraduate education. Biological Control 38: 47-53.

Kim, D-J. and Forst, S. 2005. Xenorhabdus nematophila: Mutualist and Pathogen. ASM News. 71: 174-178.

Kim, D-J.P. and Forst, S. 2005. The OmpR-FlhDC regulatory circuit and flagella regulon in Xenorhabdus spp. In Pruss, B.M and Matsumura, P. (eds): Global Regulatory Networks in Enteric Bacteria. Research Signpost. Kerala, India.

He, H., Holly A. Snyder, and Steven Forst. 2004 Unique organization and regulation of the mrx fimbrial operon in Xenorhabdus nematophila. Microbiology 150: 1439-1446.

Kim, D-J. Boylan, B. George, N. and Forst, S. 2003. Inactivation of ompR Promotes Precocious Swarming and flhDC Expression in Xenorhabdus nematophila. J. Bacteriol. 185 5290-5294.

Forst, S. and Boylan, B. 2002. Characterization of the pleiotropic phenotype of an ompR strain of Xenorhabdus nematophila. Antonie van Leeuwenhoek 81: 43-49.

P. Prohinar, S. A. Forst, D. Reed, I. Mandic-Mulec & J. Weiss. 2002. OmpR-dependent and OmpR-independent responses of Escherichia coli to sublethal attack by the neutrophil bactericidal/permeability increasing protein. Molecular Microbiology, 43: 1493-1500.

Forst, S. and Clarke, D. 2002. Bacteria-nematode symbioses. In Gaugler (ed.), Entomopathogenic Nematology CABI Publishing, Wallingford, UK.

Delihas N. and Forst S. 2001. MicF: An Antisense RNA Gene Involved in Response of Escherichia coli to Global Stress Factors. Journal of Molecular Biology 313 (1):1-12.

Kim, D-J and Forst, S. 2001. Genomic analysis of the histidine kinase family in bacteria and archaea. Microbiology 147: 1197-1212.

Volgyi, A, Fordor, A and Forst, S. 2000. Inactivation of a novel gene produces a phenotypic variant cell and affects symbiotic behavior in Xenorhabdus nematophilus. Appl. Environ. Microbiol. 66: 1622-1628.

Waukau, J. and Forst, S. 1999. Identification of a conserved sequence involved in transmebrane signal transduction in E. coli . J. Bacteriol. 181: 5534-5538.

Volgyi, A., Fodor, A., Szentirmai, A. and Forst, S. 1998. Phase variation in Xenorhabdus nematophilus. Appl. Environ. Microbiol. 64:1188-1193.

Forst, S. , Dowds, B., Boemare, N. and Stackebrandt, E. 1997. Xenorhabdus spp. and Photorhabdus spp. : Bugs that kill bugs. Ann. Rev. Microbiol. 51: 47-72

Forst, S. and Tabatabai, N. 1997. Role of the histidine kinase, EnvZ, in the production of outer membrane proteins in the symbiotic-pathogenic bacterium, Xenorhabdus nematophilus. Appl. and Environ. Microbiol. 63: 962-968.

Skarphol, K., Waukau, J. and Forst, S. 1997. The role of His-243 in the phosphatase activity of EnvZ in Escherichia coli. J. Bacteriol. 179:1413-1416.

Leonardo, M.R. and Forst, S. 1996. Reexamination of the role of the periplasmic domain of EnvZ in sensing osmolarity signals in Escherichia coli. Mol. Microbiol. 22:405-413.

Forst S, and Nealson K. 1996. Molecular biology of the symbiotic-pathogenic bacteria. Xenorhabdus spp. and Photorhabdus spp. Microbiol. Rev. 60:21-43.

Forst S, Waukau J, Leisman G, Exner M, and Hancock RW. 1995. Functional and regulatory analysis of the OmpF-like porin, OpnP, of the symbiotic bacterium, Xenorhabdus nematophilus. Mol. Microbiol. 18:779-789.

Tabatabai N and Forst S. 1995. Molecular analysis of the ompR or envZ genes in the symbiotic bacterium, Xenorhabdus nematophilus. Mol. Microbiol. 17:643-652.

Forst S, Kalve I, and Durski W. 1995. Molecular analysis of OmpR binding sequences involved in the regulation of ompF in Escherichia coli. FEMS. Micro. Let. 131:147-151.