Michael D L Johnson
Assistant Professor, Immunobiology
Assistant Professor, BIO5 Institute
Primary Department
Department Affiliations
(520) 626-3779
Work Summary
Metals such as calcium and iron are essential to living organisms. Some metals in excess, like copper, are detrimental to bacteria. My laboratory studies this phenomenon in Streptococcus pneumoniae to find novels method for killing pathogenic bacteria.
Research Interest
Metals serve as vital nutrients to all biological systems. During infections, bacteria must not only acquire all metals necessary for survival from within the host, such as calcium or manganese, but must also efflux metals that are toxic or in excess such as copper. The overall goal of my laboratory is to investigate how bacteria maintain homeostasis within the metal milieu. This goal involves determining how metals are processed, the orchestrated response during metal sensing, and the role that the host plays in this process during infection. Understanding how bacteria interact with metals during infections will identify novel therapeutic strategies against bacterial infections. Keywords: Infectious Diseases, Antibiotic resistance, Bacterial Pneumonia

Publications

Johnson, M. D., & Ayers, K. A. (2016). Science Sound Bites, a Podcast for STEM Curriculum Supplementation. Journal of microbiology & biology education, 17(2), 286-7.
Johnson, M. D., Garrett, C. K., Bond, J. E., Coggan, K. A., Wolfgang, M. C., & Redinbo, M. R. (2011). Pseudomonas aeruginosa PilY1 binds integrin in an RGD- and calcium-dependent manner. PloS one, 6(12), e29629.

PilY1 is a type IV pilus (tfp)-associated protein from the opportunistic pathogen Pseudomonas aeruginosa that shares functional similarity with related proteins in infectious Neisseria and Kingella species. Previous data have shown that PilY1 acts as a calcium-dependent pilus biogenesis factor necessary for twitching motility with a specific calcium binding site located at amino acids 850-859 in the 1,163 residue protein. In addition to motility, PilY1 is also thought to play an important role in the adhesion of P. aeruginosa tfp to host epithelial cells. Here, we show that PilY1 contains an integrin binding arginine-glycine-aspartic acid (RGD) motif located at residues 619-621 in the PilY1 from the PAK strain of P. aeruginosa; this motif is conserved in the PilY1s from the other P. aeruginosa strains of known sequence. We demonstrate that purified PilY1 binds integrin in vitro in an RGD-dependent manner. Furthermore, we identify a second calcium binding site (amino acids 600-608) located ten residues upstream of the RGD. Eliminating calcium binding from this site using a D608A mutation abolished integrin binding; in contrast, a calcium binding mimic (D608K) preserved integrin binding. Finally, we show that the previously established PilY1 calcium binding site at 851-859 also impacts the protein's association with integrin. Taken together, these data indicate that PilY1 binds to integrin in an RGD- and calcium-dependent manner in vitro. As such, P. aeruginosa may employ these interactions to mediate host epithelial cell binding in vivo.

Honsa, E. S., Johnson, M. D., & Rosch, J. W. (2013). The roles of transition metals in the physiology and pathogenesis of Streptococcus pneumoniae. Frontiers in cellular and infection microbiology, 3, 92.

For bacterial pathogens whose sole environmental reservoir is the human host, the acquisition of essential nutrients, particularly transition metals, is a critical aspect of survival due to tight sequestration and limitation strategies deployed to curtail pathogen outgrowth. As such, these bacteria have developed diverse, specialized acquisition mechanisms to obtain these metals from the niches of the body in which they reside. To oppose the spread of infection, the human host has evolved multiple mechanisms to counter bacterial invasion, including sequestering essential metals away from bacteria and exposing bacteria to lethal concentrations of metals. Hence, to maintain homeostasis within the host, pathogens must be able to acquire necessary metals from host proteins and to export such metals when concentrations become detrimental. Furthermore, this acquisition and efflux equilibrium must occur in a tissue-specific manner because the concentration of metals varies greatly within the various microenvironments of the human body. In this review, we examine the functional roles of the metal import and export systems of the Gram-positive pathogen Streptococcus pneumoniae in both signaling and pathogenesis.

Johnson, M. D., Echlin, H., Dao, T. H., & Rosch, J. W. (2015). Characterization of NAD salvage pathways and their role in virulence in Streptococcus pneumoniae. Microbiology (Reading, England), 161(11), 2127-36.

NAD is a necessary cofactor present in all living cells. Some bacteria cannot de novo synthesize NAD and must use the salvage pathway to import niacin or nicotinamide riboside via substrate importers NiaX and PnuC, respectively. Although homologues of these two importers and their substrates have been identified in other organisms, limited data exist in Streptococcus pneumoniae, specifically, on its effect on overall virulence. Here, we sought to characterize the substrate specificity of NiaX and PnuC in Str. pneumoniae TIGR4 and the contribution of these proteins to virulence of the pathogen. Although binding affinity of each importer for nicotinamide mononucleotide may overlap, we found NiaX to specifically import nicotinamide and nicotinic acid, and PnuC to be primarily responsible for nicotinamide riboside import. Furthermore, a pnuC mutant is completely attenuated during both intranasal and intratracheal infections in mice. Taken together, these findings underscore the importance of substrate salvage in pneumococcal pathogenesis and indicate that PnuC could potentially be a viable small-molecule therapeutic target to alleviate disease progression in the host.

Orans, J., Johnson, M. D., Coggan, K. A., Sperlazza, J. R., Heiniger, R. W., Wolfgang, M. C., & Redinbo, M. R. (2010). Crystal structure analysis reveals Pseudomonas PilY1 as an essential calcium-dependent regulator of bacterial surface motility. Proceedings of the National Academy of Sciences of the United States of America, 107(3), 1065-70.

Several bacterial pathogens require the "twitching" motility produced by filamentous type IV pili (T4P) to establish and maintain human infections. Two cytoplasmic ATPases function as an oscillatory motor that powers twitching motility via cycles of pilus extension and retraction. The regulation of this motor, however, has remained a mystery. We present the 2.1 A resolution crystal structure of the Pseudomonas aeruginosa pilus-biogenesis factor PilY1, and identify a single site on this protein required for bacterial translocation. The structure reveals a modified beta-propeller fold and a distinct EF-hand-like calcium-binding site conserved in pathogens with retractile T4P. We show that preventing calcium binding by PilY1 using either an exogenous calcium chelator or mutation of a single residue disrupts Pseudomonas twitching motility by eliminating surface pili. In contrast, placing a lysine in this site to mimic the charge of a bound calcium interferes with motility in the opposite manner--by producing an abundance of nonfunctional surface pili. Our data indicate that calcium binding and release by the unique loop identified in the PilY1 crystal structure controls the opposing forces of pilus extension and retraction. Thus, PilY1 is an essential, calcium-dependent regulator of bacterial twitching motility.