Justin D. Radolf, MD

Lyme Disease/Borrelia burgdorferi The Lyme disease spirochete B. urgdorferi is maintained in nature via an enzootic cycle in which it is transmitted by the nymphal stage of its vector, the deer tick Ixodes scapularis, to a mammalian host (typically the white-footed mouse Peromyscus leucopus) and then is acquired when a naïve larval tick feeds on an infected mouse. Infection of humans with B. burgdorferi is accidental and is not required for persistence of the spirochete in nature. In order to transit between its arthropod and mammalian hosts, spirochetes must decipher complex environmental cues delivered at the feeding site and, in response, undergo dramatic changes in their transcriptomes and proteomes. The principal objective of Lyme disease research conducted in the Radolf Laboratory is to understand these processes. The alternative sigma factor RpoS is a unifying genetic feature of this project. Signals delivered by the blood meal induce the expression of RpoS which, as the promoter-reading subunit of RNA polymerase, induces far-reaching changes in the bacterium’s transcriptome: (1) upregulation of approximately 100 B. burgdorferi genes that we believe are required for transmission of spirochetes from tick to mouse and/or the establishment of infection once within the mouse and (2) downregulation of approximately 30 tick-phase genes that spirochetes no longer need once they are inoculated into mice. Critical to this work has been our development of green fluorescent protein (GFP) reporters that enable us to track live spirochetes in ticks and mice. Our live-imaging studies have fundamentally changed our understanding of the transmission process. In order to reach the mouse, spirochetes disseminate through the midgut into the salivary glands in order to access the salivary stream which they “ride” into the vertebrate host. We have found that dissemination of spirochetes in ticks is actually biphasic. In the first phase, which we have termed “adherence-mediated migration, spirochetes replicate in close association with differentiating midgut epithelial cells, “working” their way as aggregates or networks to the base of the epithelium. In the second phase, they transition into typically motile spirochetes, complete the penetration through the midgut, and then move on to the salivary glands en route to the mouse. Most recently, we have found that spirochetes lacking RpoS are deficient in this process and we are developing various strategies to identify the RpoS-dependent genes involved. Syphilis/Treponema pallidum The syphilis spirochete T. pallidum harbors many resemblances to B. burgdorferi but actually employs a markedly different parasitic strategy. Whereas B. burgdorferi is an enzootic pathogen, T. pallidum is an obligate pathogen of humans which cannot be cultivated in artificial medium. The modes of transmission of the two bacteria differ markedly as well: T. pallidum is transmitted from person-to-person by sexual activity, whereas B. burgdorferi is transmitted by an arthropod vector. Once within the host, T. pallidum begins to replicate locally, eventually causing a genital ulcer, called a chancre, the clinical hallmark of the primary stage of the disease. As the chancre develops, treponemes begin to make their way towards draining lymph nodes and blood vessels in order to spread systemically. Once within the blood, T. pallidum is extremely adept at invading virtually every organ system in the body, including the central nervous system, and establishing persistent infection that can cause serious, even life threatening, complications months to years later. We have designated T. pallidum “the stealth pathogen” because of its remarkable ability to evade host immune defenses. Efforts in the Radolf Laboratory to explain T. pallidum’s stealth pathogenicity have focused on the bacterium’s unusual molecular architecture. Over the years, we have generated abundant evidence that the T. pallidum outer membrane differs markedly in structure and composition from its gram-negative counterparts (e.g., Escherichia coi). Not only does it lack lipopolysaccharide, the highly inflammatory glycolipid in the outer membranes of gram negative bacteria, it also contains a much lower density of integral membrane proteins that present few surface antigenic targets to the host immune system. Situated below the outer bilayer, where they are inaccessible to antibodies in intact organisms, are the bacterium’s major immunogens, many of which are periplasmic proteins tethered by N-terminal lipids to the cytoplasmic membrane. This work ushered in what we have termed “the quest” for T. pallidum outer membrane proteins, a project that has been ongoing for more than 20 years. Why a quest? Because identifying rare outer membrane proteins is so difficult and requires extraordinary commitment. Fortunately, we now have much more powerful tools to fulfill the quest, among which is the complete genomic sequence of T. pallidum. Genome mining, however, isn’t as easy as it sounds because, with one exception, there are no proteins in the T. pallidum genome with sequence relatedness to well characterized outer membrane proteins of gram-negatives. This work is complicated further by the fragility of the treponeme’s outer membrane. Our genereal strategy is to use bioinformatics algorithms to identify outer membrane protein candidates that then must be cloned, expressed, purified, structurally characterized, and localized in live treponemes. Why is this quest important? Two reasons. First, outer membrane proteins provide channels through which bacteria obtain nutrients. Second, we believe that these surface-exposed proteins, few as they are, are likely vaccine candidates. Given the explosive increase in syphilis cases in the United States and the world during the past decade, a vaccine would be a major weapon in our battle against this centuries old affliction of humans. In order to fulfill its genetic destiny as a stealth pathogen, T. pallidum must acquire nutrients in every milieu within its obligate human host in which it finds itself, while fending off the host’s attempts to undermine its homeostasis. Recognition of this metabolic war between pathogen and host led us to explore other facets of T. pallidum virulence. One has been transition metal acquisition. Transition metals, such as iron, manganese, and zinc, are essential for life but are present in mammalian body fluids at exceedingly low concentrations. Bacterial pathogens, T. pallidum being no exception, employ highly developed strategems to obtain these nutrients. Our work along these lines has centered about characterizing two ABC transporters within the cytoplasmic membrane (Tro and Znu) that work cooperatively to meet the bacterium’s metal requirements. Lastly, our immune system uses toxic compounds called reactive oxygen species, to kill bacteria. T. pallidum has extremely robust enzymatic mechanisms for detoxifying reactive oxygen species. Understanding how these enzymes work and are regulated in response to host defenses is relevant to all bacterial diseases, not just syphilis.

Journal Articles High prevalence of and low testing for sexually transmitted infections among transgender women in Cali, Colombia. Garcia-Luna, Jonny A; Silva-Peña, Sebastián A; Hurtado, Juan S; Ramírez-Ayala, José D; Aguirre-Martínez, Laura M; Ramírez, Lady G; Martínez-Valencia, Alvaro J; Hawley, Kelly L; Smith, Adrian D; Salazar, Juan C; Radolf, Justin D Sexually transmitted diseases 2026 Mar; Estimating the sensitivity of non-treponemal and treponemal antibody tests in primary syphilis. Sweitzer, Stephanie F; Chen, Jane S; Matoga, Mitch M; Yang, Ligang; Lopez-Medina, Eduardo; Garcia-Luna, Jonny A; Hoffman, Irving F; Yang, Bin; Salazar, Juan C; Radolf, Justin D; Parr, Jonathan B; Seña, Arlene C Sexually transmitted diseases 2026 Mar; DNA Binding by BosR Controls RpoS-Dependent and -Indep