Fluorescent Proteins, Promoters, and Selectable Markers for Applications in the Lyme Disease Spirochete Borrelia burgdorferi

Abstract

Lyme disease is the most widely reported vector-borne disease in the United States. Its incidence is rapidly increasing, and disease symptoms can be debilitating. The need to understand the biology of the disease agent, the spirochete <i>Borrelia burgdorferi</i>, is thus evermore pressing. Despite important advances in <i>B. burgdorferi</i> genetics, the array of molecular tools available for use in this organism remains limited, especially for cell biological studies. Here, we adapt a palette of bright and mostly monomeric fluorescent proteins for versatile use and multicolor imaging in <i>B. burgdorferi</i> We also characterize two novel antibiotic selection markers and establish the feasibility of their use in conjunction with extant markers. Last, we describe a set of promoters of low and intermediate strengths that allow fine-tuning of gene expression levels. These molecular tools complement and expand current experimental capabilities in <i>B. burgdorferi</i>, which will facilitate future investigation of this important human pathogen. To showcase the usefulness of these reagents, we used them to investigate the subcellular localization of BB0323, a <i>B. burgdorferi</i> lipoprotein essential for survival in the host and vector environments. We show that BB0323 accumulates at the cell poles and future division sites of <i>B. burgdorferi</i> cells, highlighting the complex subcellular organization of this spirochete.<b>IMPORTANCE</b> Genetic manipulation of the Lyme disease spirochete <i>B. burgdorferi</i> remains cumbersome, despite significant progress in the field. The scarcity of molecular reagents available for use in this pathogen has slowed research efforts to study its unusual biology. Of interest, <i>B. burgdorferi</i> displays complex cellular organization features that have yet to be understood. These include an unusual morphology and a highly fragmented genome, both of which are likely to play important roles in the bacterium's transmission, infectivity, and persistence. Here, we complement and expand the array of molecular tools available for use in <i>B. burgdorferi</i> by generating and characterizing multiple fluorescent proteins, antibiotic selection markers, and promoters of varied strengths. These tools will facilitate investigations in this important human pathogen, as exemplified by the polar and midcell localization of the cell envelope regulator BB0323, which we uncovered using these reagents.