title: Virulence of the Lyme disease spirochete before and after the tick bloodmeal: a quantitative assessment.
abstract: BACKGROUND
  Borrelia burgdorferi, the tick-transmitted agent of Lyme disease, adapts to different environments as it cycles between an arthropod vector and vertebrate host. Signals encountered during nymphal tick feeding prior to transmission activate a regulon that is controlled by the alternative sigma factors RpoN and RpoS, which are required for mammalian infection. The ingested bloodmeal also provides nutrients that stimulate spirochete replication. Although the influence of tick feeding on spirochete growth and gene expression is well documented, a quantitative assessment of spirochete virulence before and after tick feeding has not been made.
  
  
  METHODS
  Homogenates were prepared from unfed and fed infected Ixodes scapularis nymphs that had acquired B. burgdorferi as larvae. Serially diluted tick homogenates were needle-inoculated into mice to determine the infectious dose of tick-derived spirochetes before and after the bloodmeal. Mouse infection was assessed by sero-reactivity with B. burgdorferi whole cell lysates on immunoblots and attempted isolation of spirochetes from mouse tissues. Viable spirochetes in tick-derived inocula were quantified by colony formation in solid media.
  
  
  RESULTS
  We found that an inoculum containing as many as 10(4) B. burgdorferi from unfed ticks is largely non-infectious, while the calculated ID50 for spirochetes from fed ticks is ~30 organisms. Engineered constitutive production of the essential virulence factor OspC by spirochetes within unfed ticks did not confer an infectious phenotype.
  
  
  CONCLUSIONS
  Conditional priming of B. burgdorferi during tick feeding induces changes in addition to OspC that are required for infection of the mammalian host.
authors.affiliation: Laboratory of Zoonotic Pathogens, Rocky Mountain Laboratories, National Institute of Allergy and Infectious Diseases, National Institutes of Health, Hamilton, MT, 59840, USA. ikasumba@medicine.umaryland.edu.|Current address: Center for Vaccine Development, Department of Geographic Medicine, University of Maryland School of Medicine, Baltimore, MD, 21201, USA. ikasumba@medicine.umaryland.edu.
authors.forename: Irene N
authors.initials: IN
authors.lastname: Kasumba
authors.affiliation: Laboratory of Zoonotic Pathogens, Rocky Mountain Laboratories, National Institute of Allergy and Infectious Diseases, National Institutes of Health, Hamilton, MT, 59840, USA. bestora@niaid.nih.gov.
authors.forename: Aaron
authors.initials: A
authors.lastname: Bestor
authors.affiliation: Laboratory of Zoonotic Pathogens, Rocky Mountain Laboratories, National Institute of Allergy and Infectious Diseases, National Institutes of Health, Hamilton, MT, 59840, USA. ktilly@niaid.nih.gov.
authors.forename: Kit
authors.initials: K
authors.lastname: Tilly
authors.affiliation: Laboratory of Zoonotic Pathogens, Rocky Mountain Laboratories, National Institute of Allergy and Infectious Diseases, National Institutes of Health, Hamilton, MT, 59840, USA. prosa@niaid.nih.gov.
authors.forename: Patricia A
authors.initials: PA
authors.lastname: Rosa
mesh_terms: D000818:Animals; D058491:Bacterial Load; D001769:Blood; D025065:Borrelia burgdorferi / Q000187:drug effects* / Q000472:pathogenicity*; D004195:Disease Models, Animal; D018884:Ixodes / Q000382:microbiology*; D008193:Lyme Disease / Q000382:microbiology / Q000473:pathology; D051379:Mice; D014774:Virulence