Virulence of the Lyme disease spirochete before and after the tick bloodmeal: a quantitative assessment.

Irene N. Kasumba,Aaron Bestor,Patricia A. Rosa,Kit Tilly

doi:10.1186/s13071-016-1380-1

Abstract

BackgroundBorrelia 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.MethodsHomogenates 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.ResultsWe found that an inoculum containing as many as 104B. 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.ConclusionConditional priming of B. burgdorferi during tick feeding induces changes in addition to OspC that are required for infection of the mammalian host.

Highlights

Borrelia burgdorferi, the tick-transmitted agent of Lyme disease, adapts to different environments as it cycles between an arthropod vector and vertebrate host
Our results demonstrate that viable B. burgdorferi in unfed ticks are highly attenuated in their ability to infect mice, even with an inoculum of >7×103 organisms engineered to constitutively produce the virulence factor OspC
The ID50 in mice varies with B. burgdorferi source Our previous studies calculated an ID50 in mice of ~500 in vitro-cultivated B. burgdorferi, while that of bacteria derived from nymphs fed for 72 h was ~10 spirochetes [37]

Summary

Introduction

The tick-transmitted agent of Lyme disease, adapts to different environments as it cycles between an arthropod vector and vertebrate host. As B. burgdorferi cycles between vector and host, it senses external cues and adapts by making gene products appropriate for each environment (reviewed in [5, 6]). This ability to detect sudden changes in external stimuli and modulate gene expression is mediated by a relatively small set of known regulatory proteins and sigma factors [7]. When larvae feed on an infected mammal and acquire spirochetes, the RpoN-RpoS cascade is deactivated; it is re-activated when molted nymphs take a blood meal and spirochetes in the tick midgut sense accompanying stimuli, such as an influx of nutrients and changes in temperature and pH (reviewed in [5]). OspC is an outer surface lipoprotein that is required by B

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