loses essential genetic elements and cell proliferative potential during stationary phase in culture but not in the tick vector.

The Lyme disease agent is a polyploid bacterium with a segmented genome in which both the chromosome and over 20 distinct plasmids are present in multiple copies per cell. This pathogen can survive at least nine months in its tick vector in an apparent dormant state between blood meals, without losing cell proliferative capability when re-exposed to nutrients. Cultivated cells grown to stationary phase or resuspended in nutrient-limited media are often used to study the effects of nutrient deprivation. However, a thorough assessment of the spirochete's ability to recover from nutrient depletion has been lacking. Our study shows that starved cultures rapidly lose cell proliferative. Loss of genetic elements essential for cell proliferation contributes to the observed proliferative defect in stationary phase. The gradual decline in copies of genetic elements is not perfectly synchronized between chromosomes and plasmids, generating cells that harbor one or more copies of the essential chromosome but lack all copies of one or more non-essential plasmids. This phenomenon likely contributes to the well-documented issue of plasmid loss during in vitro cultivation of . In contrast, cells from ticks starved for 14 months showed no evidence of reduced cell proliferative ability or plasmid loss. Beyond their practical implications for studying , these findings suggest that the midgut of the tick vector offers a unique environment that supports the maintenance of 's segmented genome and cell proliferative potential during periods of tick fasting.