Dissection of amino acid acquisition pathways demonstrates that amino acid starvation of Borrelia burgdorferi results in a (p)ppGpp-independent maladaptive response

Borrelia burgdorferi, the causative agent of Lyme disease, is well known for its unique physiology and enzootic cycle. Building on previous work showing peptide transport is essential for viability, we endeavored to clearly define the impact of peptide starvation on the spirochete and directly compare peptide starvation to targeted free amino acid starvation. Herein, we confirm the ability of a putative glutamate transporter, BB0401, to transport glutamate and aspartate as well as demonstrate its requirement for viability. Using conditional mutants for both peptide transport and BB0401, we characterize these systems throughout the enzootic cycle, confirming their essential role during murine infection and revealing that they are dispensable during prolonged colonization of the tick midgut. We broadly define the metabolic perturbations resulting from these starvation models and show, even under the most severe amino acid stress, B. burgdorferi is unable to modulate its physiological response via the canonical (p)ppGpp-driven stringent response.