Natural reversion promoting LPS elongation of an attenuated Coxiella burnetii

Lipopolysaccharide (LPS) phase variation is a critical aspect of virulence in many Gram-negative bacteria and is of particular importance to Coxiella burnetii, the biothreat pathogen that causes Q fever. Here, we demonstrate that Nine Mile II (NMII), a strain considered to be avirulent and exempted from select agent status, was recovered from the spleens of infected guinea pigs and further shown to exhibit a previously unrecognized form of LPS modification, facilitating the production of elongated LPS and increasing virulence. We further identified the mechanism, showing that the reversion of a 3 bp mutation in cbu0533 directly led to LPS elongation. Addressing potential safety concerns, we introduce a NMII strain unable to produce elongated LPS.