We tested the effect of fosX when Hpt-mediated fosfomycin transport is active using two “infection-mimicking” in vitro conditions: (i) supplementation of BHI medium with an adsorbent (activated charcoal or Amberlite XAD-4), which causes the partial activation of the PrfA regulation system by an as yet not fully understood mechanism [37]; and (ii) use of a constitutively activated prfA* allele, where a single amino acid substitution (e.g. PrfA*G145S) locks PrfA in “On” state, causing constitutive activation of the PrfA-regulated virulence genes [38]. The gene discussed is MRS2; the disease is infection.