Differential effects of inosine monophosphate dehydrogenase (IMPDH/GuaB) inhibition in <i>Acinetobacter baumannii</i> and <i>Escherichia coli</i>

Inosine 5'-monophosphate dehydrogenase (IMPDH), known as GuaB in bacteria, catalyzes the rate-limiting step in <i>de novo</i> guanine biosynthesis and is conserved from humans to bacteria. We developed a series of potent inhibitors that selectively target GuaB over its human homolog. Here, we show that these GuaB inhibitors are bactericidal, generate phenotypic signatures that are distinct from other antibiotics, and elicit different time-kill kinetics and regulatory responses in two important Gram-negative pathogens: <i>Acinetobacter baumannii</i> and <i>Escherichia coli</i>. Specifically, the GuaB inhibitor G6 rapidly kills <i>A. baumannii</i> but only kills <i>E. coli</i> after 24 h. After exposure to G6, the expression of genes involved in purine biosynthesis and stress responses change in opposite directions while siderophore biosynthesis is downregulated in both species. Our results suggest that different species respond to GuaB inhibition using distinct regulatory programs and possibly explain the different bactericidal kinetics upon GuaB inhibition. The comparison highlights opportunities for developing GuaB inhibitors as novel antibiotics.IMPORTANCE<i>A. baumannii</i> is a priority bacterial pathogen for which development of new antibiotics is urgently needed due to the emergence of multidrug resistance. We recently developed a series of specific inhibitors against GuaB, a bacterial inosine 5'-monophosphate dehydrogenase, and achieved sub-micromolar minimum inhibitory concentrations against <i>A. baumannii</i>. GuaB catalyzes the rate-limiting step of <i>de novo</i> guanine biosynthesis and is highly conserved across bacterial pathogens. This study shows that inhibition of GuaB induced a bacterial morphological profile distinct from that of other classes of antibiotics, highlighting a novel mechanism of action. Moreover, our transcriptomic analysis showed that regulation of <i>de novo</i> purine biosynthesis and stress responses of <i>A. baumannii</i> upon GuaB inhibition differed significantly from that of <i>E. coli</i>.