Methionine Limitation Impairs Pathogen Expansion and Biofilm Formation Capacity

Multidrug-resistant bacterial pathogens are becoming increasingly prevalent, and novel strategies to treat bacterial infections caused by these organisms are desperately needed. Bacterial central metabolism is crucial for catabolic processes and provides precursors for anabolic pathways, such as the biosynthesis of essential biomolecules like amino acids or vitamins. However, most essential pathways are not regarded as good targets for antibiotic therapy since their products might be acquired from the environment. This issue raises doubts about the essentiality of such targets during infection. A putative target in bacterial anabolism is the methionine biosynthesis pathway. In contrast to humans, almost all bacteria carry methionine biosynthesis pathways which have often been suggested as putative targets for novel anti-infectives. While the growth of methionine auxotrophic strains can be stimulated by exogenous methionine, the extracellular concentrations required by most bacterial species are unknown. Furthermore, several phenotypic characteristics of methionine auxotrophs are only partly reversed by exogenous methionine. We investigated methionine auxotrophic mutants of Staphylococcus aureus, Pseudomonas aeruginosa, and Escherichia coli (all differing in methionine biosynthesis enzymes) and found that each needed concentrations of exogenous methionine far exceeding that reported for human serum (∼30 µM). Accordingly, these methionine auxotrophs showed a reduced ability to proliferate in human serum. Additionally, S. aureus and P. aeruginosa methionine auxotrophs were significantly impaired in their ability to form and maintain biofilms. Altogether, our data show intrinsic defects of methionine auxotrophs. This result suggests that the pathway should be considered for further studies validating the therapeutic potential of inhibitors.IMPORTANCE New antibiotics that attack novel targets are needed to circumvent widespread resistance to conventional drugs. Bacterial anabolic pathways, such as the enzymes for biosynthesis of the essential amino acid methionine, have been proposed as potential targets. However, the eligibility of enzymes in these pathways as drug targets is unclear because metabolites might be acquired from the environment to overcome inhibition. We investigated the nutritional needs of methionine auxotrophs of the pathogens Staphylococcus aureus, Pseudomonas aeruginosa, and Escherichia coli We found that each auxotrophic strain retained a growth disadvantage at methionine concentrations mimicking those available in vivo and showed that biofilm biomass was strongly influenced by endogenous methionine biosynthesis. Our experiments suggest that inhibition of the methionine biosynthesis pathway has deleterious effects even in the presence of external methionine. Therefore, additional efforts to validate the effects of methionine biosynthesis inhibitors in vivo are warranted.