The impact of species, respiration type, growth phase and genetic inventory on absolute metal content of intact bacterial cells

Abstract

Metal ions are abundant in microbial proteins and have structural, catalytic or electron-transferring roles. Metalloproteins are especially prevalent in respiratory chains where they couple electron flow with proton translocation across the membrane. Here, we explore the hypothesis that anaerobic respiratory chains can be investigated by quantitative whole-cell metallomics of the key metals Fe, Co, Ni and Mo. Sensitive and strictly quantitative data were obtained by inductively-coupled plasma mass spectrometry when using a triple quadrupole instrument (ICP-QqQ-MS). Our experiments provide data on the absolute cellular metal content of E. coli, an enrichment culture of “Ca. Kuenenia stuttgartiensis”, Dehalococcoides mccartyi, Desulfovibrio vulgaris, Geobacter sulfurreducens and Geobacter metallireducens. A major obstacle in whole-cell metallomics is the interference caused by metal precipitates, observed for G. metallireducens and D. vulgaris. In the other investigated organisms, whole-cell metallomics gave biologically meaningful information, e.g. high Fe and Co content in “Ca. K. stuttgartiensis” and higher Mo content in E. coli when grown under nitrate-reducing conditions. The content of all four metals was almost constant in E. coli from the late exponential phase allowing precise measurements independent of the exact duration of cultivation. Deletion or overexpression of genes involved in metal homeostasis (Ni transport or Mo-cofactor metabolism) was mirrored by dramatic changes in whole-cell metal content. Deletion of genes encoding abundant metalloproteins or heterologous overexpression of metalloproteins was also reflected in the whole-cell metal content. Our study provides a reference point for absolute microbial metallomics and paves the way for the development of fast and easy mutation screens.