Metabolism is essential for life. The metabolism of an organism defines its capabilities to take up nutrients from the environment and to convert these into its essential building blocks, such as nucleic acids and amino acids (Lazar and Birnbaum, 2012). Cellular metabolism can be described as a system of biochemical conversions (reactions), most of which are catalyzed by metabolic enzymes. Typically, a genome encodes a few thousand metabolic enzymes (Yilmaz and Walhout, 2017). Each enzyme acts on a selection of substrates and converts these into products, typically by adding or removing reactive groups. Reactions that share substrates or products can be considered functionally connected. Consequently, the collection of biochemical reactions within a cell forms a large interconnected network, representing the routes by which the organism converts simple nutrients into complex metabolites and vice-versa. This network is distributed over different subcellular compartments (organelles). Transporter proteins and channels facilitate the transport of metabolites across the lipid bilayers that surround the cell and the organelles (Sahoo et al., 2014). The overall system is subject to many parameters, such as variability in substrates, temperature, or the pH, not only between cells and the extracellular space but also within cells. Cells regulate this system to maintain homeostasis, i.e. the ability to perform important cellular functions despite variations (perturbations), and this provides robustness (Eberl, 2018; Nijhout et al., 2019). The ability to sense environmental variations and metabolic cues and to adapt metabolism accordingly, depends on a tightly interlinked regulatory system (Watson et al., 2015). This involves regulatory feedback loops embedded in interaction networks crossing metabolic, protein, transcript, and (epi)genetic levels (Figure 1). Pathogen G e n o m e T ra n s c ri p to m e P ro te o m e M e ta b o lo m e Host FIGURE 1 | The molecular layers of a cell are all interconnected, and form a complex and integrated system. In the symbiosis of a pathogen and a host, their systems are connected, and interactions occur between all molecular layers. As such, the phenotype of the cell is an emergent property of the system's complexity (Aderem, 2005). The rates of individual metabolic reactions are a direct consequence of the overall state General introduction