Plasticity of growth laws tunes resource allocation strategies in bacteria.

Bacteria grow at very different rates on different substrates. Therefore, the substrates themselves are often denoted as 'rich' substrates versus 'poor' substrates, referring to their nutrient quality. Nutrient quality is also a key parameter in bacterial growth laws that determines substrate-specific growth rate. However, it remains unclear which properties make a specific carbon source a 'good' substrate or a bad 'substrate' and a host of different explanations have been suggested, such as the energy content of the nutrient, the protein investment required for efficient catabolism of the substrate, or limitations in membrane capacity for transporters of the substrate, all of which are actively debated. Here, we show instead that nutrient quality can be a plastic property that can be dialed by evolution or by genetic perturbations. While we report a correlation between nutrient quality and protein cost of substrate utilization, this correlation is not causal. Instead, nutrient quality is encoded by the proteome fraction of a core catabolic sector within a much larger co-regulated catabolic sector. Different nutrients thus enable bacteria to grow, but they also serve as a major signal that allows microbes to infer information about their environment. We propose that nutrient quality encoded in a combination regulatory architecture and enzymatic properties, serves as both as a map of the safety and reliability of the environment and as a regulatory mechanism implementing proteome allocation decisions.