PhD ceremony Mr. P. Puri: On the regulation of the metabolic shift and protein synthesis in Lactococcus lactis

PhD ceremony: Mr. P. Puri

Dissertation: On the regulation of the metabolic shift and protein synthesis in Lactococcus lactis

Promotor(s): prof. B. Poolman

Faculty: Mathematics and Natural Sciences

The research work described in this thesis is aimed at understanding the regulation of metabolic shift in the lactic acid bacterium Lactococcus lactis. We choose to study this bacterium as it is important for the dairy industry.

Metabolic shift refers to a switch from one mode of cellular metabolism to another, which occurs in response to a change in growth conditions. Depending on the amount of available sugar, L. lactis can grow at different speed or growth rate. At low growth rates L. lactis produces mainly formate, acetate and ethanol (mixed-acid fermentation), while at high growth rates the major metabolic end-product is lactate (homolactic fermentation).

In order to make these fermentation products it requires different set of enzymes and a pool of ribosomes to synthesize these enzymes. The hypothesis that we tested in this thesis states that depending on the proteins involved in the different metabolic pathway branches, investment of proteins (enzymes) varies with substrate availability and consequently growth rate, which alters the metabolic profile of the microorganism.

We performed detailed experiments to find out the reason behind this metabolic shift. Our analysis showed that the bacterium keeps a large capacity of both enzymes and ribosomes required to generate the fermentation products, to allow a rapid response when nutritional conditions change. Instead of changing the levels of different enzymes and ribosomes, L. lactis uses various small molecules to control their activity. Our results indicate that this bacterium is always ready for fast growth.