Natural Saccharomyces cerevisiae cells have long been utilized as very efficient biocatalysts, thanks to their native enzymatic capabilities. Ethanol, single cell proteins, flavours and fragrances are among the most traditional examples.
Since about three decades budding yeast can also be engineered and has been used to efficiently produce simple as well as complex molecules. Prominent examples are proteins with pharmaceutical applications, industrial enzymes, organic acids, new bio-fuels, biopolymers, vitamins and steroids, in a single fermentation step [1–8].
Glucose, either derived from starch and/or cellulosic materials, is the main carbon and energy source today available.
An economically sustainable bioprocess leading to the production of a homologous or heterologous low molecular compound requires a high yield (grams of product obtained by gram of substrate), high production titer (g/L) and high productivity (g/L/h) values.
It has been shown that high yields and high production titres can be obtained by recombinant redirection of the carbon flow towards the desired compound. In this respect, prominent examples are the production of lactic-, pyruvic- and malic- acid, glycerol and resveratrol [9–13]. Theoretically, high productivities could be obtained by increasing the carbon consumption rates itself (i.e., essentially the glycolytic flux rate). It should be also underlined that an increased productivity, and therefore a reduction of the process duration, is not only implying a reduction in terms of costs. Cell factory viability (and therefore production) has also to be taken into account: very often the production process involves a stressful environment, leading to cell death during fermentation, as in the case of ethanol production, gradually reducing cell viability and thereby biocatalyst concentration .
Glucose transport, hexose phosphorylation, phosphofructokinase and pyruvate kinase activities have all been proposed to play central roles in the control of glycolysis flux rates [15–20].
Individual or simultaneous overproduction of glycolytic enzymes resulted either in no increases in glycolytic flux or in only incremental increases [21–25]. Furthermore, attempts to correlate glycolytic flux with enzyme levels under different physiological conditions have generally failed [26–28]. This is likely because the control of glucose catabolism is distributed over several different metabolic controls; in this context, glucose transport has been suggested to be one of the most important players [29, 30].
Glucose transport in S. cerevisiae relies on a multi-factorial uptake system. More in details, the uptake of glucose by S. cerevisiae is controlled by multiple hexose transporters (Hxts) . At least 20 HXT genes encoding these transporters have been identified [31, 32]. Many and different studies were done to determine the respective biochemical features of these transporters (affinity and capacity), as well as to construct strains deleted in one or more HXT genes and to construct chimera proteins combining affinity and capacity of different transporters [33–35]. Remarkably, Otterstedt et al.  showed that a simple manipulation of the glucose uptake can strongly alter the mode of metabolic control.
Essentially, the various hexose transporters differ considerably in substrate specificity and affinity. Hxt1 and Hxt3 are low-affinity transporters (Km for glucose, ~50 to 100 mM), Hxt4 is a moderately low-affinity transporter, and Hxt2, Hxt6, and Hxt7 are high-affinity transporters (Km for glucose, ~1 to 4 mM) [36, 37]. Hxt5 has been shown to be a transporter with intermediate to high affinity [38, 39]. Both high- and low-affinity carriers have been shown to have a higher affinity for glucose than for fructose . Analyses of the effect of HXT gene inactivation have shown that the hexose carriers Hxt1 to Hxt7 are the main transporters . In this respect, it has been already shown that the ethanol (and CO2) productivity and yield (grams of ethanol produced per gram of glucose consumed) can be improved by overexpression of HXT 1 transporter in S. cerevisiae [40–43].
S. cerevisiae has long been studied for the production of organic acids like lactic, ascorbic, pyruvic and malic [[4, 8, 11] and ]. Indeed, yeast can grow and survive at low pH values, avoiding the accumulation of the respective salts [4, 11].
Here we show improved lactic acid productivities, induced by an increase of the glucose consumption rate. Hxt1 and Hxt7 have been selected for this study. In spite of their different biochemical properties, the overexpression of HXT 1 or HXT 7 genes does lead to very similar results in the tested conditions. Moreover, we demonstrate that the increase of the glucose consumption rate has a positive effect not only in respect to microbial productivity and metabolite production, but also on biomass accumulation. Said phenomenon is more or less evident in respect to the yeast background.