## 3 Discussion
Thereby, many cellular processes that are directly or indirectly connected to the host cell metabolism are affected such as the proteasome, mitochondrial activity and intracellular signalling. A common host reaction to intracellular pathogens is the upregulation of HIF-1 [40] which leads to enhanced glucose uptake and glycolytic activity. Indeed, evidence for an increased glucose metabolism via glycolysis by the infected host were found in various studies with _Mycobacterium tuberculosis_[41; 42], _Listeria monocytogenes_[43] and _Chlamydia_ spp. [44]. In our study, however, we observed a reduced glucose uptake and lactate secretion within the first 12 h of infection, which does not indicate a special role of HIF-1 activation or upregulation in A549 cells during _S. aureus_ infection. We furthermore detected an increased amount of intracellular amino acids during the later time points after the infection in A549 cells. Since the uptake of amino acids was strongly reduced, enhanced nutrient consumption can be excluded. However, as an explanation cellular processes like autophagy or protein degradation seem appropriate. Proteasomal activity by the immunoproteasome upon infection stress can be induced by the cytokine IFN-\(\gamma\)[45]. Moreover, IFN-\(\gamma\) is released in response to _S. aureus_ and immunoproteasomal mRNAs were found expressed in infected A549 cells [46]. Therefore the combination of an early alpha-toxin activity and a late host-response might lead to time dependent changes in the intracellular amino acid profile. From the perspective of the pathogen, this would result in a beneficial situation due to the elevated amount of free amino acids. Indeed, the chemical environment of the mammalian host cytosol supports growth rather of adapted bacteria [22]. Various intracellular pathogens have mechanisms to elevate the intracellular amino acid content to their benefit, indicating that especially intracellular amino acids are not concentrated enough for bacterial replication. _Francisella tulareis_ induces macroautophagy in the host and _Legionella pneumophila_ promotes proteasomal degradation to elevate the intracellular amino acid amount [47; 48]. It is not clear, whether _S. aureus_ induces one of the above-mentioned processes for the same reason, but _S. aureus_ is known to induce autophagy in host cells [19; 35; 49]. In line with these findings we detected strongly reduced amounts of intracellular _myo_-inositol, which was demonstratedto induce autophagy in an mTOR-independent manner [50]. However the second metabolic marker for this alternative autophagy induction namely inositol-1,4,5-trisphosphate was detected only in trace amounts under both conditions at all times. The stringent response is a bacterial indicator for a non-sufficient amino acid supply in the current environment, by which the alarmone (p)ppGpp is synthesized and regulates more than 150 genes in _S. aureus_[51]. Three (p)ppGpp synthetases RSH, RelP and RelQ are present in _S. aureus_. RSH contains an amino acid sensing domain, whereas the latter two enzymes lack this domain and are most likely involved in the response to cell wall stress [52]. Recently, it was shown, that RSH is downregulated in intracellular _S. aureus_ cells but the other two (p)ppGpp synthetases are upregulated [46]. This is in line with our finding that the amount of amino acids is increasing in the host cytosol, whereas cell wall stress due to phagosomal activity is likely to occur. Our study revealed that, beside amino acids, the amounts of the nucleotides GTP and CTP are temporarily elevated in cells exposed to _S. aureus_. Recently, the CTP-synthease of T-cells was found to play an important role in proliferation after T-cell activation [53]. Moreover, in murine lung cells infected with _Mycobacterium tuberculosis_ nucleotide concentrations were elevated and also connected to active replication of the host cells [41]. To test the activity of nucleotide biosynthesis pathways, we used azaserine and DON as metabolic inhibitors.