Abstract
Staphylococcus aureus is a worldwide pathogen that colonizes the human nasal cavity and is a major cause of respiratory and cutaneous infections. In the nasal cavity, S. aureus thrives with high concentrations of nitric oxide (NO) produced by the innate immune effectors and has available for growth slow-metabolizing free hexoses, such as galactose. Here, we have used deep sequencing transcriptomic analysis (RNA-Seq) and 1H-NMR to uncover how S. aureus grown on galactose, a major carbon source present in the nasopharynx, survives the deleterious action of NO. We observed that, like on glucose, S. aureus withstands high concentrations of NO when using galactose. Data indicate that this resistance is, most likely, achieved through a distinct metabolism that relies on the increased production of amino acids, such as glutamate, threonine, and branched-chain amino acids (BCAAs). Moreover, we found that under NO stress the S. aureus α-acetolactate synthase (ALS) enzyme, which converts pyruvate into α-acetolactate, plays an important role. ALS is proposed to prevent intracellular acidification, to promote the production of BCAAs and the activation of the TCA cycle. Additionally, ALS is shown to contribute to the successful infection of murine macrophages. Furthermore, ALS contributes to the resistance of S. aureus to beta-lactam antibiotics such as methicillin and oxacillin.
Highlights
Staphylococcus aureus is an opportunistic human pathogen that is persistently found in the epithelial cells of the nasal cavity of patients suffering recurrent sinusitis (Weidenmaier et al, 2012)
These results show that the resistance of S. aureus to high concentrations of nitric oxide (NO) is similar for cells grown on galactose or on glucose
To elucidate how S. aureus sustains the nitrosative stress, the metabolic state of S. aureus grown on galactose and exposed to NO was studied by 1H-nuclear magnetic resonance (NMR) quantification of the glycolytic carbon source consumption, the citrate uptake from the growth medium to feed the tricarboxylic acid cycle (TCA) cycle, and the excreted end products, namely those formed downstream of the pyruvate node (Figure 1)
Summary
Staphylococcus aureus is an opportunistic human pathogen that is persistently found in the epithelial cells of the nasal cavity of patients suffering recurrent sinusitis (Weidenmaier et al, 2012). Even in the absence of symptoms, the staphylococcal nasal carriage is a known risk factor as S. aureus exhibits high capacity to evade and thwart the innate and adaptive immune responses. As for other pathogens, the synthesis and acquisition of certain nutrients are critical to support the infection capacity of S. aureus, glucose being one of the preferred carbon sources of this bacterium. In the nasal cavity the glucose levels have been proposed to be substantially lower than those determined in human plasma, and in this niche, disaccharides and other free hexoses are accessible (Krismer et al, 2014). Galactose is one of the major carbohydrates present in the mucus mucins that coat the nasopharyngeal epithelium of the nasal cavity, and studies done with the nasal colonizer S. pneumoniae showed that: (i) galactose is released by hydrolysis of mucins promoted by bacterial exoglycosidases; (ii) free galactose is abundantly present in the nasal lavage fluid, whether healthy individuals carry or not the pneumococcus; and (iii) glucose was not detected in S. pneumoniae carriers and non-carriers (King, 2010; Blanchette et al, 2016)
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