Acid shock responses in Staphylococcus aureus

We present a general overview of the changes in the genetic expression along a time curve through the first 20 minutes after acidification to pH 4.5 of exponentially growing cultures of the food pathogenic strain Staphylococcus aureus 50583. A newly developed method for statistical significance testing was used to detect significant gene expression responses. Most responses showed an increase or decrease from time zero to 10 minutes after acidification and then generally a stabilisation in expression level from 10 to 20 min. Increased urease activity appeared to be an important factor in the acid defence along with proton excretion by NADH dehydrogenase and macromolecule repair mechanisms. Oxidative stress responses such as increased expression of thioredoxin genes and upregulation of pentose phosphate pathway genes to generate more reducing power were also induced. A general reduction in the expression of genes encoding ribosomal proteins and genes involved in nucleotide synthesis as well as fatty acid and lipoprotein metabolism reflected the lowered growth rate after acidification. The pH shock did not appear to trigger major virulence responses or biofilm formation. The metal ion regulation and transport was affected by the acid shock and production of several cofactors such as molybdopterin was increased. Many of the presented observations could be explained, while some represent still unknown mechanisms. The patterns of regulation were confirmed by real time reverse transcriptase PCR. Together these results showed the main responses of S. aureus and will be a good starting point for future more specific in-depth studies of specific gene responses in conjunction with acid stress defence of S. aureus. Keywords: Time course, stress response S. aureus was inoculated in TSB and and incubated at 200 rpm and 37 oC. At an OD600 of 1.0 corresponding to 108 CFUml-1 the control samples were withdrawn immediately prior to the addition of HCl to directly bring the pH down to 4.5. Samples were withdrawn 2, 5, 10 and 20 min after addition of acid. All samples were directly transferred to RNA Protect Bacteria reagent (Qiagen). This procedure was repeated at three different days for a total of three sample sets (parallels). Separate array experiments were performed on all three parallels.