0 and pH 5.5, which was also found in strain SA45. The same expression pattern has been found for the prophage-encoded Panton-Valentine leukocidin (PVL, luk-PV) of S. aureus . Maximal expression of luk-PV in the late exponential growth phase was followed by a rapid decline post-exponentially. Our observation could partially be explained by the induction of the prophage carrying the toxin gene. The sea-phage copy numbers of S. aureus Mu50 at pH 6.0 remained constant during the first part of cultivation. In the late stationary growth phase, however,
the number had increased four times (average GDC-0994 cost increase of two biological replicates) compared to levels in early stationary growth phase. The phage copy numbers might have increased further if growth was allowed to continue. An acetic-acid induced intracellular drop in pH, leading to oxidative stress  would activate the SOS response system inducing the prophage . Sumby and Waldor showed that upon prophage induction in S. aureus, the phage DNA was replicated, resulting in an increase in sea gene copy number, and that a second prophage-regulated sea promoter was also activated, resulting in increased sea expression . Similar enhanced transcription
of phage-encoded virulence genes upon prophage induction has also been observed for PVL in S. aureus and the Shiga toxins in E. coli [28, 31]. Mitomycin C, a well-known https://www.selleckchem.com/products/Adriamycin.html prophage inducer, was used in this study. The more MC added, the more SEA was produced per CFU for all three strains PU-H71 in vitro tested, supporting
the association between prophage induction and SEA production. However, the expected boost in extracellular SEA levels accompanying the increased sea mRNA levels and sea gene copy levels observed at pH 5.5 was not found. This could be because of the pronounced phage production at pH 5.5 seen as a rapid increase in extracellular sea-phage copy number (Figure 3). The window for phage-encoded SEA-biosynthesis prior to phage-release could be too narrow in the bacteria at this pH level. The relative phage copy number generally increased over time at all pH levels investigated. At pH 5.5, the relative phage copy number was increasing dramatically over time, suggesting that substantial prophage induction had occurred. The sea gene copy number, however, was decreasing over acetylcholine time at pH 5.5. This could be due to cell lysis occurring upon prophage induction at this pH. At pH 5.0 and 4.5, a big increase in relative sea gene copy number was observed between the two last sampling points. This suggests that the prophage has been induced and the replicative form of the phage DNA is produced. However, at these low pH values, no great increase in SEA or phage copies were observed, suggesting protein synthesis was impaired. In addition, the reason why the sea expression of S. aureus Mu50 at pH 5.5 was not as high as at pH 6.