3.4. Fe2þ concentrationSince PT cau

3.4. Fe2þ concentrationSince PT causes host cell death and subsequent lysis, releasing Fe3þ-containing compounds, PT production could be seen as a means to acquire iron. Hence, PT expression might be modified by the levels of available Fe3þ. While total absence of iron would cause iron-limited growth, possibly a low Fe3þ concentration, corresponding with the situation in the host organism, could enhance toxin production. This hy-pothesis was tested in medium containing various amounts of iron, shown in Fig. 4.The medium that contained no added iron showed a much slower growth than the cultures that contained 10e200 mM added iron (Fig. 4a). The cultures with no added iron yielded a dry weight of 0.19 0.002 g/L, significantly less than the average dry weight of the 8 other cultures, which was 0.75 0.029 g/L (no data shown). The spent culture medium of the cultures without added iron showed that less than half of the added lactate was used while all glutamate was consumed (no data shown). Apparently Fe3þ present asacontaminant and from carry over (