The antimicrobial activity of the new dithiolopyrrolone antibiotics (PR2, PR8, PR9 and PR10) is shown in Table 1. The antibiotic PR8 showed higher activity than other compounds against Gram-positive bacteria. The antibiotics PR2 and PR9 were not active against Aspergillus carbonarius and the phytopathogenic fungi Fusarium oxysporum f. sp. lini, Fusarium graminearum and Fusarium moniliforme. However, the antibiotics PR8 and PR10 showed a moderate activity against all fungi and yeasts tested. None of the new induced antibiotics showed activity against Gram-negative bacteria. Dithiolopyrrolones are known to be produced by several species of Streptomyces, Xenorhabdus

and Alteromonas. The actinomycete S. algeriensis produces five dithiolopyrrolones in the basic medium (without precursors): thiolutin, iso-butyryl-pyrrothine, butanoyl-pyrrothine, senecioyl-pyrrothine and tigloyl-pyrrothine Rapamycin supplier (Lamari et al., 2002b). This actinomycete has a great ability to produce a wide range of dithiolopyrrolone derivatives that, depending on the composition

of the Nutlin-3a supplier culture medium, nature and concentration of precursors added and an enzymatic system, are involved in attaching a variety of radicals (R) into pyrrothine ring (Bouras et al., 2006a, b, 2007, 2008; Chorin et al., 2009). The data presented above show that the addition of sorbic acid at a concentration of 5 mM to the SSM as a precursor has induced the production of four new peaks, as revealed by HPLC analysis. These induced compounds did not correspond to known dithiolopyrrolones with respect to retention time, but they were identified as dithiolopyrrolone derivatives by their spectral characteristics (UV spectra, EIMS and NMR). From MS and 1H- and 13C-NMR spectroscopic analyses, as well as by comparison with all dithiolopyrrolone derivatives reported in the literature, the structures of the four new dithiolopyrrolones (PR2, PR8, PR9 and PR10) were characterized as N-acyl derivatives of 6-amino-4,5-dihydro-4-methyl-5-oxo-1,2-dithiolo[4,3-b]pyrrole.

The four compounds Bay 11-7085 showed a prominent fragment ion of m/z 186 and indicated by the EIMS spectrum an extra methyl group in the heterocyclic ring (corresponding to the empirical formula C6H6N2OS2) as reported for other dithiolopyrrolones (McInerney et al., 1991; Lamari et al., 2002b). On the basis of NMR and MS data, the molecular formula of PR2 was determined as C10H10N2O2S2 (Fig. 3). The antibiotic PR8 was determined as C12H12N2O2S2, suggesting an intact direct incorporation of the sorbic acid into pyrrothine ring. The results of Bouras et al. (2008) showed that addition of precursors into the culture medium, such as organic acids, led to precursor-directed biosynthesis of new dithiolopyrrolone analogues. In the same context, Chorin et al. (2009) suggest that the enzymatic reaction of pyrrothine acylation takes part in the dithiolopyrrolone biosynthetic pathway in S.