Surface-enhanced Raman spectroscopy and ultrastructural analysis of penicillin-producing Penicillium rubens strains

Raman spectroscopy, transmission electron microscopy (TEM) and atomic force microscopy (AFM) techniques can perform chemical analyses and acquire high-resolution images of cell samples. For this reason, in this study, semi-thin sections of a single Penicillium rubens cell were analysed by Raman enhanced surface spectroscopy. The spectra showed peaks corresponding to the macromolecules that make up the cellular components. In addition, the various organelles were analysed by TEM and AFM to observe the cellular nanostructures. With the use of these techniques, it is possible to identify molecules in semi-thin sections, which provides a wide potential for biomedical applications and for the analysis of cell dynamics. The observation of the most detailed possible structure of cells is used as a starting point in numerous studies to identify and localise some biochemical processes. Given that the function of eukaryotic cells depends on the location, shape, structure and function of the subcellular organelles (and on the interaction between them), the sum of the data obtained allows a complete analysis of what happens in the cell. This article addresses, from a multidisciplinary point of view, what happens in a single cell of a filamentous fungus (Penicillium rubens) while it is in a physiological moment (secondary metabolism) that allows the biosynthesis of an antibiotic (penicillin). For this purpose, different types of microscopies were used (TEM: transmission electron microscopy, and AFM: atomic force microscopy, which allow visualising small details in the cell) and a spectroscopy method (Raman, which allows detecting certain characteristics of the macromolecules and some stretching bonds). Regarding the results, during the synthesis of penicillin, the antibiotic-producing Penicillium rubens cells showed significant changes compared to the non-producing cells: the cell wall is observed to be significantly thickened in the production phase, organelles such as peroxisomes grow in number and size since it is known that the final route of metabolite synthesis takes place in them. When penicillin is released from peroxisomes, they must be degraded to release the load from the cell; this is done by vacuoles, which are active and engulf peroxisomes. The newly synthesised penicillin is found within secretory vesicles that travel towards the cell membrane and both membranes fuse creating ripples. On the other hand, and given that a single cell is being studied, it is essential to increase the signal to detect biomolecules employing the Raman-SERS technique, using a silver substrate to obtain the increased signal.