Viability staining and terminal deoxyribonucleotide transferase-mediated dUTP nick end labelling of the mycelium in submerged cultures of Streptomyces antibioticus ETH7451

Recent advances in recombinant protein expression by Corynebacterium, Brevibacterium, and Streptomyces: from transcription and translation regulation to secretion pathway selection

Gram-positive bacteria are widely used to produce recombinant proteins, amino acids, organic acids, higher alcohols, and polymers. Many proteins have been expressed in Gram-positive hosts such as Corynebacterium, Brevibacterium, and Streptomyces. The favorable and advantageous characteristics (e.g., high secretion capacity and efficient production of metabolic products) of these species have increased the biotechnological applications of bacteria. However, owing to multiplicity from genes encoding the proteins and expression hosts, the expression of recombinant proteins is limited in Gram-positive bacteria. Because there is a very recent review about protein expression in Bacillus subtilis, here we summarize recent strategies for efficient expression of recombinant proteins in the other three typical Gram-positive bacteria (Corynebacterium, Brevibacterium, and Streptomyces) and discuss future prospects. We hope that this review will contribute to the development of recombinant protein expression in Corynebacterium, Brevibacterium, and Streptomyces.

Distribution of live and dead cells in pellets of an actinomycete Amycolatopsis balhimycina and its correlation with balhimycin productivity

Secondary metabolites such as antibiotics are typically produced by actinomycetes as a response to growth limiting stress conditions. Several studies have shown that secondary metabolite production is correlated with changes observed in actinomycete pellet morphology. Therefore, we investigated the correlation between the production of balhimycin and the spatio-temporal distribution of live and dead cells in pellets of Amycolatopsis balhimycina in submerged cultures. To this end, we used laser scanning confocal microscopy to analyze pellets from balhimycin producing and nonproducing media containing 0.2 and 1.0 g l−1 of potassium di-hydrogen phosphate, respectively. We observed a substantially higher fraction of live cells in pellets from cultures yielding larger amounts of balhimycin. Moreover, in media that resulted in no balhimycin production, the pellets exhibit an initial death phase which commences from the centre of the pellet and extends in the radial direction. A second growth phase was observed in these pellets, where live mycelia are seen to appear in the dead core of the pellets. This secondary growth was absent in pellets from media producing higher amounts of balhimycin. These results suggest that distribution of live and dead cells and its correlation with antibiotic production in the non-sporulating A. balhimycina differs markedly than that observed in Streptomycetes.

A two-step mechanism for the activation of actinorhodin export and resistance in Streptomyces coelicolor

Many microorganisms produce secondary metabolites that have antibiotic activity. To avoid self-inhibition, the producing cells often encode cognate export and/or resistance mechanisms in the biosynthetic gene clusters for these molecules. Actinorhodin is a blue-pigmented antibiotic produced by Streptomyces coelicolor. The actAB operon, carried in the actinorhodin biosynthetic gene cluster, encodes two putative export pumps and is regulated by the transcriptional repressor protein ActR. In this work, we show that normal actinorhodin yields require actAB expression. Consistent with previous in vitro work, we show that both actinorhodin and its 3-ring biosynthetic intermediates [e.g., (S)-DNPA] can relieve repression of actAB by ActR in vivo. Importantly, an ActR mutant that interacts productively with (S)-DNPA but not with actinorhodin responds to the actinorhodin biosynthetic pathway with the induction of actAB and normal yields of actinorhodin. This suggests that the intermediates are sufficient to trigger the export genes in actinorhodin-producing cells. We further show that actinorhodin-producing cells can induce actAB expression in nonproducing cells; however, in this case actinorhodin is the most important signal. Finally, while the "intermediate-only" ActR mutant permits sufficient actAB expression for normal actinorhodin yields, this expression is short-lived. Sustained culture-wide expression requires a subsequent actinorhodin-mediated signaling step, and the defect in this response causes widespread cell death. These results are consistent with a two-step model for actinorhodin export and resistance where intermediates trigger initial expression for export from producing cells and actinorhodin then triggers sustained export gene expression that confers culture-wide resistance. IMPORTANCE Understanding the links between antibiotic resistance and biosynthesis is important for our efforts to manipulate secondary metabolism. For example, many secondary metabolites are produced at low levels; our work suggests that manipulating export might be one way to enhance yields of these molecules. It also suggests that understanding resistance will be relevant to the generation of novel secondary metabolites through the creation of synthetic secondary metabolic gene clusters. Finally, these cognate resistance mechanisms are related to mechanisms that arise in pathogenic bacteria, and understanding them is relevant to our ability to control microbial infections clinically.

Mycelium differentiation and antibiotic production in submerged cultures of Streptomyces coelicolor

Despite the fact that most industrial processes for secondary metabolite production are performed with submerged cultures, a reliable developmental model for Streptomyces under these culture conditions is lacking. With the exception of a few species which sporulate under these conditions, it is assumed that no morphological differentiation processes take place. In this work, we describe new developmental features of Streptomyces coelicolor A3(2) grown in liquid cultures and integrate them into a developmental model analogous to the one previously described for surface cultures. Spores germinate as a compartmentalized mycelium (first mycelium). These young compartmentalized hyphae start to form pellets which grow in a radial pattern. Death processes take place in the center of the pellets, followed by growth arrest. A new multinucleated mycelium with sporadic septa (second mycelium) develops inside the pellets and along the periphery, giving rise to a second growth phase. Undecylprodigiosin and actinorhodin antibiotics are produced by this second mycelium but not by the first one. Cell density dictates how the culture will behave in terms of differentiation processes and antibiotic production. When diluted inocula are used, the growth arrest phase, emergence of a second mycelium, and antibiotic production are delayed. Moreover, pellets are less abundant and have larger diameters than in dense cultures. This work is the first to report on the relationship between differentiation processes and secondary metabolite production in submerged Streptomyces cultures.

Viability analysis of alginate encapsulated micro-organisms using fluorescent stains

The encapsulation of micro-organisms such as bacteria and fungi in biopolymers is currently being evaluated as delivery systems in many fields. Information about the viability and morphology of the organisms in the microparticle is often required to ascertain the longevity of the systems. A rapid method using fluorescent stains for microbial viability has been validated for organisms within alginate microparticles. Usually viability is assessed by dissolving the microparticles and cell culturing. This new method is advantageous for slow growing or filamentous organisms because these are not quickly or accurately enumerated by plate counts. In addition, the technique also allows the morphology of the organism to be monitored over time.

A death round affecting a young compartmentalized mycelium precedes aerial mycelium dismantling in confluent surface cultures of Streptomyces antibioticus

Development-associated cell-death processes were investigated in detail during the growth and differentiation of Streptomyces antibioticus ATCC 11891 on confluent surface cultures, by using fluorescent viability probes, membrane and activity fluorescence indicators, and electron microscopy analysis. A previously unsuspected complexity was revealed, namely the presence of a very young compartmentalized mycelium that dies following an orderly pattern, leaving alternating live and dead segments in the same hypha. This death round is followed by the growth of a second mycelium which develops rapidly from the live segments of the first mycelium and dies massively in a second death round, which extends over the phases of aerial mycelium formation and sporulation.

Mycelium development in Streptomyces antibioticus ATCC11891 occurs in an orderly pattern which determines multiphase growth curves

Background

The current model for the developmental cycle of Streptomyces confluent cultures on agar surface is based on the assumption that the only differentiation takes place along the transverse axis (bottom-up): a vegetative (substrate) mycelium grows completely live and viable on the surface and inside the agar until it undergoes a death process and differentiates to a reproductive (aerial) mycelium which grows into the air. Hence, this vertical description assumes that the development in the pre-sporulating phases is more or less homogeneous in all zones of the plate surface.

Results

The work presents a detailed analysis of the differentiation cycle in Streptomyces antibioticus ATCC11891 considering a different spatial dimension: the longitudinal axes, represented by the plate surface. A previously unsuspected complexity during the substrate mycelial phase was detected. We have demonstrated that the young substrate hyphae suffer an early death round that has not been previously described. Subsequently, the remaining mycelium grows in successive waves which vary according to the density of the spore inoculum. In the presence of dense inocula (1.5 × 10⁶ spores per plate), the hyphae develop in regular circles, approximately 0.5 cm in diameter. By contrast, with highly diluted inocula (6 × 10³ spores per plate), aerial mycelium develops initially in the form of islands measuring 0.9 mm in diameter. Further mycelial development occurs between the circles or islands until the plate surface is totally covered. This pattern persists throughout the entire developmental cycle including the sporulation phases.

Conclusion

An early death round during the substrate mycelial phase of Streptomyces antibioticus ATCC11891 takes place prior to successive growth periods in surface cultures. These developmental periods in turn, determine the shape of the complex multiphase growth curves observed. As shown here, these results also apply to other Streptomyces strains and species. Understanding these peculiarities of the Streptomyces developmental cycle is essential in order to properly interpret the morphological/biochemical data obtained from solid cultures and will expand the number of potential phenotypes subject to study.

Cytological and biochemical evidence for an early cell dismantling event in surface cultures of Streptomyces antibioticus

A process of programmed cell death taking place late in the aerial mycelium was previously reported in surface cultures of Streptomyces antibioticus ATCC11891. In this study, we present evidence for the occurrence of a similar process taking place early in the vegetative mycelium of surface cultures of the same strain. Several indicators, such as cell wall and membrane disruption, DNA degradation and release of the cytoplasmic content into the exocellular medium, support the existence of active, highly regulated cell suicide involving specific enzymes. Calcium-dependent proteolytic activation of a precursor of nucleases and the nucleolytic formation of a ladder of chromosomal bands are conspicuous events associated with the initiation of the death process.

Cloning and characterization of a Streptomyces antibioticus ATCC11891 cyclophilin related to Gram negative bacteria cyclophilins

Cyclophilins are folding helper enzymes and represent a family of the enzyme class of peptidyl-prolyl cis-trans isomerases. Here, we report the molecular cloning and biochemical characterization of SanCyp18, an 18-kDa cyclophilin from Streptomyces antibioticus ATCC11891 located in the cytoplasm and constitutively expressed during development. Amino acid sequence analysis revealed a much higher homology to cyclophilins from Gram negative bacteria than to known cyclophilins from Streptomyces or other Gram positive bacteria. SanCyp18 is inhibited weakly by CsA, with a K(i) value of 21 microM, similar to cyclophilins from Gram negative bacteria. However, this value is more than 20-fold higher than the K(i) values reported for cyclophilins from other Gram positive bacteria, which makes SanCyp18 unique within this group. The presence of SanCyp18 in Streptomyces is likely due to horizontal gene transmission from Gram-negative bacteria to Streptomyces.

Nuclease activities and cell death processes associated with the development of surface cultures of Streptomyces antibioticus ETH 7451

The presence and significance of developmentally regulated nucleases in Streptomyces antibioticus ETH 7451 has been studied in relation to the lytic processes occurring during differentiation. The cell-death processes have been followed in surface cultures by a propidium iodide viability assay. This has allowed the visualization of dead (membrane-damaged, red fluorescent) and live (membrane-intact, green fluorescent) mycelium during development, and has facilitated the analysis of the role of nucleases in these processes. A parallel activity-gel analysis showed the appearance of 20-22 kDa, 34 kDa and 44 kDa nucleases, the latter appearing only when aerial mycelium is formed. The appearance of these nucleases shows a remarkable correlation with the death process of the mycelium during differentiation and with chromosomal DNA degradation. The 20-22 kDa enzymes are possibly related to the lytic phenomena taking place in the vegetative substrate mycelium before the emergence of the reproductive aerial mycelium, whereas the function of the 44 kDa nuclease seems to be related to the sporulation step. The 20-22 kDa nucleases require Ca2+ for activity and are inhibited by Zn2+. The nucleases are loosely bound to the cell wall from where they can be liberated by simple washing. Conceivably, these enzymes work together and co-ordinate to achieve an efficient hydrolysis of DNA from dying cells. The results show that the biochemical reactions related with the lytic DNA degradation during the programmed cell death are notably conserved in Streptomyces. Some of the features of the process and the biochemical characteristics of the enzymes involved are analogous to those taking place during the DNA fragmentation processes in eukaryotic apoptotic cells.