Assessing the potential of an induced-mutation strategy for avermectin overproducers

Screening of a high-yield strain of avermectin B1a by colony analysis in situ

Avermectin, an agricultural antibiotic, is widely used as an agricultural insecticide and an important lead compound of antibiotics. It is manufactured by Streptomyces avermitilis through fermentation. Manufacturers pay special attention to screening for strains with high fermentation capacity based on morphological properties of the colony and by the result of shake flask fermentation. These traditional screening methods are time-consuming and labor-intensive and require specialized equipment. Moreover, evaluation of colony appearance is highly subjective. To improve and accelerate the screening process, we developed a rapid in situ screening method. Forty-four strains isolated naturally from the spores of industrial high-yielding strains were studied. The data show that the colony fermentation titer is highly correlated with the yield from the shake flask fermentation of avermectin, and the Pearson's R is 0.990. The total titer of avermectins by shake flask fermentation is also highly correlated with the B_1a titer (Pearson's R is 0.994). This result also shows that strains can be quickly screened by analyzing the colony titer. Pigment rings of the colonies that appeared after growing and maturing on the new medium plate were analyzed. The chosen colonies were directly marked and punched and then extracted with methanol. The fermentation ability can be evaluated by measuring the absorbance at 245 nm. This methodology can be applied in both natural breeding and mutation breeding conditions. By continuously breeding from 2008 to 2020, the flask titer of avermectin B_1a increased from 4582 ± 483 to 9197 ± 1134 μg/mL.

Recent advances in the research of milbemycin biosynthesis and regulation as well as strategies for strain improvement

Milbemycins, a group of 16-membered macrocylic lactones with excellent acaricidal, insecticidal and anthelmintic activities, can be produced by several Streptomyces species. For the reason that they have low toxicity in mammals, milbemycins and their derivatives are widely used in agricultural, medical and veterinary industries. Streptomyces bingchenggensis, one of milbemycin-producing strains, has been sequenced and intensively investigated in the past decades. In this mini-review, we comprehensively revisit the progress that has been made in research efforts to elucidate the biosynthetic pathways and regulatory networks for the cellular production of milbemycins. The advances in the development of production strains for milbemycin and its derivatives are discussed along the strain-generation technical approaches of random mutagenesis, metabolic engineering and combinatorial biosynthesis. The research progress made so far indicates that strain improvement and generation of novel milbemycin derivatives will greatly benefit from future development of enabling technologies and deeper understanding of the fundamentals of biosynthesis of milbemycin and the regulation of its production in S. bingchenggensis. This mini-review also proposes that the overproduction of milbemycins could be greatly enhanced by genome minimization, systematical metabolic engineering and synthetic biology approaches in the future.

Implication of mutagenesis and precursor supplementation towards the enhancement of lipstatin (an antiobesity agent) biosynthesis through submerged fermentation using Streptomyces toxytricini

In the present study, lipstatin production was studied from different mutants of Streptomyces toxytricini which were developed using ultraviolet radiation (exposure time 30 s, 1, 2, 5 and 10 min), ethyl methane sulfonte, methyl methane sulfonate (MMS) and N-methyl-N'-intro-N-nitrosoguanidine (NTG) treatments (50, 100, 200, 500, 1000 µM, respectively). Highest yielding mutants were provided precursor supplementation of citric acid, thiamine and biotin (each 1 g/L) at idiophase for further enhancement in the production of lipstatin. Screened mutants produced biomass in the range of 5.8-7.16 g/L which were lesser than control. Screened mutants also exhibited pellet morphology in submerged culture. Out of these mutants, NTG8 mutant produced highest amount of lipstatin (1383.25 mg/L) with 9.606 mg/L/h productivity. Precursor supplementation to this mutant further increased the production to 2387.81 mg/L. Mutant was validated in 5 L bioreactor and lipstatin production was enhanced to 2519.34 mg/L.

A plate method for rapid screening of Ketogulonicigenium vulgare mutants for enhanced 2-keto-l-gulonic acid production

A new plate method was developed for rapid screening of Ketogulonicigenium vulgare mutants overproducing 2-keto-l-gulonic acid (2-KLG). The screening methodology took the advantage of the acidity caused by 2-KLG, which changes the color of bromothymol blue (pH indicator) from blue to yellow. Using the proposed method, a mutant, K. vulgare 65, was selected from 20,000 colonies produced by a strain subjected to spaceflight mutagenesis. When co-cultured with Bacillus megaterium 2980 in 20-L fermenters, K. vulgare 65 showed a high conversion rate (94.45%) of l-sorbose to 2-KLG. In contrast to the traditional screening method, this one significantly improved the frequency of obtaining positive mutants. The proposed plate screening method is cost-effective and easy to run and is thus useful for the isolation and screening of K. vulgare mutants overproducing 2-KLG.

Interrogation of Streptomyces avermitilis for efficient production of avermectins

The 2015 Nobel Prize in Physiology or Medicine has been awarded to avermectins and artemisinin, respectively. Avermectins produced by Streptomyces avermitilis are excellent anthelmintic and potential antibiotic agents. Because wild-type strains only produce low levels of avermectins, much research effort has focused on improvements in avermectin production to meet the ever increasing demand for such compounds. This review describes the strategies that have been widely employed and the future prospects of synthetic biology applications in avermectin yield improvement. With the help of genome sequencing of S. avermitilis and an understanding of the avermectin biosynthetic/regulatory pathways, synthetic and systems biotechnology approaches have been applied for precision engineering. We focus on the design and synthesis of biological chassis, parts, devices, and modules from diverse microbes to reconstruct and optimize their dynamic processes, as well as predict favorable effective overproduction of avermectins by a 4Ms strategy (Mine, Model, Manipulation, and Measurement).

Synthetic biology of avermectin for production improvement and structure diversification

Natural products are still key sources of current clinical drugs and innovative therapeutic agents. Since wild-type microorganisms only produce natural products in very small quantities, yields of production strains need to be improved by breaking down the precise genetic and biochemical circuitry. Herein, we use avermectins as an example of production improvement and chemical structure diversification by synthetic biology. Avermectins are macrocyclic lactones produced by Streptomyces avermitilis and are well known and widely used for antiparasitic therapy. Given the importance of this molecule and its derivatives, many efforts and strategies were employed to improve avermectin production and generate new active analogues. This review describes the current status of synthetic strategies successfully applied for developing natural-product-producing strains and discusses future prospects for the application of enhanced avermectin production.

Secondary metabolism in simulated microgravity and space flight

Space flight experiments have suggested that microgravity can affect cellular processes in microorganisms. To simulate the microgravity environment on earth, several models have been developed and applied to examine the effect of microgravity on secondary metabolism. In this paper, studies of effects of space flight on secondary metabolism are exemplified and reviewed along with the advantages and disadvantages of the current models used for simulating microgravity. This discussion is both significant and timely to researchers considering the use of simulated microgravity or space flight to explore effects of weightlessness on secondary metabolism.

Magnetic field is the dominant factor to induce the response of Streptomyces avermitilis in altered gravity simulated by diamagnetic levitation

Background

Diamagnetic levitation is a technique that uses a strong, spatially varying magnetic field to simulate an altered gravity environment, as in space. In this study, using Streptomyces avermitilis as the test organism, we investigate whether changes in magnetic field and altered gravity induce changes in morphology and secondary metabolism. We find that a strong magnetic field (12T) inhibit the morphological development of S. avermitilis in solid culture, and increase the production of secondary metabolites.

Methodology/Principal Findings

S. avermitilis on solid medium was levitated at 0 g*, 1 g* and 2 g* in an altered gravity environment simulated by diamagnetic levitation and under a strong magnetic field, denoted by the asterix. The morphology was obtained by electromicroscopy. The production of the secondary metabolite, avermectin, was determined by OD_{245 nm}. The results showed that diamagnetic levitation could induce a physiological response in S. avermitilis. The difference between 1 g* and the control group grown without the strong magnetic field (1 g), showed that the magnetic field was a more dominant factor influencing changes in morphology and secondary metabolite production, than altered gravity.

Conclusion/Significance

We have discovered that magnetic field, rather than altered gravity, is the dominant factor in altered gravity simulated by diamagnetic levitation, therefore care should to be taken in the interpretation of results when using diamagnetic levitation as a technique to simulate altered gravity. Hence, these results are significant, and timely to researchers considering the use of diamagnetic levitation to explore effects of weightlessness on living organisms and on physical phenomena.