Aims:  To evaluate the influence of environmental parameters on the production of antibiotics (xenocoumacins and nematophin) by Xenorhabdus nematophila and enhance the antibiotic activity. Methods and Results:  Response surface methodology (RSM) was employed to study the effects of five parameters (the initial pH, medium volume in flask, rotary speed, temperature and inoculation volume) on the production of antibiotics in flask cultures by X. nematophila YL001. A 25−1‐factorial central composite design was chosen to explain the combined effects of the five parameters and to design a minimum number of experiments. The experimental results and software‐predicted values of production of antibiotics were comparable. The statistical analysis of the results showed that, in the range studied, medium volume in flask, rotary speed, temperature and inoculation volume had a significant effect (P < 0·05) on the production of antibiotics at their individual level, medium volume in flask and rotary speed showed a significant influence at interactive level and were most significant at individual level. The maximum antibiotic activity was achieved at the initial pH 7·64, medium volume in 250 ml flask 25 ml, rotary speed of 220 rev min−1, temperature 27·8°C and inoculation volume of 15·0%. Maximum antibiotic activity of 331·7 U ml−1 was achieved under the optimized condition. Conclusions:  As far as known, there are no reports of production of antibiotic from X. nematophila by engineering the condition of fermentation using RSM. The results strongly support the use of RSM for fermentation condition optimization. The optimization of the environmental parameters resulted not only in a 43·4% higher antibiotic activity than unoptimized conditions but also in a reduced amount of the experiments. The chosen method of optimization of fermentation condition was efficient, relatively simple and time and material saving. Significance and Impact of the Study:  This study should contribute towards improving the antibiotics activity of X. nematophila. Integrated into a broader study of the impact of environmental factors on the production of antibiotic, this work should help to build more rational control strategy, possibly involving scale‐up of production of antibiotics by X. nematophila.

In this study, we evaluated the phylogenetic diversity of the cecal microbiota of 3-week-old ducklings fed three diets differing in metabolizable energy. The contents of the ceca were collected from ducklings of different groups. The ceca bacterial DNA was isolated and the V3 to V4 regions of 16S rRNA genes were amplified. The amplicons were subjected to high-throughput sequencing to analyze the bacterial diversity of different groups. The predominant bacterial phyla were Bacteroidetes (~65.67%), Firmicutes (~17.46%), and Proteobacteria (~10.73%). The abundance of Bacteroidetes increased and that of Firmicutes decreased with increasing dietary energy level. The diversity decreased (Simpson diversity index and Shannon diversity index) with the increase in dietary energy level, but the richness remained constant. Notably, Brachyspira bacteria were detected with a very high relative abundance (4.91%) in ceca of ducks fed a diet with 11.30 MJ/kg metabolizable energy, suggesting that low energy content may affect their colonization in cecum.