Research Article Assessment of Performance, Microbial Community, Bacterial Food Quality, and Gene Expression of Whiteleg Shrimp (Litopenaeus vannamei) Reared under Different Density Biofloc Systems Mohamed M. Said, 1 Y. A. El-barbary, 2 and O. M. Ahmed 3 1 Department of Aquaculture, Faculty of Fish Resources, Suez University, Suez, Egypt 2 Department of Fish Health and Diseases, Faculty of Fish Resources, Suez University, Suez, Egypt 3 Department of Fish Processing and Technology, Faculty of Fish Resources, Suez University, Suez, Egypt Correspondence should be addressed to O. M. Ahmed; omaima.maamoun@gmail.com Received 19 July 2022; Accepted 8 September 2022; Published 21 September 2022 Academic Editor: Yanjiao Zhang Copyright © 2022 Mohamed M. Said et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Biofloc shrimp culture, as a way of improving shrimp production, gains worldwide consideration. However, the effects of the biofloc system on shrimp culture at high densities could be a challenge. Here, this study is aimed at identifying a better stocking density of whiteleg shrimp (Litopenaeus vannamei) between two intensive biofloc systems of 100 and 300 org./m 2 . Achieving that was done by comparing growth performance, water quality, feed utilization, microbial loads from water and shrimps, and gene expression of growth, stress, and immune-related genes. Shrimp postlarvae with a mean weight of 35:4±3:7 mg were reared in six indoor cement tanks (36 m 3 total capacity each) at two stocking densities (3 replicates each) for a rearing period of 135 days. Better final weight, weight gain, average daily weight gain, specific growth rate, biomass increase percentage, and survival rate were associated with lower density (100/m 2 ), whereas high-density showed significantly higher total biomass. Better feed utilization was found in the lower density treatment. Lower density treatment enhanced water quality parameters, including higher dissolved oxygen and lower nitrogenous wastes. Heterotrophic bacterial count in water samples was recorded as 5:28 ± 0:15 and 5:11 ± 0:28 log CFU/ml from the high- and low-density systems, respectively, with no significant difference. Beneficial bacteria such as Bacillus spp. were identified in water samples from both systems, still, the Vibrio-like count was developed in the higher density system. Regarding shrimp food bacterial quality, the total bacterial count in the shrimp was recorded as 5:09 ± 0:1 log CFU/g in the 300 org./m 2 treatment compared to 4:75 ± 0:24 log CFU/g in the lower density. Escherichia coli was isolated from the shrimps in a lower density group while Aeromonas hydrophila and Citrobacter freundii were associated with shrimps from a higher density system. Immune-related genes including prophenoloxidase, superoxide dismutase (SOD), and lysozyme (LYZ) expressions were all significantly higher expressed in the shrimp from the lower density treatment. Toll receptor (LvToll), penaiedin4 (PEN4), and stress-related gene (HSP 70) showed a decreased gene expression in the shrimp raised in the lower density. Significant upregulation of growth-related gene (Ras- related protein-RAP) expression was associated with the lower stocking density system. In conclusion, the current study found that applying high stocking density (300 org./m 2 ) contributes negatively to performance, water quality, microbial community, bacterial food quality, and gene expression of immune, stress, and growth-related genes when compared with the lower stocking density system (100 org./m 2 ) under biofloc system. Hindawi Aquaculture Nutrition Volume 2022, Article ID 3499061, 13 pages https://doi.org/10.1155/2022/3499061