Bacteria capable of producing different extracellular enzymes of potential relevance in digestive processes were isolated from the stomach, hepatopancreas and intestine of Pacific white shrimp Litopenaeus vannamei. A total of 64 strains with proteolytic activity were isolated and grouped into 16 clusters based on morphological characteristics: 4 groups were isolated from the intestine; 5 from the hepatopancreas; and 7 from the stomach. Molecular methods (16S rRNA gene amplification and sequencing) and phenotypic criteria (Gram stain, catalase and oxidase tests, cell and colony morphology) were used to identify strains, which corresponded to Pseudoalteromonas and Vibrio genera. These genera are reported to form part of the digestive tract microbial community in shrimp. Both genera were isolated from all three tested tissues. One member of each morphologic group was selected for analysis of the presence of amylases, lipases/esterases and chitinases. Most of the strains had all the tested enzymes, indicating that the L. vannamei digestive tract microbiotic flora includes groups which have the potential to contribute to the degradation of dietary components.

This study aimed to evaluate possible modification of some immune‐related genes (hemocyanin, alpha2‐macroglobulin, prophenoloxidase and penaeidin3a) at the transcriptional level and signs of disease and mortality in Litopenaeus vannamei previously reared in biofloc and challenged with different Vibrio harveyi densities (10³–10⁸ CFU per ml). Gene expression was up‐ or downregulated based on bacterial density, indicating that defence mechanisms were activated to remove the pathogen. The genes analysed were not affected in the control group (without bacteria) or in the 10³ CFU per ml group. The prophenoloxidase gene was expressed in the 10⁴ CFU per ml group, whereas transcripts of the alpha2‐macroglobulin gene were detected in the 10⁵ and 10⁶ CFU per ml groups. However, the penaeidin3a and hemocyanin genes were upregulated in the 10⁷ and 10⁸ CFU per ml groups, in which signs of disease and mortality in shrimp were evident. These results suggest that the genes involved in the shrimp immune response fluctuate according to pathogen density, and when this density increases (e.g. 10⁷ and 10⁸ CFU per ml), the activated disease tolerance and defence mechanisms are inefficient. For this reason, disease and mortality were observed in the shrimp regardless of the previous condition of the biofloc.