Genome-free bacteria vaccine against motile Aeromonas septicaemia in tilapia (Oreochromis sp.)


Aquaculture is a rapidly growing food-producing sector, accounting for 52% of the fish consumed globally in 2020 and generating over £200 billion in economic value. Low-and-middle income countries (LMICs) have become increasingly reliant on fishery trade, with an estimated 56.6 million people employed in the industry, with the vast majority (94%) located in Asia.

One of the most significant challenges facing the aquaculture industry is bacterial disease, which has costed the industry over £4.8 billion in loss-of-yield each year. One of the most common bacterial diseases affecting cultured aquatic organisms is motile Aeromonas septicaemia (MAS), caused by Aeromonas spp. MAS usually infecting cultured tilapia, catfish and carps causing up to 50-70% of mortality, where these species are highly cultured and consumed in LMICs.

Antimicrobial resistance (AMR) further complicates the treatment of this bacterial disease, as Aeromonas spp. is among the most reported AMR organisms in various fish species. As a result, treating these pathogens is becoming increasingly challenging and expensive, which puts the livelihoods of many aquafarmers in LMICs at risk.

To address this issue, this project aims to develop whole-cell bacterial vaccines using the SimCell technology. SimCells are genome-free bacterial cells produced by enzymatically shearing the bacterial genome, rendering the bacterium replication-deficient while preserving immunogenic cell-surface features. Developing SimCell vaccines against MAS could offer an effective solution for protecting fish from the pathogen, potentially reducing economic losses and supporting the livelihoods of small-scale farmers and fishing communities in LMICs.

Project outcomes

Aquaculture, a vital component of global fish consumption and contributing £200 billion annually to the economy, faces substantial threats from bacterial diseases like motile Aeromonas septicaemia (MAS). Particularly impactful in low- and middle-income countries (LMICs), these diseases, compounded by antimicrobial resistance, present challenges to both aquafarmers' livelihoods and global food safety.

This study explored the immuno-protective efficacy of a novel Aeromonas hydrophila-based SimCells vaccine against MAS in tilapia. Engineered via a switchable restriction pathway using a homing endonuclease, A. hydrophila SimCells were produced in quantities of up to 1012 cells through a proprietary manufacturing process. These SimCells were then incorporated into commercial fish feed using palm oil as an adjuvant.

Tilapia (Oreochromis sp.) were divided into three groups: negative control, A. hydrophila SimCells vaccine, and formalin-killed A. hydrophila vaccine. Oral vaccination was administered on three consecutive days on day 0, with a booster on day 14. On day 21, fish were intraperitoneally challenged with virulent A. hydrophila. Various parameters and samples were collected for growth performance and immune response analysis, with protective efficacy was assessed by bacterial challenge.

No significant differences (p ≥ 0.05) in fish weight and length were observed post-vaccination across all treatment groups. A. hydrophila SimCells provided a 40% RPS, compared to 30% with formalin-killed A. hydrophila and 0% with the control following bacterial challenge. Serum lysozyme production and IgM antibody levels were significantly higher (p ≤ 0.05) in vaccinated fish than controls.

Gene expression analysis revealed differing patterns between the two vaccines: formalin-killed A. hydrophila elicited higher peak expression of IL-1β, MHCII, CD4, IgT, and IgM in the fish hindgut, while A. hydrophila SimCells induced higher peak expression of immune-related genes in the head kidney. In addition, A. hydrophila SimCells were found to induce a longer-lasting immune genes expression in both organs when compared to formalin-killed A. hydrophila. Histopathological assessment demonstrated fewer pathological changes in vaccinated fish organs compared to controls.

The study underscores the efficacy of feed-based A. hydrophila SimCell vaccination in tilapia, with potential enhancements such as an additional booster to further improve protective efficacy throughout the fish culture period.



Boon LimDr Boon Lim
Chief Technology Officer
Oxford SimCell Ltd


Assoc. Professor. Dr. Mohammad Noor Amal Azmai, Head, Aquatic Animal Health and Therapeutics Laboratory, Institute of Bioscience, Universiti Putra Malaysia, Malaysia