Delivering the pain: an overview of the type III secretion system with special consideration for aquatic pathogens

Gram-negative bacteria are known to subvert eukaryotic cell physiological mechanisms using a wide array of virulence factors, among which the type three-secretion system (T3SS) is often one of the most important. The T3SS constitutes a needle-like apparatus that the bacterium uses to inject a diverse set of effector proteins directly into the cytoplasm of the host cells where they can hamper the host cellular machinery for a variety of purposes. While the structure of the T3SS is somewhat conserved and well described, effector proteins are much more diverse and specific for each pathogen. The T3SS can remodel the cytoskeleton integrity to promote intracellular invasion, as well as silence specific eukaryotic cell signals, notably to hinder or elude the immune response and cause apoptosis. This is also the case in aquatic bacterial pathogens where the T3SS can often play a central role in the establishment of disease, although it remains understudied in several species of important fish pathogens, notably in Yersinia ruckeri. In the present review, we summarise what is known of the T3SS, with a special focus on aquatic pathogens and suggest some possible avenues for research including the potential to target the T3SS for the development of new anti-virulence drugs.

Antibacterial Activity of Microalgae Dunaliella salina, Tetraselmis chuii and Isochrysis galbana Against Aquatic Pathogens

Recently, expanded consideration has been paid to the business and potentiality of microalgae. Some microalgae are at present being studied for their capacity to find important metabolites for the drug industry or aquacultural applications. Concerning these biotechnological challenges, there is a consistent exertion accommodated in both finding and taking advantage of new microalgal assets and fostering their putative business results or modern valorizations. The crude methanol extracts of three microalgae, Dunaliella salina, Tetraselmis chuii, and Isochrysis galbana, have been investigated for antibacterial activity using the disk diffusion method against aquatic pathogens of fish, shrimp, and shellfish e.g. Pseudomonas fluorescence and Vibrio harveyi. This research aimed to analyze the antibacterial activity of crude extract of these microalgae against aquatic pathogens. The disk diffusion method was used to investigate the antibacterial activity. The result showed that only T. chuii has the inhibition zone for both tested bacteria P. fluorescence and V. harveyi with the inhibition zone of 3.0 ± 0.6 mm on 100 µg. g–1 of extract concentration and 4.20 ± 1.1 mm against P. fluorescence on 10000 µg. g–1 of extract concentration. While D. salina and I. galbana have inhibition zone only for V. harveyi with the inhibition zone of 4.4 ± 0.6 mm and 3.2 ± 0.7 mm on 10000 µg. g–1 of extract concentration. These three microalgae may have potential use in aquatic pathogens as antimicrobial agents. It would be possible to develop biologically active compounds of microalgae as a functional feed for aquaculture.

Linalool Nanoemulsion Preparation, Characterization and Antimicrobial Activity against Aeromonas hydrophila

Aeromonas hydrophila is one of the most important aquatic pathogens causing huge economic losses to aquaculture. Linalool, a vital ingredient of a variety of essential oils, was proved as a good antimicrobial agent in our previous studies. However, the low solubility and volatility of Linalool obstruct its application in the field of aquatic drugs. Thus, in this study, Linalool nano-emulsion (LN) was prepared to solve these obstructions. We investigated the physicochemical properties, antibacterial activity, and mode of action of LN against A. hydrophila. LN with different medium chain triglycerides (MCT) concentrations were prepared by ultrasonic method. The results showed that the emulsion droplet size of LN was the smallest when MCT was not added to the formulation. Nano-emulsions are usually less than 500 nm in diameter. In our study, LN in this formulation were spherical droplet with a diameter of 126.57 ± 0.85 nm and showed good stability. LN showed strong antibacterial activity, the MIC and MBC values were 0.3125% v/v and 0.625% v/v, respectively. The bacterial population decreased substantially at 1 × MIC of LN. LN exhibited disruptive effect on cell membranes by scanning electron microscope (SEM) and transmission electron microscope (TEM). The present study provided a formulation of Linalool nano-emulsion preparation. Moreover, the good antibacterial activity of LN showed in our study will promote the application of Linalool for the control and prevention of A. hydrophila in aquaculture.

APPLICATION OF COMPARATIVE GENOME IN AQUACULTURE DISEASES DIAGNOSIS

The sustainability of aquaculture industry is critical both for global food security and economic welfare. However, the massive wealth of pathogenic bacteria poses a key challenge to the development of a sustainable bio-control method. Recent advances in genome sequencing study combined with pan-genome analysis can be an efficacious management applied to numerous aquatic pathogens. Thus, routine comparative genome analyses of aquatic pathogens will deduce the phylogenomic diversity and possible evolutionary trendsof aquatic bacterial pathogen strains, elucidate the mechanisms of pathogenesis, as well as estimate patterns of pathogen transmission across epidemiological scales. This study also reviews comparative pan-genome analysis with a particular focus on controlling aquatic diseases, especially for: (i) re-identifying the previously misidentified strain with high accuracy and discovering novel isolates that may be associated with high rateof fish mortalities, (ii) developing routine pan-PCR based on highly informative identified genetic targets that are capable of distinguishing all the clinical isolates, and finally (iii) studying the multivalent vaccine following reverse vaccinology towards the prevention of numerous aquatic animal diseases.