A new study reviews the pathogen Aeromonas hydrophila, its impact on aquaculture and its relationship to environmental factors.
Dr. Bahaa Abdella and colleagues from Kavrel Sheikh University in Egypt recently published a study in Aquaculture International discussing the pathogenic bacterium Aeromonas hydrophila, its impact on aquaculture and its relationship to various environmental factors.
1. What is Aeromonas hydrophila
Dr Bahaa Abdella of Kavrel Sheikh University in Egypt and senior author of the study said: "Our study highlights the importance of understanding how environmental conditions affect the toxicity of Aeromonas hydrophila in aquaculture. It involves complex and dynamic interactions between bacteria and their environment. Critical points provided, including: monitoring temperature, pH, and ammonia concentrations to prevent disease outbreaks. Proper management of these factors can improve fish and shrimp health and reduce bacterial infections. ”
Diseases caused by Aeromonas species in fish include hemorrhagic sepsis, ulcer disease, and motor aeromonas sepsis. These diseases can lead to high mortality rates and high economic losses in the aquaculture industry. Aeromonas hydrophila is the main pathogenic species of this genus in farmed freshwater fish and other species worldwide. Due to their genetic diversity, it is difficult to serotype or genotype Aeromonas hydrophila strains because of the complexity of the types and numbers of virulence genes present in each isolate. In addition to being a fish pathogen, Aeromonas hydrophila can cause diseases in humans, such as gastroenteritis, wound infections, and sepsis in immunocompromised people.
Environmental factors such as mineral availability, salinity, dissolved oxygen concentration, pH and temperature in hatchery facilities, as well as poor management (malnutrition, overfeeding and overcrowding) can stress farmed animals, making them more susceptible to Aeromonas hydrophila infection.
In addition, the Aeromonas hydrophila genome encodes different types of virulence factors. The expression and regulation of these genes can vary depending on the bacterial strain, host, and environmental conditions. How environmental conditions affect the intrinsic properties of bacteria and the expression of virulence genes is not fully understood, so this is an open question. Therefore, understanding the diversity, function, and environmental regulation of these genes is essential to develop effective strategies for the prevention and treatment of Aeromonas hydrophila infections in farms.
"The genome coding of Aeromonas hydrophila presents a significant challenge for aquaculture due to its ability to thrive in a wide range of environmental conditions and infect a variety of hosts," said Dr Abdella. Factors such as temperature, dissolved oxygen, pH, ammonia, and nutrient levels affect the expression of virulence genes and the production of toxins. Understanding these mechanisms is essential for developing strategies to control bacterial infections in aquatic and to promote healthier and more sustainable aquaculture practices. ”
2. Factors affecting Aeromonas hydrophila
Physical factors affect the growth rate of microorganisms, such as temperature and pH. However, little is known about how these factors alter the expression of virulence genes in pathogenic microorganisms. It is critical to understand the factors that trigger the expression of virulence factors in a particular pathogen, or at least the most important factor.
For example, it has been reported that the expression of some virulence genes in Aeromonas ichthyosa is temperature-dependent. However, the molecular mechanisms and regulatory pathways involved in this process are not fully understood. In addition, increased temperatures cause stress on the host, which increases the expression of certain steroids, which increases the vulnerability of fish to infection.
Fluctuations in temperature and pH significantly affect the metabolic activity and growth rate of Aeromonas hydrophila, thereby regulating its virulence and overall pathogenicity. Overall, the expression of virulence genes in Aeromonas varies with water temperature and host animals, and the expression patterns of many virulence factors in the same strain are inconsistent in response to temperature changes, which highlights the difficulty and necessity of identifying the main virulence factors that contribute to the pathogenesis of susceptible hosts.
Aeromonas are facultative anaerobes and, like all other facultative microorganisms, they can switch between aerobic and anaerobic metabolism. This switching behavior is controlled by gene expression and triggered by molecular sensing of oxygen concentration. Not surprisingly, it has developed a molecular mechanism to achieve this through gene expression control, including virulence genes. Oxygen can regulate virulence genes by regulating the activity of transcription factors, which are proteins that bind to specific DNA sequences and control gene expression.
Overall, oxygen can be considered as a key environmental factor influencing the expression of virulence genes and their pathogenicity in many strains of bacteria, including Aeromonas hydrophila. However, there are few studies on the effect of oxygen concentration on the expression of virulence genes in fish.
Figure 1 depicts the interaction between the environment, microorganisms, and the host.
3. The relationship between feed formula and Aeromonas hydrophila
The formulation of feeds has a considerable impact on the microbial ecology of the aquatic environment as well as the pathogenicity, interaction and abundance of microorganisms, and the infection of Aeromonas hydrophila is associated with contamination of aquatic water systems by feeds with high organic matter content and nutrient abundance in the water body, e.g., unused feed can lead to an increase in Aeromonas in pond water.
Precise formulation of the feed and its utilization in fish is essential for infection control and inhibition of activation of the virulence gene of Aeromonas hydrophila. More research is needed to confirm the link between various feed components and overexpression of the virulence gene of Aeromonas hydrophila. However, feeding fish rich in omega-3 fatty acids can help improve the immune system and make fish less susceptible to infections.
The toxic environmental regulation of Aeromonas hydrophila is characterized by complex and dynamic interactions between bacteria and their environment. Temperature, dissolved oxygen, pH, ammonia, and nutrient availability play a regulatory role, coordinating the expression of virulence genes and the production of toxins. This remarkable adaptation allows Aeromonas hydrophila to thrive in a variety of aquatic environments and establish infections in a variety of hosts, posing a significant challenge to aquaculture.
"Unraveling the mechanisms underlying the regulation of environmental virulence provides a compelling goal for future research," the study authors concluded. By articulating these pathways, we can pave the way for the development of targeted interventions to mitigate Aeromonas hydrophila infections, ultimately contributing to a healthier and more sustainable aquaculture industry. ”