Some of the most remarkable and fascinating qualities of Aeromonas hydrophila are its adaptations to its unpredictable environment. For example, it is able to thrive even in polluted, oxygen-poor environments and resist antibiotics. Because it utilizes so many of these adaptations or resistance mechanisms, it is sometimes referred to as the “Jack of all trades”.
Bacterial Morphologies. Image found in public domain.
·         It is a rod shaped cell rather than a spherical cell which allows it to maximize surface area to increase nutrient uptake. In the image on the right, you can see that most bacteria modify their shape in some way to increase their surface area. In this species, the morphology most closely represents the bacillus morphology.

·         It produces an autoinducer, specifically the second autoinducer (AI-2) which is also present in Salmonella enterica and E. coli. Autoinducers allow a species to communicate in order to regulate population density. For example, it can control antibiotic production and biofilm formation. AI-2 is thought to be the universal signal for interspecies communication.

·         It possesses a type II secretion system (T2SS). This is a pathway that has evolved for the purpose of secreting extracellular proteins, one of the major mechanisms of bacterial infection. This particular system secretes proteases, cellulases, pectinases, phospholipases, lipases, and toxins. These proteins are responsible for destruction of various tissues which causes cell damage and disease in the host. The autoinducers are involved in regulating the secretion so that they will only be secreted when the bacteria have reached their correct location and obtained a critical mass, typically after it has entered the host.

·         Some strains of Aeromonas hydrophila also have a type III secretion system (T3SS). This system secretes the extracellular components of the flagella. Many motile plant and animal pathogens utilize a T3SS because it allows the organism to deliver effectors, such as toxins, to host cells. It also allows the production of complex protein machineries that connect the bacterial cell envelope to the eukaryotic (host) cell membrane.

·         It has developed a type VI secretion system (T6SS) which regulates secretion for survival and fitness inside of the host environment. For example, it regulates the degree of virulence (its ability to infect or cause disease), the production of biofilm, and inducing cytotoxicity in hosts. Environmental factors inside the Mechanism of flagella and cillia. Permission granted by the terms of the GNU Free Documentation Licensehost, especially pH, trigger this T6SS.

·         Some strains have an additional lateral flagellar system. Many have additional surface structures such as type IV pili and adhesions. These can all enhance adhesion to the eukaryotic cell surfaces, aid in recognition of host cell receptors, and thus initiate the colonization process. The image on the left displays the different types of flagella that bacteria are capable of, but Aeromonas hydrophila typically has the Type A flagella.

·         It is resistant to antibacterial factors because it is able to induce β-lactamases and other proteins that resist lactam antibiotics such as penicillins, cephalosporins, and several others.

·         Extracellular transducers are dominated over intracellular signal transduction proteins. This means that the organism expends more energy responding to their external environment rather than their intracellular environment. Thus, this can help the organism respond to the host environment such as a host immune response

·         It is able to survive in highly polluted waters because it likely contains transporters that move heavy metals or toxic compounds out of the cell. It may also produce enzymes that alter the toxic compounds and disinfecting chemicals to maintain environmental stability within the cell.

·        A microcompartment is specifically utilized in order to metabolize toxic by-products away from other more sensitive cell structures.

·         It secretes acetoin which helps it maintain pH homeostasis.

·         The Aeromonas hydrophila genome contains DNA repair enzymes since DNA damage is common in aquatic and host environments. This mechanism is common of the gammaproteobacteria.

·         It has polysaccharide capsules which act as a protective coating to help bacteria survive from host immune responses such as serum killing and desiccation, as well as phagocytes.

Most importantly, its adaptation of enterotoxins significantly increases its virulence once inside its host.

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