CHITIN DEGRADATION AND METABOLISM BY VIRULENT Aeromonas hydrophila

Aeromonas hydrophila is the causal agent of motile Aeromonas septicemia (MAS) in catfish and other warm-water fishes. Severe outbreak of MAS caused by virulent A. hydrophila (vAh) was reported in 2009 in the Southeastern United States; the disease has since resulted in loss of millions of pounds of market-size catfish annually. The persistence of MAS outbreaks may suggest that certain environmental conditions promote vAh maintenance or growth in pond water. One such environmental factor of interest is chitin, a linear polymer of N-acetylglucosamine, which may benefit the water-borne pathogen.  

Analysis of draft genome sequence of a vAh strain, ML-10-51K, reveals that the bacterium has four genes coding for chitinases with calculated molecular weight (MW) of 106.6, 92.8, 68.7, and 53.2 kDa, respectively, and two genes for chitobiases with MW of 98.2 and 69.0 kDa, respectively. In addition, there are many other chitin and N-acetylglucosamine utilization genes in the pathogen's genome, such as transcriptional regulator of N-acetylglucosamine utilization, chitin binding protein, phosphatase NagD, N-acetylglucosamine regulated methy-accepting chemotaxis protein, N-acetylglucosamine ABC transport system, glucosamine-6-phosphate deaminase, and N-acetylglucosamine-regulated outer membrane porin.

In in vitro assays with M9 minimal salts, ML-10-51K could efficiently use N-acetylglucosamine, colloidal chitin or crystalline chitin as sole carbon source. Extracellular proteins (ECPs) from ML-10-51K culture could degrade colloidal chitin and generate N-acetylglucosamine and N,N'-diacetyl chitobiose. Mass spectrometric analysis of chitin binding proteins in ECPs revealed that all four chitinases found in ML-10-51K genome participated in chitin degradation.

Since chitin is abundantly available in natural aquatic ecosystems (resulting from exoskeletons of crustaceans and other arthropods, as well as the cell wall of fungi), it makes it possible for vAh readily to colonize and use the substrate and sustain high population level in fish ponds. Additionally, there is growing evidence that N-acetylglucosamine plays diverse roles in cell signaling pathways that impact the virulence of microbes and the sensitivity of host cells. It would be of importance to further investigate mechanisms associated with chitin metabolism and pathogenesis in virulent Aeromonas hydrophila.