Submitted to: Archives of Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/7/2016
Publication Date: 12/29/2016
Citation: Zhang, D., Xu, D., Qiu, J., Rasmussen-Ivey, C.R., Liles, M.R., Beck, B.H. 2016. Chitin degradation and utilization by virulent Aeromonas hydrophila strain ML10-51K. Archives Of Microbiology. 199(4):573-579. doi:10.1007/s00203-016-1326-1.

Interpretive Summary: Aeromonas hydrophila, a Gram-negative bacterium, is the causal agent of motile Aeromonas septicemia (MAS) in catfish and other warm-water fishes. A severe outbreak of MAS in the Southeastern United States was reported in 2009; the disease has since resulted in loss of millions of pounds of market-size catfish annually. Emergence of a new pathotype of virulent A. hydrophila (vAh) was associated with the outbreaks but certain aquaculture practices also had an impact on severity of disease severity. Earlier studies found that A. hydrophila was more abundant in waters with a high organic load than in relatively unpolluted water and MAS outbreaks caused by vAh in crucian carp in China occurred primarily in fish ponds with high organic loads. Since chitin is the major component of organic matter in aquatic ecosystems, understanding the role of chitin in vAh pathogenesis is of great importance. In this study, we examined the capability of chitin utilization by ML10-51K, a vAh strain. We provided evidence that ML10-51K is a highly chitinolytic bacterium, which can efficiently use chitin as substrate for growth. This capability would make it possible for the bacterium to maintain high population densities when abundant chitin is available in aquatic ecosystems. The ability to use chitin could promote the persistence and growth of vAh, and may also be important in their production of virulence factors which could enhance their pathogenesis in fish.

Technical Abstract: Virulent Aeromonas hydrophila (vAh) is one of the most important bacterial pathogens that causes persistent outbreaks of motile Aeromonas septicemia (MAS) in warm-water fishes. Among factors associated with MAS outbreaks, the survivability of this pathogen in aquatic environments is of great concern. The aim of this study was to determine the capability of the vAh strain ML10-51K to degrade and utilize chitin, a widely available polysaccharide that is composed of N-acetylglucosamine (GlcNAc) monomers. Genome-wide analysis revealed that ML10-51K encodes a suite of proteins (enzymes) for chitin substrate metabolism. Assays in vitro showed that four chitinases (2 in glycosyl hydrolase (GH) family 18 and 2 in GH19), one chitobiase (in GH20) and one chitin binding protein were secreted extracellularly, which participated in chitin degradation and yielded GlcNAc and/or (GlcNAc)2. ML10-51K was shown to be able to use GlcNAc as the sole carbon source for growth, resulting in growth comparable to that observed using glucose as a sole carbon source with cell density increasing more than 3 orders of magnitude within 24 h. Furthermore, the bacterium was shown to utilize both insoluble colloidal chitin and chitin flakes as sole carbon sources resulting in similarly rapid growth. This study indicates that ML10-51K is a highly chitinolytic bacterium and suggests that the capability of effective chitin utilization could enable the bacterium to attain high densities when abundant chitin is available in aquatic niches, thereby presenting a potent threat to susceptible fish.