Stenotrophomonas maltophilia, an emerging opportunist pathogen for cultured channel catfish, Ictalurus punctatus, in China

Stenotrophomonas maltophilia has emerged as an important opportunistic pathogen in hospitalized and debilitated hosts, especially among immunocompromised patients. Recently, S. maltophilia was isolated from channel catfish, Ictalurus punctatus, with infectious intussusception syndrome (IIS), which is the most common disease of unknown etiology of channel catfish in Sichuan Province, China. The goals of this study were to identify the bacterial strains isolated from fish with IIS collected between 2006 and 2009 in Sichuan Province, China; to evaluate the pathogenicity of the pathogen in channel catfish; and to determine the susceptibility of the pathogen strains to many currently available antimicrobial agents.All isolated strains acquired from diseased fish with IIS were identified as S. maltophilia by API 20NE identification and 16s rDNA sequencing analysis. Susceptibility of the isolates to 24 antibiotics was tested using the agar dilution method. All isolates showed a similar antibiotic susceptibility, which was characterized by resistance to all penicillins, cephalosporins, and aminoglycosides and to ambramycin, erythromycin, and rifampicin; the strains were susceptible only to some fluoroquinolones and trimethoprim + sulfamethoxazole. Virulence tests with isolated strains were conducted and pathogenicity was demonstrated: 90–100%, 65–70%, 40–50% and 0–5% mortalities were achieved via intraperitoneal injection of channel catfish with the different isolates at doses of 1.5 × 106 CFU/fish, 1.5 × 105 CFU/fish, 1.5 × 104 CFU/fish and 1.5 × 103 CFU/fish, respectively. This is the first report of S. maltophilia as a pathogen of channel catfish.