Isolation and characterization of Laribacter hongkongensis from freshwater fish : histopathological and antimicrobial susceptibility findings

Abstract

Laribacter hongkongensis is a gram-negative emerging bacterium that has been isolated from patients with underlying liver diseases and is associated with gastroenteritis and traveler’s diarrhea. Freshwater fish are postulated to be the reservoir of this bacterium and a possible source for human infections. Previous study showed that the bacterium targets liver and causes histopathological changes in adult zebrafish. The first part of the present study therefore aims to evaluate the histopathological effect of the bacterium in liver of grass carps and to investigate the association between recovery of L. hongkongensis and liver histopathological changes. The livers of the grass carps were grossly unremarkable and they were categorized into either necrosis, steatosis, or no abnormality detected upon professional examinations of the tissue sections. Statistical analysis indicated that there was no association between the recovery of L. hongkongensis from the intestines and the histopathological changes in the livers of grass carps. The second part of the study aims to study the extended-spectrum ß- lactamase (ESBL) phenotype and ESBL genes in L. hongkongensis fish isolates. Using the ESBL phenotypic test recommended by the Clinical and Laboratory Standards Institute (CLSI), 8 out of 14 fish isolates were shown to be ESBL-positive. However, further PCR tests could not detect the presence of any ESBL genes (TEM, SHV, OXA-1 or CTX-M). Among these 8 isolates, ampC gene was detected in 6 of 8 isolates using PCR. The inconsistent phenotypic and genotypic results prompted the investigation of the role of AmpC on the ESBL phenotype. To examine the effect of AmpC on ESBL phenotype, boronic acid (BA), which was shown to be an effective AmpC inhibitor, was included in the ESBL phenotypic test. Addition of BA diminished the ESBL phenotype in all the 8 isolates. The absence of AmpC amplification in 2 of 8 isolates with ESBL phenotype may be due to primer mismatch and deserves further follow-up to confirm its presence. Altogether, these results suggested that the CLSI ESBL phenotypes previously identified were possibly due to the presence of AmpC β-lactamase.