Novel macrolide-resistance genes, mef(C) and mph(G), carried by plasmids from Vibrio and Photobacterium isolated from sediment and seawater of a coastal aquaculture site.

UNLABELLED: The aim of this study was to determine whether mef(C) and mph(G), originally found on the transferable multi-drug plasmid pAQU1 from Photobacterium damselae subsp. damselae isolated from seawater of a fish farm, are responsible for conferring macrolide resistance. Since these genes are localized head-to-tail on pAQU1 and only four nucleotides exist between them, the single- and combination-effect of these genes was examined. When mph(G) alone was introduced to Escherichia coli, the minimum inhibitory concentrations (MICs) against erythromycin, clarithromycin and azithromycin increased, whereas introduction of mef(C) alone did not influence macrolide susceptibility. Introduction of both mef(C) and mph(G) dramatically increased the MICs to the same three macrolides, i.e. >512 µg ml(-1) , >512 µg ml(-1) and 128 µg ml(-1) respectively. These results suggest that the macrolide phosphotransferase encoded by mph(G) is essential for macrolide resistance, while the efflux pump encoded by mef(C) is required for high-level macrolide resistance. The tandem-pair arrangements of the mef(C) and mph(G) genes were conserved on plasmids ranging in size from 240 to 350 kb of the 22 erythromycin-resistant strains belonging to Vibrio and Photobacterium obtained from the fish farm. Sixteen of 22 plasmids ranged in size from 300 to 350 kb. This is the first report of novel macrolide resistance genes originating from a marine bacterium. SIGNIFICANCE AND IMPACT OF THE STUDY: In this study, mef(C) and mph(G) were found to be novel macrolide-resistance genes, and this is the first report of macrolide-resistance genes originating from a marine bacterium. These genes may be responsible for previously reported cases of the emergence of erythromycin-resistant bacteria in aquaculture sites by an unknown mechanism. The introduction of the tandem arrangement of the mef(C) and mph(G) genes in Escherichia coli increased the MICs to erythromycin, clarithromycin and azithromycin, suggesting a novel mechanism conferring high-level macrolide resistance via combined expression of the efflux pump and macrolide phosphotransferase.

Quasi-immune response of Penaeus japonicus to penaeid rod-shaped DNA virus (PRDV).

A quasi-immune response was demonstrated in kuruma prawn Penaeus japonicus infected naturally or experimentally with PRDV (penaeid rod-shaped DNA virus, also called white spot syndrome virus or WSSV), the causative agent of PAV (penaeid acute viremia). In the first step of this study, natural survivors 4 mo after a PAV outbreak demonstrated 94 % relative percent survival (RPS) upon experimental PRDV challenge. Mortalities after challenge were confirmed by PRDV detection to be due to PAV using a PCR method. In the second step, experimental PAV survivors were produced by intramuscular (IM) injection of PRDV into naive shrimp subsequently reared collectively in a tank (A group) or individually in chamber units (B group). Survival was 41 and 90% in the A and B groups, respectively. A subsequent IM re-challenge of these PRDV survivor groups with PRDV made 32 d after the first challenge revealed a protective response with high RPS of 77 and 64%, respectively. These high survival rates suggested that PAV survivors (natural or experimental) were able to resist PRDV infection and that the resistance was not due to selection of naturally resistant shrimp during a PAV outbreak, but due to enhancement of an immune-like system (quasi-immune response) after exposure to PRDV. No PRDV neutralizing activity was revealed in the serum of the 4 mo natural survivors of the PRDV outbreak. However, it was found in their serum 17 d after they had been experimentally challenged with PRDV.