Inapparent Streptococcus agalactiae infection in adult/commercial tilapia

Abstract

We report on inapparent infections in adult/commercial tilapia in major tilapia fish farms in Guangdong. A total of 146 suspected isolates were confirmed to be S. agalactiae using an API 20 Strep system and specific PCR amplification. All isolates were identified as serotype Ia using multiplex serotyping PCR. An MLST assay showed single alleles of adhP (10), atr (2), glcK (2), glnA (1), pheS (1), sdhA (3) and tkt (2), and this profile was designated 'unique ST 7'. The analysis of virulence genes resulted in 10 clusters, of which dltr-bca-sodA-spb1-cfb-bac (62, 42.47%) was the predominant virulence gene profile. The PFGE analysis of S. agalactiae yielded 6 distinct PFGE types (A, B, C, D, F and G), of which Pattern C (103) was the predominant type, accounting for approximately 70.55% (103/146) of the total S. agalactiae strains. Therefore, unlike what has been found in juvenile tilapia, in which PFGE pattern D/F is the major prevalent pattern, we found that pattern C was the major prevalent pattern in inapparent infected adult/commercial tilapia in Guangdong, China. In conclusion, we close a gap in the current understanding of S. agalactiae epidemiology and propose that researchers should be alert for inapparent S. agalactiae infections in adult/commercial tilapia to prevent a potential threat to food safety.

Figure 1. Locations from which tilapia were sampled in Guangdong.

The 5 isolation sites are marked in Guangdong Province, China. The location of Guangdong is also shown in the figure. The geographical map was generated using ArcGIS software (version 10.2, Environmental Systems Research Institute, Inc., Redlands, USA) (www.esri.com).

Figure 2

Adult tilapia (A), lesions in tissue samples (B) and positive cultures grown on blood medium (C).

Figure 3. Gel electrophoresis was performed to obtain multiplex PCR amplification products from partial S. agalactiae isolates.

Direct analysis of amplicon sizes and band patterns allowed the determination of the molecular serotypes of the strains, as follows: lane 1, the molecular marker, a 100 bp DNA ladder; lane 2, positive control (ATCC27956, Ia); lanes 3-12, randomly selected isolates from 146 S. agalactiae strains.

Figure 4. The eBURST illustration that was generated using MLST international database data.

No. of isolates = 1376, no. of STs = 308, no. of re-samplings for bootstrapping = 1000, and no. of loci per isolate = 7. Founding and subgroup founding genotypes are shown in blue and yellow, respectively. The size of the dots is representative of the number of isolates belonging to that ST.

Figure 5. The PFGE dendrogram that was constructed using similarity and clustering analyses for the 146 S. agalactiae strains that were isolated from tilapia grown in concentrated aquaculture areas in China.

The digitalized genotyping patterns were analysed using the Dice coefficient and UPGMA methods.