Results and discussion
A nested-PCR was used for detecting S. agalactiae DNA in red tilapia tissue and screening a number of fish specimens from farms which had been previously diagnosed as being S. agalactiae-positive in Colombia. An intergene DNA fragment from the S. agalactiae ST 991 16S-23S rRNA gene was successfully amplified from frozen or paraffin-wax embedded juvenile tilapia tissue or individuals (40 individuals) which had been experimentally infected with S. agalactiae or from paraffin-wax embedded tissues from 4 naturally-infected adult tilapia ( Fig. 1). By contrast, no amplification was found in 444 frozen or paraffin-wax embedded tissues from randomly sampled larvae and fry from S. agalactiae-positive farms as diagnosed by microbiology, histopathology and immunohistochemistry. All 100 randomly-selected samples proving negative for S. agalactiae DNA 16S-23S rRNA subunit amplification showed amplification of the Oreochromis niloticus β-actin gene 135 bp fragment, thereby confirming that the extracted DNA was optimal for PCR amplification and that inhibitors were not present in the DNA samples ( Fig. 2A–C). These results were consistent with and corroborated prior observations by our group where no S. agalactiae disease or infection in tilapia weighing less than 20 g could be demonstrated by the same techniques ( Hernández et al., 2009). The nested-PCR's reliability and reproducibility make it the technique of choice for diagnosis and epidemiological survey purposes. It was successfully applied in four independent diagnostic cases concerning paraffin-wax-embedded adult tilapia tissue which had been previously diagnosed as being S. agalactiae-positive by the aforementioned techniques