Identification of an unusual Streptococcus agalactiae growing on MacConkey Agar and its confirmation by biochemical tests, qPCR and nanopore sequencing

Streptococcus agalactiae belongs to Group B Streptococcus (GBS), an important Gram-positive pathogen attributed to mastitis and elevated somatic cell counts (SCC) in dairy cows, which invariably exhibits complete β-haemolysis on blood agar (BA) / Polymixin Nalidixic acid Blood Agar (PNBA) and fails to grow on MacConkey agar (MCA). The present study conducted in the Advanced Agricultural Laboratories under the Almarai company, Riyadh, KSA, reports the isolation, identification and characterization of β-haemolytic Strepotococcus agalactiae which exhibited lactose fermenting colonies on MCA. Though the colony characteristics on BA/ PNBA were suggestive of Streptococcus, the observation of pink colored colonies on MCA was deemed unusual and was subjected to further confirmatory tests viz., Gram’s staining, Lancefield grouping, catalase, oxidase and CAMP tests. Further, real-time PCR and Oxford nanopore sequencing of the 16S ribosomal RNA gene were performed for molecular characterization. The findings of the study would add to the existing knowledge on cultural characteristics of Streptococcus agalactiae for its presumptive identification. Keywords: Streptococcus agalactiae, MacConkey agar, nanopore sequencing