Polynucleobacter necessarius, an obligate bacterial endosymbiont of the hypotrichous ciliate Euplotes aediculatus, is a member of the beta-subclass of Proteobacteria.

An almost full length 16S rRNA gene of the obligate bacterial endosymbiont Polynucleobacter necessarius was amplified using the polymerase chain reaction in combination with site-specific primers. The amplified DNA was directly sequenced and compared with other bacterial 16S rRNA sequences. P. necessarius belongs to the beta-subclass of Proteobacteria and shows the closest relationship to Alcaligenes eutrophus, Burkholderia solanacearum, and B. pickettii. In Proteobacteria and shows the closest relationship to Alcaligenes eutrophus, Burkholderia solanacearum, and B. pickettii. In situ hybridization with a specific oligonucleotide probe corroborated the assignment of the retrieved sequence to P. necessarius.