The invention concerns single stranded oligonucleotides comprising a sequence of at least 12 consecutive nucleotide motifs contained in one of the sequences SEQ ID NO: 1 to SEQ ID NO: 4 wherein "n" represents an identical or different nucleotide selected among inosine or an equimolar mixture of 4 different nucleotides selected among a, t, c or g, and among oligonucleotides complementary thereto. The invention also concerns using the oligonucleotides to detect bacteria of the Spirochaetales order. The invention is based on the use of specifically defined sequences in the rpoB gene of spirochetes coding for the beta subunit of the bacterial RNA polymerase.