Abstract: Modified recombinant protein derived from Streptococcus pneumoniae PBP2x, referred to as mini-PBP2x, consisting of a concatenation of the fragments corresponding, respectively, to the amino acids located between positions 74 to 90, 186 to 199, 218 to 228 and 257-750, with reference to the sequence of the PBP2x protein of the strain R6 (SWISSPROT P14677 or GENBANK 18266817), each one of said fragments being preceded by a peptide fragment of 1 to 7 amino acids, and uses of said mini-PBP2x protein for selecting and identifying antibiotics which are active on ?-lactam-resistant strains of S. pneumoniae.