Transfer of penicillin resistance from S treptococcus oralis to S treptococcus pneumoniae identifies mur E as resistance determinant

Summary Beta‐lactam resistant clinical isolates of S treptococcus pneumoniae contain altered penicillin‐binding protein ( PBP ) genes and occasionally an altered mur M , presumably products of interspecies gene transfer. MurM and MurN are responsible for the synthesis of branched lipid II , substrate for the PBP catalyzed transpeptidation reaction. Here we used the high‐level beta‐lactam resistant S . oralis   U o5 as donor in transformation experiments with the sensitive laboratory strain S . pneumoniae R6 as recipient. Surprisingly, piperacillin‐resistant transformants contained no alterations in PBP genes but carried mur E Uo5 encoding the UDP ‐ N ‐acetylmuramyl tripeptide synthetase. Codons 83–183 of mur E Uo5 were sufficient to confer the resistance phenotype. Moreover, the promoter of mur E Uo5 , which drives a twofold higher expression compared to that of S . pneumoniae R6, could also confer increased resistance. Multiple independent transformations produced S . pneumoniae   R 6 derivatives containing mur E Uo5 , pbp2x Uo5 , pbp1a Uo5 and pbp2b Uo5 , but not mur M Uo5 sequences; however, the resistance level of the donor strain could not be reached. S . oralis   U o5 harbors an unusual mur M , and mur N is absent. Accordingly, the peptidoglycan of S . oralis   U o5 contained interpeptide bridges with one L ‐ A la residue only. The data suggest that resistance in S . oralis   U o5 is based on a complex interplay of distinct PBPs and other enzymes involved in peptidoglycan biosynthesis.