Different modifications in PBPs may mediate high level resistance in clinical isolates of S. mitis and S. pneumoniae

AMOROSO A; DEMARES D; MOLLERACH M; THAMM I; GUTKIND G; COYETTE J

Los Angeles, USA

Congreso; 100th General Meeting of the American Society for Microbiology; 2000

American Society for Microbiology

Today recognized as a common cause of endocarditis, oral streptoccoci are increasingly reported as a cause of bacteriemia in neutropenic patients, and a rising resistance to ß-lactams incidence is being reported in this group of microorganism. Two biochemically and genetically identified isolates of Streptococcus mitis, one sensitive and one resistant to ß-lactams were comparatively studied. The resistant strain was isolated from a newborn suffering a severe neumophaty, who previously went through surgery for a congenital cardiophaty. MICs for penicillin and cephalosporins were approximately 5000 times higher in this than those in the sensitive one. Three HMM and one LMM PBPs could be detected in both strains using 125 I-Penicillin-X as radiotracer. In the resistant isolates, HMM PBPs showed strong alterations both in their affinity and electrophoretical mobility, compared with the sensitive one. We sequenced the genes encoding the HMM PBPs in both strains, detecting mosaic structure in all of the genes belonging to the resistant isolate. Aminoacide changes, inside or nearby the cavity forming the active site of PBPs, were found when compared with published sequences of homologous genes of S. mitis. Essentially, they are not coincident with those changes associated with affinity changes and a resistant phenotype in Streptococcus pneumoniae, despite the high phylogenetic proximity between these species. Resistance determinants could be easily transferred to S. pneumoniae; cefotaxime MICs were increased 20-fold in transformants obtained only alter a single transformation round. In these transformants, low affinity to penicillin can be detected in; at least, PBP 2X and a pbpX gene displaying mosaic structure. The implicated mechanism suggests that many more different subtle modifications than those already described could be in involved as a mechanism for resistance acquisition.