GENETIC DIVERSITY OF PENICILLIN-BINDING PROTEIN-2B AND PROTEIN-2X GENES FROM STREPTOCOCCUS-PNEUMONIAE IN SOUTH-AFRICA

Streptococcus pneumoniae (the pneumococcus) is believed to have develo ped resistance to penicillin by the production of altered forms of pen icillin-binding proteins (PBPs) that have decreased affinity for penic illin. Sixty-eight clinical isolates of serogroup 6 and 19 pneumococci (MICs, <0.015 to 8 mug/ml) were randomly selected from hospitals acro ss South Africa which are at substantial geographic distance from each other. The polymerase chain reaction was used to isolate the penicill in-binding domain of PBPs 2B and 2X from the chromosomal DNAs of the b acteria; the purified PBP DNA was digested with restriction enzymes, t he fragments were end-labelled and separated on polyacrylamide gels, a nd the DNA fingerprints were visualized following autoradiography. Fin gerprint analysis revealed that at least 19 PBP 2B gene variants occur in the serogroup 6 and 19 pneumococci. The PBP 2B gene revealed a uni form profile among penicillin-susceptible isolates, with variation fro m this profile occurring only in isolates for which MICs were greater- than-or-equal-to 0.06 mug/ml. Analysis of the PBP 2X gene revealed a g reater diversity in the population with 26 variant genes, including so me diversity among susceptible isolates. Discrete profiles of both gen es were found only within narrow bands of the penicillin MIC, so that the gene pattern predicted the MIC. PBP 2X gene variation and the lack of variability among PBP 2B genes in pneumococci inhibited at low MIC s confirm that PBP 2X alteration may be responsible for low-level peni cillin resistance, while alterations in both PBP 2B and PBP 2X are req uired for high-level resistance. The extensive diversity of PBP genes in South African serogroup 6 and 19 strains suggests that altered PBP genes have arisen frequently in this population.