Drastic changes in the peptidoglycan composition of penicillin resistant laboratory mutants of

Abstract

The penicillin MIC of 2 Streptococcus pneumoniae clinical isolates was increased 100-fold (from 0.02 to 2.0 micromilligrams) and 20-fold (from 0.5 to 10.0 micromilligrams) through gradual exposure of the bacteria to increasing concentrations of penicillin in the laboratory. In both mutants the affinity of all four high molecular mass penicillin binding proteins (PBPs) for penicillin was drastically reduced accompanied by major changes in the composition of peptidoglycan as resolved by HPLC. The ratio of crosslinked to monomeric peptides became virtually inverted in the resistant cell walls with monomers representing two-thirds of the muropeptide species. The proportion of the crosslinked tri-tetra dimer, a major component of the cell wall of the original isolates, decreased to one-third or one-sixth of its normal representation, while the amounts of tripeptide monomers with an alanyl-serine substitution on the lysine epsilon amino group increased by close to a factor of two. The growth rates of both mutants decreased by a factor of approximately two, as compared to the original bacteria.

Inhibition of the expression of penicillin resistance in Streptococcus pneumoniae by inactivation of cell wall muropeptide branching genes

“…Of particular interest were genetic determinants of the branched structured muropeptides that appear as minor components in the species-specific peptidoglycan of penicillin-susceptible pneumococci but become major building blocks of the cell wall in penicillin-resistant strains (2)(3)(4). The frequent association between the increased proportion of branched cell wall components and penicillin resistanceboth in laboratory mutants (15) and clinical isolates (2-4)-suggested a mechanistic connection between antibiotic resistance and wall structure (2). Support for such a possibility came from genetic crosses in which the abnormal branched-peptiderich wall structure of the penicillin-resistant South African DNA donor strain 8249 was found to be transferred along with the antibiotic resistance trait into a penicillin-susceptible recipient during genetic transformation (2).…”