Abstract
The recently identified
murMN
operon is essential for the production of branched-structured muropeptides in the cell wall and also for the expression of the resistant phenotype in penicillin-resistant
strains of
Streptococcus pneumoniae
. The purpose of studies described in this communication was to understand better the role of
murMN
in penicillin resistance. Deletion of
murM
in
the penicillin-resistant strain Pen6, which causes reduction in the penicillin MIC from 6.0 to 0.03
μ
g/ml, was successfully complemented to recover the original high level of penicillin resistance
in transformants that received functional
murM
alleles cloned in plasmid pLS578. Inactivation of penicillin resistance was not accompanied by any detectable change in the low affinity or abnormal
molecular size pattern of the penicillin-binding proteins (PBPs) nor in the mosaic sequence of PBP2X typical of resistant strain Pen6. Exposure of strain Pen6 with inactivated
murM
to 0.05
μ
g/ml
of penicillin (
i.e.
, a concentration more than 100 times below the MIC of the parental strain) initiated a phenotypic response typical of penicillin-susceptible strains of pneumococci: inhibition
of growth followed by rapid and extensive loss of viability and lysis. Unexpectedly, inactivation of
murMN
also caused hypersensitivity to lysis by low concentrations of a variety of cell wall active
antibiotics such as fosfomycin, D-cycloserine, and nisin, suggesting that the
murMN
operon may perform an important regulatory role in the control of the irreversible antimicrobial effects of cell
wall inhibitors.