β-lactam resistance in Streptococcus pneumoniae: Penicillin-binding proteins and non-penicillin-binding proteins

The β-lactams are by far the most widely used and efficacious of all antibiotics. Over the past few decades, however, widespread resistance has evolved among most common pathogens. Streptococcus pneumoniae has become a paradigm for understanding the evolution of resistance mechanisms, the simplest of which, by far, is the production of β-lactamases. As these enzymes are frequently plasmid encoded, resistance can readily be transmitted between bacteria. Despite the fact that pneumococci are naturally transformable organisms, no β-lactamase-producing strain has yet been described. A much more complex resistance mechanism has evolved in S. pneumoniae that is mediated by a sophisticated restructuring of the targets of the β-lactams, the penicillin-binding proteins (PBPs); however, this may not be the whole story. Recently, a third level of resistance mechanisms has been identified in laboratory mutants, wherein non-PBP genes are mutated and resistance development is accompanied by deficiency in genetic transformation. Two such non-PBP genes have been described: a putative glycosyltransferase, CpoA, and a histidine protein kinase, CiaH. We propose that these non-PBP genes are involved in the biosynthesis of cell wall components at a step prior to the biosynthetic functions of PBPs, and that the mutations selected during β-lactam treatment counteract the effects caused by the inhibition of penicillin-binding proteins.