Chapter 62 : Mechanisms of Resistance to β-Lactam Antibiotics

Interaction between the β-lactam ring and the active-site serine of penicillin-interactive enzymes. The chemical reaction for binding of penicillin and other β-lactams to PBPs and β-lactamases is represented by the equation

Ribbon figure of the type A β-lactamase of S. aureus, illustrating key motifs of penicillin-interactive enzymes. S70 indicates the active-site serine in the SxxK motif, S130 and N132 are in the SDN loop, and K234 begins the KTG triad. R244 is a highly conserved residue in class A β-lactamases that is important in catalysis. Differences in amino acids positioned near the active-site cleft at residues 128 and 216 that are responsible for the kinetic differences observed among the wild-type variants of S. aureus β-lactamase are summarized in Table 2 .

Mechanisms of induction of β-lactamase and PBP2a in S. aureus. Normally the penicillin- interactive proteins are produced in only small amounts because of the binding of a blaIencoded repressor to the promoters (P) of blaZ and blaR1. Induction of β-lactamase is initiated by the binding of a β-lactam to the sensor-transducer encoded by blaR1. Binding induces autocatalytic cleavage within blaR1, which unmasks a metalloprotease domain that cleaves the blaI repressor into two fragments, either directly or via interaction with another factor. Thus, the repressor can no longer bind to P, and β-lactamase and blaR1 sensor-transducer production are induced via derepression. The induced β-lactamase is excreted to the extracellular space, where it inactivates the β-lactam. The induction of the low-affinity PBP2a, encoded by mecA, is regulated in a fashion similar to that regulating β-lactamase, and PBP2a mediates resistance by cross-linking peptidoglycan in the presence of β-lactams. There is homology between mecI and blaI, mecR1 and blaR1, and the promoter and N-terminal portions of blaZ and mecA. The zigzag arrows denoting the products of mecI and mecR1 indicate that these genes encode homologs of the blaI and blaR1 products, respectively, with similar functions in induction signaling. The homology is strong enough that a plasmidderived blaI-blaR1-blaZ genomic element can restore the normal inducible phenotype to MRSA clonotypes that produce large amounts of PBP2a constitutively because of deletions in the mecImecR1 region.

Model for the formation of mosaic PBPs in S. pneumoniae. An isolate of S. pneumoniae in the nasopharynx or oropharynx of a colonized person becomes transformed with PBP-encoding DNA from S. oralis, S. mitis, or another commensal streptococcal species that has accumulated point mutations related to exposure to β-lactams. Recombination occurs between regions of identity or high homology between species (black) such that heterologous DNA from the donor (gray) is inserted into the PBP (white) of S. pneumoniae, thereby producing a mosaic PBP that contains regions of both PBPs and exhibits reduced β-lactam-binding affinity.