pneumoniae R6 sequence (accession number AE008520.1) ( 16). It was noticed that numbering for the PBP2B amino acid sequence was off by 6 amino acids compared to that of the original S. pneumoniae R6 genes and named with an alphabet letter based on homology to the R6 sequence, e.g., allele A has the highest degree of homology to the R6 sequence, and allele L has the lowest degree of homology to the R6 sequence. PBP alleles were identified based on comparison to sequences of analyzed S. The obtained sequences were analyzed by BLAST and ClustalW ( 29). Fragments of the genes pbp1A (nucleotide region 870 to 1950), encoding 350 amino acids pbp2B (nucleotide region 655 to 2028), encoding 458 amino acids and pbp2X (nucleotide region 301 to 2034), encoding 578 amino acids were amplified using primers and conditions described previously ( 17, 24) and directly sequenced with a CEQ8000 genetic analysis system (Beckman Coulter, Fullerton, CA). Twelve clinical, clonally unrelated pneumococcal isolates from different countries (Table (Table1) 1) classified by penicillin G susceptibility according to old CLSI criteria ( 4) (three penicillin G susceptible, three penicillin G intermediate, and six penicillin G resistant) were tested by the CLSI macrodilution method ( 3, 4). In the present study, we examined the antipneumococcal activities of faropenem by comparing the affinities of faropenem for all pneumococcal PBPs in strains with different β-lactam resistance phenotypes with those of representatives of the penicillin (penicillin G and amoxicillin), carbapenem (imipenem), and cephem (cefuroxime) classes. pneumoniae strains, which are spreading in the United States ( 5, 22). Faropenem demonstrates lower MICs (MIC 90, 1 μg/ml) than other β-lactams and macrolides for panresistant non-vaccine serotype 19A S. pneumoniae, β-lactamase-producing Haemophilus influenzae, and β-lactamase-producing Moraxella catarrhalis isolates ( 6, 12, 25, 28). Since the expanded-spectrum cephalosporins do not interact with PBP2B ( 15), only PBP2X and PBP1A play a role in resistance to these compounds ( 20).įaropenem is an oral penem with excellent activity against the primary bacterial pathogens responsible for community-acquired respiratory tract infections, including non-penicillin-susceptible S. Additionally, the presence of a low-affinity PBP1A is essential for high-level resistance but requires a modified PBP2B and/or PBP2X ( 20, 23). have also suggested that PBP2A is a naturally resistant form of PBP and that PBP2A may be able to take over the activities of other PBPs in the presence of clinically relevant concentrations of β-lactam antibiotics, which may therefore account for its role in resistance to β-lactams ( 18, 30). have also observed that carbapenems have high affinities for all PBPs in a penicillin-susceptible isolate but show reduced binding affinities for PBPs, particularly PBP2X and PBP2B, in a penicillin-resistant isolate ( 9).Īmong all PBPs, pneumococcal PBP2X has one of the highest affinities for penicillin G ( 19).
![davies pbp3 davies pbp3](https://journals.asm.org/cms/10.1128/AAC.01529-07/asset/3158550c-42cf-47f8-8d6e-69ace8f4b2ba/assets/graphic/zac0040872290001.jpeg)
Additional alterations in PBP1A raise penicillin G MICs to ≥1 μg/ml and cefotaxime MICs to ≥0.5 μg/ml ( 21). Alterations in the conserved motifs in PBP2B are associated with resistance to penicillin G, and alterations in PBP2X mediate low-level resistance to cephalosporins. Changes in these motifs or in the positions flanking these motifs are associated with low-affinity PBP variants ( 21). The active sites of PBPs are formed by three conserved amino acid motifs, SXXK, SXN, and KT(S)G. Altered PBPs in pneumococci have greatly decreased affinities for almost all β-lactams, including expanded-spectrum cephalosporins ( 27).
![davies pbp3 davies pbp3](https://journals.asm.org/cms/10.1128/mBio.03058-20/asset/29ade993-83da-4311-a377-aa5293619ca9/assets/images/medium/mbio.03058-20-f0006.gif)
The most highly penicillin G-resistant pneumococcal clinical isolates (for which MICs are 2 to 16 μg/ml) produce altered forms of PBP1A, PBP2X, PBP2B, and PBP2A that have reduced drug affinities ( 13, 18, 20, 27). It is well known that these different PBPs have different affinities for β-lactams ( 1, 7, 9, 11, 21, 26).
![davies pbp3 davies pbp3](https://www.researchgate.net/publication/352102645/figure/fig2/AS:1031736397930496@1622996534283/PBP3-targeting-substituted-pyrrolidin-2-3-dionesThe-pyrrolidine-2-3-dione-core-was_Q320.jpg)
pneumoniae: five high-molecular-mass PBPs (PBP1A, PBP1B, PBP2X, PBP2A, and PBP2B) and one low-molecular-mass PBP, PBP3. The mechanism of increased β-lactam MICs for Streptococcus pneumoniae strains is sequential alterations in the penicillin-binding proteins (PBPs).