Antimicrobial Research Centre and School of Biochemistry and Microbiology, University of Leeds, Leeds LS2 9JT, UK
Received 14 October 2004; returned 2 December 2004; revised 6 December 2004; accepted 6 December 2004
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Abstract |
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Methods: Using established procedures, we examined the activity of CB-181963 against methicillin-susceptible (MSSA) and methicillin-resistant (MRSA) strains of Staphylococcus aureus in both planktonic and biofilm culture. We also determined whether CB-181963 exhibited a post-antibiotic effect (PAE). A radioactive competition assay with 3H-labelled benzylpenicillin was used to determine penicillin-binding protein (PBP) affinities of CB-181963, including binding to PBP2a from MRSA. The potential for emergence of CB-181963-resistant mutants in MSSA and MRSA strains was examined using plating procedures.
Results: CB-181963 showed excellent activity against MRSA strains resistant to other cephalosporins in both planktonic and biofilm cultures. However, in common with other cephalosporins it was unable to eradicate biofilms. CB-181963 had a short PAE compared with other ß-lactam antibiotics. CB-181963 retained activity against a strain expressing type A ß-lactamase and demonstrated affinity for PBP2a of MRSA. Mutants resistant to CB-181963 were not recovered in either MSSA or MRSA.
Conclusions: CB-181963 is a potent antistaphylococcal agent with better activity against MRSA than other cephalosporins. The anti-MRSA activity is correlated with elevated binding to PBP2a. CB-181963 may have a role in the treatment of staphylococcal infections, including those caused by MRSA and in the prophylaxis of biofilm-associated MSSA and MRSA infections. However, because of its short PAE, CB-181963 may have to be administered more frequently than other ß-lactam antibiotics, or given via prolonged infusion.
Keywords: methicillin-resistant Staphylococcus aureus , penicillin-binding proteins
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Introduction |
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CB-181963 (formerly CAB-175) (Figure 1) is a novel cephalosporin, possessing an azomethine substituent at position 3 of the cephem nucleus.3 The activity of CB-181963 against MRSA is one of the most interesting properties of this cephalosporin.3 Other cephalosporins active against MRSA possess enhanced binding to penicillin-binding protein (PBP) 2a, which is uniquely expressed in MRSA.1,2 However, there is no published information on the affinity of CB-181963 for the PBPs of S. aureus. In this paper, we report binding affinities of CB-181963 for staphylococcal PBPs and conclude that the anti-MRSA activity of CB-181963 is indeed related to potent binding activity for PBP2a.
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Materials and methods |
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S. aureus strains 83254, SH1000 and PC1 (all MSSA), and EMRSA-15 and EMRSA-16 are maintained in a culture collection belonging to the Division of Microbiology, University of Leeds.
Antibiotics, chemicals and growth media
Chemicals were from Sigma-Aldrich (Poole, UK) unless stated otherwise. Antibiotics were also from Sigma-Aldrich (Poole, UK), with the exception of CB-181963, samples of which were gifts from Sandoz GmbH (Vienna, Austria) and Cubist Pharmaceuticals (Lexington, MA, USA). MuellerHinton broth (MHB) and agar (MHA) were from Fisher (Loughborough, UK).
Determination of susceptibility to antimicrobial agents
MICs were determined by agar dilution according to BSAC guidelines5 on MHA. The MIC was defined as the lowest concentration of antibiotic completely inhibiting visible growth after 1824 h of incubation at 37°C.
Determination of susceptibility to antimicrobial agents in biofilms
Biofilm MICs and minimum biofilm eradication concentrations (MBEC) were determined in the Calgary Biofilm Device (Nunc Inc., Roskilde, Denmark) as described by Ceri et al.6
This involves the following steps. Aliquots (200 µL) of exponential phase culture were added to each well of a 96-well microtitre tray. The lid assembly, which has 96 polystyrene pegs corresponding to each well, was then replaced and the system incubated for 24 h at 37°C on a rocking platform. Following this, a biofilm of 107 cfu matured on each peg. The lid was then washed twice in phosphate-buffered saline (PBS) to remove residual planktonic growth and then placed into a microtitre tray with fresh media containing doubling dilutions of the test antibiotic. The system was then incubated for 24 h at 37°C on a rocking platform. The MIC was defined as the lowest concentration of antibiotic completely inhibiting visible growth after this incubation. After the MIC was recorded, the lid assembly was again washed twice in PBS to remove planktonic cells and remaining antibiotic, and then placed into fresh drug-free media. The system was incubated for a further 24 h and the MBEC was defined as the lowest concentration of antibiotic completely inhibiting the re-establishment of planktonic growth.
Measurement of post-antibiotic effect (PAE)
PAEs were determined with strain 83254 using a standard viable count method in triplicate, as previously described.7 Bacteria were exposed to x5 the MIC of the antimicrobial agent for 60 min in MHB before removal of the drug by a 103 dilution.
Determination of mutation frequencies for resistance to antibiotics
This was performed as described by O'Neill et al.8 using MH growth media. In each case, cultures were started from single colonies of the test strain that were shown to be sensitive to the antibiotic under investigation. Antibiotic-resistant mutants were identified on medium containing the selective antibiotic at a concentration that was four-fold higher than the respective MIC value for an individual strain. Mutation frequencies were expressed as the number of resistant mutants recovered as a fraction of total viable bacteria.8
PBP binding assays
PBP assays were performed using membranes prepared from S. aureus 83254 (MSSA) according to Komatsuzawa et al.,9 or purified soluble PBP2a kindly provided as a gift by Dr W. P. Lu10 (Proctor and Gamble Pharmaceuticals, Mason, USA). The use of soluble PBP2a was required because of difficulty in separating PBP2a from other PBPs during electrophoresis of membranes prepared from MRSA.
Membranes from S. aureus 83254 (MSSA) were used as a source of PBPs 14 for binding studies with CB-181963 and the comparator agent cefotaxime. Membranes prepared according to Komatsuzawa et al.9 were resuspended in PBS containing DNAse1 (25 mg/L) and stored at 80 °C. For PBP binding assays, 10 µL of membrane preparation (50 µg of protein) was added to 10 µL of ß-lactam solution (various concentrations) in PBS. Samples were incubated at 35 °C for 20 min, then 2 µL of [3H]benzylpenicillin (Amersham Pharmacia Biotech, Amersham, UK) (37 mBq/mL; final concentration 0.5 mg/L) was added. After a further 20 min at 35°C, the reaction was stopped by the addition of 20 µL of SDS sample buffer [0.06 M Tris-HCl, 1.4 M 2-mercaptoethanol, 4% (w/v) sodium dodecyl sulphate, 20% (v/v) glycerol, 0.1% (w/v) Bromophenol Blue]. Samples were boiled for 5 min and then centrifuged (30000 g, 10 min, 4 °C). Proteins were separated by SDSPAGE and the gels analysed, essentially as described9 after autoradiography for 28 days, using Amplify (Amersham Pharmacia, UK) as the fluor. Binding assays with purified soluble PBP2a from MRSA were performed as described for membranes, and 1 µg aliquots applied to gels for electrophoresis.
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Results |
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CB-181963 was active against planktonic cultures of MSSA and EMRSA (MIC range 0.54 mg/L), whereas other cephalosporins had poor activity against EMRSA (MIC range 16256 mg/L) (Table 1). In addition, CB-181963 appears to be stable against degradation by staphylococcal ß-lactamases, since its MIC against S. aureus PC1, which expresses type A ß-lactamase constitutively,2 was 0.5 mg/L. Furthermore, the MIC was also lower than for other cephalosporins against EMRSA-15 and EMRSA-16, which also exhibit ß-lactamase activity.
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The activity of CB-181963 against S. aureus biofilms was compared with a range of other cephalosporins (Table 1). CB-181963 prevented biofilm formation by MSSA and EMRSA, whereas other cephalosporins were generally less active, especially for preventing biofilms formed by the EMSRA strains (Table 1). However, all cephalosporins, including CB-181963, were ineffective at eradicating the biofilm since MBEC values exceeded 256 mg/L (Table 1).
PAE of CB-181963 and comparator antibiotics
Cultures of S. aureus 83254 were exposed to x5 the MIC of the drugs for 60 min before washout, and the PAE determined, as described in the Materials and methods section. CB-181963 demonstrated a PAE of 0.5 h compared with PAEs of between 0.22 and 2.5 h for other antibiotics (Table 2).
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We were unable to recover mutants in MSSA or MRSA host backgrounds by direct selection, indicating a mutation frequency of < 109 (data not shown). We were also unable to record mutation frequencies in MSSA with other ß-lactam antibiotics. In contrast, with similar selections in the MSSA strain 83254, mutants resistant to fusidic acid arose with a frequency of 7.6 ± 1.3 x 107 and those resistant to mupirocin with a frequency of 7.2 ± 0.93 x 108.
Penicillin-binding protein affinities
Data for competition of CB-181963 and cefotaxime with [3H]benzylpenicillin for the various PBPs are presented in Table 3. As the PBP assay is based on competitive binding, these data are expressed as IC50 values, i.e. the concentration of cephalosporin required to inhibit subsequent binding of [3H]benzylpenicillin by 50%. CB-181963 demonstrated strong binding to PBP1, 2, 2a and 3 of S. aureus (Table 3). These binding affinities are consistent with the potency of CB-181963 against MRSA. In contrast, although cefotaxime bound to PBPs 14 from the MSSA strain, it failed to bind PBP2a (Table 3) consistent with its poor activity against MRSA (Table 1).
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Discussion |
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Footnotes |
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Acknowledgements |
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References |
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