This study aims to compare the resistance phenotypes conferred by various genes encoding enzymes that phosphorylate erythromycin. The mph genes were cloned into Escherichia coli AG100A susceptible to macrolides and ketolides following disruption of the AcrAB pump. An 882 bp sequence containing a premature stop codon, homologous to the three other previously described mph genes and present widely among Enterobacteriaceae, was found to confer resistance to erythromycin by phosphorylation. The mph(C) gene, as reported for mph(B), also conferred resistance to spiramycin. The mph(A) gene was unique in conferring resistance to azithromycin. The four investigated genes conferred resistance to telithromycin.
Macrolides are extensively used for the treatment of various infections of the upper and lower respiratory tracts and of skin and soft tissues, either as a first-line therapy or as an alternative to the penicillins for allergic patients. Since its discovery in the 1950s, erythromycin has been used as a therapeutic agent as well as a starting point for hemi-synthesis of derivatives with improved physiochemical and antimicrobial properties. The ketolides are the latest derivatives developed to overcome acquired macrolide resistance that is widely distributed among Gram-positive pathogens (Douthwaite, 2001).
During characterization of the resistance genes carried by plasmid pIP1204 of Klebsiella pneumoniae BM4536 (T. Lambert, M. Galimand, S. Sabtcheva, and P. Courvalin, Abstr. 44th Intersci. Conf. Antimicrob. Agents Chemother., abstr. C1-1946, 2004), a genetic structure was detected in Tn1548 (Galimand et al., 2005) identical to that in the previously sequenced pCTX-M3 plasmid (GenBank accession number AF550415) from Citrobacter freundii. This structure consisted of 897 bp (from a canonical RBS sequence AAGGA at 76975–76979 nt) annotated in the database as mph1 (ATG at 76988–76990 nt) and mph2 (ATG at 77243–77245 nt), given the high similarity scores with the N- and C-terminal portions of the Mph-deduced sequences, respectively. In order to determine whether such a structure of two mph parts can confer some resistance properties, various constructs were cloned into Escherichia coli AG100A, which is susceptible to macrolides and ketolides following disruption of the AcrAB pump (Okusu et al., 1996) and the phenotypes obtained were compared with those due to mph(A), mph(B) and mph(C) in the same strain.
Materials and methods
Bacteria and growth conditions
The strains and plasmids are listed in Table 1. Escherichia coli and S. aureus were grown in Luria–Bertani (LB) and brain–heart infusion (BHI) media (Becton-Dickinson, Le Pont-de-Claix, France), respectively. All cultures were incubated at 37°C. When required, antibiotics (Sigma-Aldrich, Saint-Quentin-Fallavier, France) were added to the media as follows: ampicillin at 100 µg mL−1 (selection of pUC19 and pRB474 derivatives in E. coli), chloramphenicol at 10 µg mL−1 (for pRB474 derivatives in S. aureus) or at 20 µg mL−1 (for pACYC184 derivatives in E. coli). Electrotransformation was performed as described (Chesneau et al., 2005).
Ap, ampicillin; Cm, chloramphenicol; Em, erythromycin; Km, kanamycin; Lc, lincomycin; Nal, nalidixic acid; Sm, streptomycin; Tc, tetracycline; I, intermediate; R, resistant; S, susceptible.
Construction of recombinant plasmids
Basic DNA manipulations, such as restriction, ligation and agarose gel electrophoresis were carried out according to standard protocols (Sambrook & Russell, 2001). Plasmid and fragment DNA was purified using commercial kits according to the manufacturer's instructions (Qiagen, Courtaboeuf, France). PCR was performed in a 2400 GeneAmp cycler (Perkin-Elmer Applied Biosystems, Cergy-Pontoise, France) using the primers, templates and assay conditions summarized in Table 2. Site-directed mutagenesis was carried out with the Quick-Change system (Stratagene, La Jolla, CA). All the plasmid constructs were generated using E. coli TOP10 as the intermediate host. The amplified fragments (Table 2) were verified by sequencing with a CEQ 2000 model automate (Beckman Instruments, Inc., Palo Alto, CA) and digested with the appropriate restriction enzymes (Roche Diagnostics GmbH, Mannheim, Germany). Ligation was carried out using the Rapid DNA Ligation kit (Roche).
↵The sequences underlined correspond to the restriction sites used for cloning and the nucleotides in bold to the mutagenized bases.
↵Programs of the thermocycler: P1, 1 × (5 min at 94°C–2 min at 48°C) 30 × (50 s at 72°C–40 s at 94°C–40 s at 48°C) 1 × (7 min at 72°C); P2, 1 × (30 s at 95°C) 18 × (30 s at 95°C–1 min at 50°C–10 min at 72°C); P3, 1 × (30 s at 95°C) 12 × (30 s at 95°C–1 min at 55°C–10 min at 72°C).
The minimum inhibition concentration (MIC) of macrolides and ketolides were determined in three independent experiments on Mueller–Hinton agar (BioRad, Marnes-la-Coquette, France) by the E-test method (AB Biodisk, Solna, Sweden) for erythromycin and azithromycin or by serial twofold dilution for spiramycin (Grünenthal GmbH, Aachen, Germany) and telithromycin (Aventis, Romainville, France) according to the recommendations of the CA-SFM (1996). Discs purchased from BioRad were used for routine susceptibility assays.
Escherichia coli cells from one liter of LB cultures were harvested by centrifugation and treated as described (O'Hara et al., 1988). Briefly, the washed cells were disrupted by sonication, and membranes were pelleted at 30 000 g. The reaction mixture was as follows: 5 mg of erythromycin (Abbot-Laboratories, Rungis, France), 8 mL of the S-30 cell fraction and 2 mL of 80 mM ATP in TMK buffer (0.06 M calcium chloride, 0.01 M magnesium acetate, 0.006 M β-mercaptoethanol and 0.1 M Tris-HCl, pH 7.8). The mixture was incubated overnight at 37°C. The reaction was stopped by boiling, and the denatured proteins were pelleted by centrifugation at 100 000 g for 30 min. The supernatant was treated twice with chloroform, to remove any remaining intact antibiotic, and three times with butanol. The upper phases were pooled, dried and resuspended in 500 µL of distilled water. Ten microliters of the butanol extracts were analysed by thin-layer chromatography.
Results and discussion
A c. 900 bp fragment containing mph1 and mph2 was amplified from pIP1204 DNA with primers mph1F and mph2R (Fig. 1), cut by BamHI and SphI and cloned in pACYC184 previously digested with the same enzymes. When introduced into E. coli AG100A or DB10, the pAT462[mph(1+2)opal] resulting plasmid (Table 1) conferred resistance to erythromycin (MIC=64 µg mL−1). The presence of a suppressor tRNA in different strains being unlikely, the 900 bp fragment was split into two parts, mph1 and mph2, based on the annotation in the database, using primers mph1F and mph1R, and mph2F and mph2R, respectively (Fig. 1). The mph1 fragment was cloned in pUC19, generating pAT464[mph1], and mph2 in pACYC184, generating pAT465[mph2] (Table 1). Neither of these recombinant plasmids conferred resistance to erythromycin in E. coli. As there was no translational start site within mph2, the 5′ portion of the 640 bp insert in pAT465[mph2] was modified by site-directed mutagenesis to generate a GTG start codon six nucleotides downstream from the GGAGG canonical RBS sequence. The pAT466[mph2RBS] resulting plasmid (Table 1) was introduced into E. coli AG100A/pAT464[mph1] to test for trans-complementation. The transformed strain remained susceptible to erythromycin (MIC=4 µg mL−1). An opal codon read-through, as observed for other genes (Mottagui-Tabar, 1998), might therefore occur within plasmid pAT462[mph(1+2)opal]. So far, this has not yet been described for a resistance gene. Synthesis of a functional enzyme was confirmed by phosphorylation of erythromycin evidenced by thin-layer chromatography using crude S30 extracts of AG100A harbouring pTZ3716[mph(B)] (Fig. 2).
Analysis of the translational read-through between mph1 and mph2. Annotation and numbering refer to AF550415. The putative RBS sequence of mph1 is underlined. The stop codons are indicated in italics. Relevant deduced amino acids are indicated above the sequence. The primers used for cloning are indicated by convergent arrows. Added restriction sites are symbolized by B for BamHI and S for SphI. The mutagenized bases are in bold face letters: TGA was replaced by TAA and CTACA by GGAGG, giving rise to plasmids pAT463 and pAT466, respectively.
Thin-layer chromatography (silica gel with CH3COOC2H5–CH3-COOH–H2O [60 : 20 : 20] as a solvent mixture) of 20 µg of erythromycin (lane 1) and 10 µL aliquots of butanol extracts of Escherichia coli AG100A harbouring pACYC184 (lane 2), pAT462[mph(1+2)opal] (lane 3) or pTZ3716[mph(B)] (lane 4). ERY, erythromycin; MF, front of solvent migration; SL, spot-line.
The efficiency of a stop codon, in particular UGA, in promoting translation termination is very sensitive to the context on both sides of the codon (Mottagui-Tabar, 1998). It has been demonstrated that a negatively charged amino acid residue at the penultimate position of the nascent polypeptide, together with an adenine just after an opal codon, can result in up to 25% read-through by the UGG reader tRNATrp. An aspartate residue at position 77 of Mph(1+2), combined with the presence of an adenine at the 3′ end of the opal codon, fulfilled all the requirements for inefficient recognition of the termination signal within the mph(1+2) structure (Fig. 1). Site-directed mutagenesis of pAT462[mph(1+2)opal] using primers OchF and OchR allowed the replacement of the TGA opal by a TAA ochre codon generating plasmid pAT463[mph(1+2)ochre]. This led to a fourfold decrease in erythromycin MIC against AG100A/pAT463[mph(1+2)ochre] and to a 16-fold decrease against DB10/pAT463[mph(1+2)ochre] (MIC=4 µg mL−1). Thus, and as expected (Nilsson & Rydén-Aulin, 2003), the stop codon appeared to be more efficient when opal was replaced by ochre in mph(1+2).
The levels of resistance conferred by mph(A), mph(B), mph(C) and mph(1+2) in E. coli AG100A to various macrolides and ketolides were compared (Table 3). The mph(C) gene, which has been reported to be silent in S. aureus in the absence of msr(A) (Matsuoka et al., 2003), was cloned in the shuttle expression plasmid pIP1840 (Table 2). The pAT461[mph(C)] resulting plasmid (Table 1) conferred resistance to the 14- and 16-membered macrolides and to telithromycin in E. coli (Table 3) but no resistance was detected in S. aureus RN4220, as already observed (Matsuoka et al., 2003). The closely related mph(B) gene conferred the same resistance phenotype as mph(C) in E. coli (Table 3) but is expressed in S. aureus (Noguchi et al., 1998). The mph(1+2) read-through conferred resistance to the 14-membered macrolides and to telithromycin. The mph(A) gene conferred resistance to the 14-, 15- and 16-membered macrolides and to telithromycin. The sequences of the four deduced polypeptides exhibited from 45% to 59.8% similarity in pairwise comparisons. Mph(B) and Mph(C) were the most closely related (48.8% identity) and conferred similar resistance phenotypes (Table 3). Residues known to be important for phosphorylation of macrolides or aminoglycosides were found at the same relative positions in the multiple sequence alignment provided by clustal v (Fig. 3). Except for the second aspartate at position 209 of Mph(B), the other four catalytic aspartates were conserved among the proteins as well as the histidine at position 205 of Mph(B) that is crucial for the activity of the Mph(2′)II enzymes (Taniguchi et al., 1999, 2004). Despite this, it is rather difficult to correlate the sequence-structure traits of these enzymes with their drug specificities.
Alignment of the deduced polypeptide sequences. Identical amino acids in at least three sequences are boxed. Residues known to be important for catalysis are indicated by asterisks. The tryptophan due to read-through between mph1 and mph2 is indicated in bold and underlined.
In conclusion, all the mph genes, including mph(1+2), conferred resistance to telithromycin while the mph(A) gene appeared to be unique by conferring resistance to azithromycin. Why mph(B) but not mph(C) is functional in S. aureus remains unknown.
K.T. was the recipient of an ESCMID grant for training in foreign laboratories. The authors thank M. Matsuoka and K. O'Hara for the gift of plasmids, and D. Cane, E. Dassa, M. Galimand and T. Lambert for helpful discussions.
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