In vitro [3H]-erythromycin binding to Staphylococcus aureus

Overcoming Acquired and Native Macrolide Resistance with Bicarbonate

The growing challenge of microbial resistance emphasizes the importance of new antibiotics or reviving strategies for the use of old ones. Macrolide antibiotics are potent bacterial protein synthesis inhibitors with a formidable capacity to treat life-threatening bacterial infections; however, acquired and intrinsic resistance limits their clinical application. In the work presented here, we reveal that bicarbonate is a potent enhancer of the activity of macrolide antibiotics that overcomes both acquired and intrinsic resistance mechanisms. With a focus on azithromycin, a highly prescribed macrolide antibiotic, and using clinically relevant pathogens, we show that physiological concentrations of bicarbonate overcome drug resistance by increasing the intracellular concentration of azithromycin. We demonstrate the potential of bicarbonate as a formulation additive for topical use of azithromycin in treating a murine wound infection caused by Pseudomonas aeruginosa. Further, using a systemic murine model of methicillin-resistant Staphylococcus aureus (MRSA) infection, we demonstrate the potential role of physiological bicarbonate, naturally abundant in the host, to enhance the activity of azithromycin against macrolide-resistant MRSA. In all, our findings suggest that macrolide resistance, observed in the clinical microbiology laboratory using standard culturing techniques, is a poor predictor of efficacy in the clinic and that observed resistance should not necessarily hamper the use of macrolides. Whether as a formulation additive for topical use or as a natural component of host tissues, bicarbonate is a powerful potentiator of macrolides with the capacity to overcome drug resistance in life-threatening bacterial infections.

Antibiotic Resistance ABC-F Proteins: Bringing Target Protection into the Limelight

Members of the ATP-binding cassette (ABC)-F protein subfamily collectively mediate resistance to a broader range of clinically important antibiotic classes than any other group of resistance proteins and are widespread in pathogenic bacteria. Following over 25 years' of controversy regarding the mechanism by which these proteins work, it has recently been established that they provide antibiotic resistance through the previously recognized but underappreciated phenomenon of target protection; they bind to the ribosome to effect the release of ribosome-targeted antibiotics, thereby rescuing the translation apparatus from antibiotic-mediated inhibition. Here we review the ABC-F resistance proteins with an emphasis on their mechanism of action, first exploring the history of the debate about how these proteins work and outlining our current state of knowledge and then considering key questions to be addressed in understanding the molecular detail of their function.

ABC-F Proteins Mediate Antibiotic Resistance through Ribosomal Protection

Members of the ABC-F subfamily of ATP-binding cassette proteins mediate resistance to a broad array of clinically important antibiotic classes that target the ribosome of Gram-positive pathogens. The mechanism by which these proteins act has been a subject of long-standing controversy, with two competing hypotheses each having gained considerable support: antibiotic efflux versus ribosomal protection. Here, we report on studies employing a combination of bacteriological and biochemical techniques to unravel the mechanism of resistance of these proteins, and provide several lines of evidence that together offer clear support to the ribosomal protection hypothesis. Of particular note, we show that addition of purified ABC-F proteins to an in vitro translation assay prompts dose-dependent rescue of translation, and demonstrate that such proteins are capable of displacing antibiotic from the ribosome in vitro. To our knowledge, these experiments constitute the first direct evidence that ABC-F proteins mediate antibiotic resistance through ribosomal protection.

Antimicrobial resistance ranks among the greatest threats currently facing human health. Elucidation of the mechanisms by which microorganisms resist the effect of antibiotics is central to understanding the biology of this phenomenon and has the potential to inform the development of new drugs capable of blocking or circumventing resistance. Members of the ABC-F family, which include lsa(A), msr(A), optr(A), and vga(A), collectively yield resistance to a broader range of clinically significant antibiotic classes than any other family of resistance determinants, although their mechanism of action has been controversial since their discovery 25 years ago. Here we present the first direct evidence that proteins of the ABC-F family act to protect the bacterial ribosome from antibiotic-mediated inhibition.

The post-antibiotic effects of rokitamycin (a 16-membered ring macrolide) on susceptible and erythromycin-resistant strains of Streptococcus pyogenes

The post-antibiotic effects (PAEs) on susceptible and erythromycin-resistant strains of Streptococcus pyogenes (M phenotype and inducibly resistant) of rokitamycin and erythromycin were investigated in vitro using microbiological impedance measurement. Exposure of susceptible S. pyogenes strains to 1/4, 1/2, 1 and 2 MIC erythromycin and rokitamycin resulted in PAEs of rokitamycin in the same order of magnitude as those of erythromycin and that were dose dependent. The duration of rokitamycin PAEs in erythromycin-resistant S. pyogenes strains (M phenotype and those with inducible resistance) were comparable with those observed in susceptible strains. This was not the case for erythromycin. The investigation showed that a 16-membered ring macrolide such as rokitamycin has different PAEs from those of a 14-membered ring macrolide such as erythromycin. They also indicated that, as the PAEs of rokitamycin on the M phenotype and inducible resistant strains were comparable with those on susceptible strains, no re-evaluation of therapeutic dosing regimens was required.

Identification of a chromosomally encoded ABC-transport system with which the staphylococcal erythromycin exporter MsrA may interact

The energy-dependent efflux of erythromycin (Er) in staphylococci is due to the presence of msr A, which encodes an ATP-binding protein. MsrA is related to the multi-component ATP-binding cassette (ABC) transporters which characteristically also contain membrane-spanning domains. Since MsrA functions in a heterologous host in the absence of other plasmid-encoded products, the requirement for a transmembrane (TM) complex might be fulfilled by hijacking a chromosomally encoded protein. Two genes, stpA and smpA, were identified upstream from msrA on the original Staphylococcus epidermidis plasmid, encoding an ATP-binding protein and a hydrophobic TM protein, respectively. Sequences highly similar to stpA and smpA (stpB and smpB) were also found adjacent to a chromosomal copy of msrA in S. hominis. In Southern blots, internal fragments of stpA or smpA hybridized to the chromosome of the Ers S. aureus RN4220. Cloning and sequence analysis of the region identified revealed the presence of two genes, stpC and smpC, related to stpA and smpA. The deduced amino-acid sequences of the gene products showed that StpA and StpC were 85% identical, whereas SmpA and SmpC were 65% identical. A gene similar to msrA was not present in the S. aureus chromosome. There was no further sequence similarity outside these conserved regions. These results indicate that the chromosomes of S. hominis and S. aureus contain sequences encoding a potential TM protein with which MsrA might interact.

Role of an energy-dependent efflux pump in plasmid pNE24-mediated resistance to 14- and 15-membered macrolides in Staphylococcus epidermidis

We have elucidated a new mechanism for bacterial resistance to the 14-membered macrolides oleandomycin and erythromycin and the 15-membered macrolide azithromycin. Plasmid pNE24, previously isolated from a clinical specimen of Staphylococcus epidermidis, was characterized as causing resistance to 14-membered but not 16-membered macrolides by a mechanism suggested to involve reduced antibiotic permeation of bacterial cells (B. C. Lampson, W. von David, and J. T. Parisi, Antimicrob. Agents Chemother. 30:653-658, 1986). Our recent investigations have demonstrated that S. epidermidis 958-2 containing plasmid pNE24 also contains an energy-dependent macrolide efflux pump which maintains intracellular antibiotic concentrations below those required for binding to ribosomes. Thus, when strain 958-2 was pretreated with the inhibitor carbonyl cyanide m-chlorophenylhydrazone (CCCP), macrolide accumulated at the same rate and to the same extent as in CCCP-treated or untreated control cells lacking plasmid pNE24 (strain 958-1). In contrast, macrolide did not accumulate in energy-competent strain 958-2 but did accumulate to levels equal to those of ribosomes immediately following CCCP addition. Furthermore, intracellular macrolide was excreted and bacteria resumed growth when CCCP but not macrolide was removed from the growth medium. As expected, the 16-membered macrolide niddamycin accumulated to the same level in energy-competent strains 958-1 and 958-2 at the same rapid rate. Macrolide incubated with lysates prepared from both strains or recovered from cells of strain 958-2 was unmodified and bound to ribosomes from strains 958-1 and 958-2 with identical affinities and kinetics, thus precluding a role for ribosome or drug alteration in the resistance mechanism. We conclude that the presence of plasmid pNE24 results in specific energy-dependent efflux of 14- and 15-membered macrolides.

Erythromycin and azithromycin transport into Haemophilus influenzae ATCC 19418 under conditions of depressed proton motive force (delta mu H)

The effect of collapsing the electrochemical proton gradient (delta mu H) on [3H]erythromycin and [14C]azithromycin transport in Haemophilus influenzae ATCC 19418 was studied. The proton gradient and membrane potential were determined from the distribution of [2-14C]dimethadione and rubidium-86, respectively. delta mu H was reduced from 124 to 3 mV in EDTA-valinomycin-treated cells at 22 degrees C with 150 mM KCl and 0.1 mM carbonyl cyanide m-chlorophenylhydrazone. During the collapse of delta mu H, macrolide uptake increased. Erythromycin efflux studies strongly suggested that this increase was not due to an energy-dependent efflux pump but was likely due to increased outer membrane permeability. These data indicated that macrolide entry was not a delta mu H-driven active transport process but rather a passive diffusion process.

Inducible erythromycin resistance in staphylococci is encoded by a member of the ATP-binding transport super-gene family

A Staphylococcus epidermidis plasmid conferring inducible resistance to 14-membered ring macrolides and type B streptogramins has been analysed and the DNA sequence of the gene responsible for resistance determined. A single open reading frame of 1.464 kbp, preceded by a complex control region containing a promoter and two ribosomal binding sites, was identified. The deduced sequence of the 488-amino-acid protein (MsrA) revealed the presence of two ATP-binding motifs homologous to those of a family of transport-related proteins from Gram-negative bacteria and eukaryotic cells, including the P-glycoprotein responsible for multidrug resistance. In MsrA, but not these other proteins, the two potential ATP-binding domains are separated by a Q-linker of exceptional length. Q-linkers comprise a class of flexible interdomain fusion junctions that are typically rich in glutamine and other hydrophilic amino acids and have a characteristic spacing of hydrophobic amino acids, as found in the MsrA sequence. Unlike the other transport-related proteins, which act in concert with one or more hydrophobic membrane proteins, MsrA appears to function independently when cloned in a heterologous host (Staphylococcus aureus RN4220). MsrA might, therefore, interact with and confer antibiotic specificity upon other transmembrane efflux complexes of staphylococcal cells. The active efflux of [14C]-erythromycin from cells of S. aureus RN4220 containing msrA has been demonstrated.

Binding studies of macrolides, lincosamides and streptogramins to Streptococcus G group using [3H]-erythromycin

Parameters of [3H]-erythromycin binding to Streptococcus are determined in vivo using both equilibrium and kinetic methods. This binding is saturable, reversible and independent of energetic systems. Whatever the methods used, the binding parameters are identical as 14 nM for the dissociation constant of the complex erythromycin-Streptococcus and a density of binding sites of 11,865 molecules/cell. Other macrolides, streptogramins and lincosamides competitively displaced bound [3H]-erythromycin suggesting that these compounds share common binding sites on the bacteria. In parallel, the MIC values of these antibiotics against Streptococcus are determined by agar dilution method in Mueller-Hinton medium with 5% of horse blood in order to compare the binding and microbiological parameters. A strong correlation (n = 0.863) has been found between the corresponding inhibition constants and MIC values. Such binding studies could be used in conjunction with microbiological assays for primary screening of active analogous or other compounds with interfere with [3H]-erythromycin binding to the bacteria.

Binding parameters and microbiological activity of macrolides, lincosamides and streptogramins against Staphylococcus aureus

Parameters of erythromycin binding to Staphylococcus aureus were measured in-vitro using an equilibrium method with [3H]erythromycin. The dissociation constant of the complex, erythromycin-S. aureus sensitive strain, was KD = 0.11 microM. The maximal binding, representing the density of binding sites was 14,847 molecules/cell. No binding was detectable on the constitutive resistant strain. Macrolides, streptogramins and lincosamides displaced bound [3H]erythromycin by a competitive process indicating that these compounds share common binding sites on the bacteria, i.e. 50 S ribosomal subunits. A good correlation (r = 0.99) was demonstrated between the corresponding inhibition constants (Ki) and the minimal inhibitory concentration. It is proposed that knowledge of the binding parameters provides a good indication of bacterial susceptibility and may serve as a useful adjunct in developing new compounds.

Determination of binding parameters of macrolides, lincosamides, and streptogramins to Legionella pneumophila

Parameters of [3H]erythromycin binding to Legionella pneumophila were determined in vitro using both an equilibrium and a kinetic method. Different L. pneumophila serogroups, 1-3, and a virulent strain serogroup, 1, were tested. All strains of bacteria exhibited the same binding pattern, with a dissociation constant of 0.15 microM. Other macrolides, streptogramin B-types, and lincosamides competitively displaced bound erythromycin suggesting that these compounds share common binding sites on the bacteria. Minimum inhibitory concentration (MIC) values for macrolides, streptogramin B-types, and lincosamides were determined with buffered charcoal yeast extract (BCYE) medium. A good correlation (r = 0.994) was found between the corresponding inhibition constants of these antibiotics and their MIC. It was also noted that for lincosamides the microbiological inactivity was associated with a very low bacterium affinity. Thus, it is concluded that binding parameters of these antibiotics reflect their efficacy against L. pneumophila in vitro and may serve as a useful adjunct in developing new compounds.