Efflux-mediated resistance to tigecycline (GAR-936) in Pseudomonas aeruginosa PAO1

MexAB-OprM hyperexpression in NalC-type multidrug-resistant Pseudomonas aeruginosa: identification and characterization of the nalC gene encoding a repressor of PA3720-PA3719

MexAB-OprM is a multidrug efflux system that contributes to intrinsic and acquired multidrug resistance in Pseudomonas aeruginosa, the latter as a result of mutational hyperexpression of the mexAB-oprM operon. While efflux gene hyperexpression typically results from mutations in the linked mexR repressor gene, it also occurs independently of mexR mutations in so-called nalC mutants that demonstrate more modest mexAB-oprM expression and, thus, more modest multidrug resistance than do mexR strains. Using a transposon insertion mutagenesis approach, nalC mutant strains were selected and the disrupted gene, PA3721, identified. Amplification and sequencing of this gene from previously isolated spontaneous nalC mutants revealed the presence of mutations in all instances and as such, PA3721 has been renamed nalC. PA3721 (nalC) encodes a probable repressor of the TetR/AcrR family and occurs upstream of an apparent two-gene operon, PA3720-PA3719, whose expression was negatively regulated by PA3721. Thus, PA3720-PA3719 was hyperexpressed in transposon insertion and spontaneous nalC mutants. The loss of PA3719 but not of PA3720 expression in a spontaneous nalC mutant reduced MexAB-OprM expression to wild-type levels and compromised multidrug resistance, an indication that hyperexpression of PA3719 only was necessary for the nalC phenotype. Introduction of PA3719 into wild-type P. aeruginosa on a multicopy plasmid was, in fact, sufficient to promote elevated MexAB-OprM expression and multidrug resistance characteristic of a nalC strain. Thus, the nalC (PA3721) mutation serves only to enhance PA3720-PA3719 expression, with expression of PA3719 (encodes a 53 amino acid protein of predicted pI 10.4) directly or indirectly impacting MexAB-OprM expression. Intriguingly, nalC strains produce markedly elevated levels of stable MexR protein suggesting that PA3720-PA3719 hyperexpression somehow modulates MexR repressor activity. The deduced products of PA3720-PA3719 show no homology to sequences presently in the GenBank databases, however, and as such provide no clues as to how this might occur.

The therapeutic challenge of Gram-negative sepsis: Prolonging the lifespan of a scarce resource

Mortality from severe bacterial sepsis remains high. The pathogenesis involves production of pro and anti-inflammatory cytokines which mediate: neutrophil adhesion to the endothelium, diffuse capillary leak, disseminated intravascular coagulation, vasodilatation and mitochondrial dysfunction, all of which culminate in microcirculatory failure. Therapy is multifaceted. As described in 'the surviving sepsis guidelines', many therapeutic interventions, such as early goal-directed resuscitation, low dose intravenous steroids, strict glucose control, recombinant activated protein C and ventilation according to ARDS- net criteria are critical to survival. However appropriate empiric antibiotic therapy initiated early is pivotal. Empiric therapy should be designed with regard to the bacterial epidemiology within the unit and the aim should be to optimise outcome while yet attempting to reduce the potential for resistance development. Antibiotic therapy for resistant organisms consists of the carbapenems, including ertapenem for ESBL's, cefepime, piperacillin/tazobactam and, on occasion, the Gram-negative quinolones, ciprofloxacin and levofloxacin. Consideration should be given to the possibility of 'collateral damage', where overuse of an antibiotic predisposes to multi-drug resistance. Antibiotics should be limited, where possible, to those organisms that are pathogens and not colonisers and should be discontinued if sepsis is not confirmed or there is rapid resolution of clinical indicators of sepsis. De-escalation strategies should be consistently employed and the duration of therapy should be tailored to clinical response. Continuation beyond 8 days is generally detrimental in terms of the potential for superinfection with resistant organisms. Failure of response necessitates, initially, a re-evaluation of source control and obsessive culturing of likely sites of sepsis prior to random antibiotic changes.

Tigecycline

Tigecycline is the first member of a new class of broad-spectrum antibacterials, the glycylcyclines, that has been specifically developed to overcome the two major mechanisms of tetracycline resistance (ribosomal protection and efflux). In vitro, tigecycline was active against a wide range of Gram-positive and -negative aerobic and anaerobic bacteria implicated in complicated skin and skin structure infections (cSSSIs) and complicated intra-abdominal infections (cIAIs). Intravenously administered tigecycline (recommended dosage regimen 100 mg initially, followed by 50 mg every 12 hours for 5-14 days) has been approved by the US FDA for the treatment of cSSSIs and cIAIs. In well designed, pivotal phase III studies, tigecycline monotherapy was noninferior to combination therapy with vancomycin 1 g plus aztreonam 2 g every 12 hours in hospitalised adult patients with cSSSIs (two trials; pooled clinical cure rates, 86.5% vs 88.6%) or broad-spectrum therapy with imipenem/cilastatin 200-500 mg/200-500 mg every 6 hours in hospitalised adult patients with cIAIs (two trials; pooled clinical cure rates, 86.1% vs 86.2%). Tigecycline was generally well tolerated in phase III studies; nausea, vomiting and diarrhoea were the most frequent adverse events in patients treated with tigecycline or an active comparator (vancomycin plus aztreonam or imipenem/cilastatin).

Tigecycline: An investigational glycylcycline antimicrobialwith activity against resistant gram-positive organisms

BACKGROUND

Bacterial resistance to currently available antimicrobials is an increasing concern, particularly among various gram-positive organisms such as drug-resistant pneumococci, methicillin-resistant staphylococci, and drug-resistant enterococci. Tigecycline is an investigational glycylcycline antibiotic that shows promising activity against these resistant gram-positive organisms.

OBJECTIVE

: This paper reviews the pharmacology, pharmacokinetic and pharmacodynamic properties, in vitro and in vivo activity, safety profile, and potential role of tigecycline in the management of gram-positive infections involving resistant microbes.

METHODS

Articles included in this review were identified through a search of MEDLINE from 1998 through 2004 using the terms tigecycline and GAR-936. Abstracts from the Interscience Conference on Antimicrobial Agents and Chemotherapy from 1998 to 2003 were searched using the same terms. The reference lists of identified articles were also reviewed for pertinent publications.

RESULTS

Whereas resistance has developed with many of the earlier tetracycline derivatives, tigecycline appears to have a reduced potential for resistance. Several reports have evaluated the in vitro activity of this agent against a number of organisms. It has exhibited pronounced activity against most gram-positive microbes, including resistant strains (eg, drug-resistant pneumococci, methicillin-resistant staphylococci, resistant enterococci). Tigecycline has also shown useful activity against many clinically important gram-negative microbes. In vivo studies of tigecycline are limited. Only 2 clinical trials have been reported to date, one in patients with complicated skin and skin-structure infections and the other in patients with complicated intra-abdominal infections. In these studies, tigecycline therapy resulted in clinical cures in more than two thirds of evaluable patients. Tigecycline was well tolerated in both studies; nausea and vomiting were the most common adverse events.

CONCLUSIONS

Although published clinical trials involving tigecycline are limited and additional trials are needed, preliminary reports on its use in the treatment of gram-positive infections are encouraging. Tigecycline has favorable pharmacokinetic properties and, apart from gastrointestinal adverse events, appears to be well tolerated.

New therapeutic agents for resistant Gram-positive infections

Gram-positive bacteria are an increasingly common cause of community acquired and nosocomial infections, and their resistance to antibiotics is increasing. The recent reports from several continents of methicillin-resistant Staphylococcus aureus with reduced glycopeptide-susceptibility is of grave concern. New agents are required to meet these threats and several classes of compounds are under development. This review focuses on agents that have been recently licensed or are presently in clinical development for the treatment of serious multidrug-resistant staphylococcal, enterococcal and pneumococcal infections, including methicillin-resistant S. aureus and vancomycin-resistant enterococci.

Effects of efflux transporter genes on susceptibility of Escherichia coli to tigecycline (GAR-936)

The activity of tigecycline, 9-(t-butylglycylamido)-minocycline, against Escherichia coli KAM3 (acrB) strains harboring plasmids encoding various tetracycline-specific efflux transporter genes, tet(B), tet(C), and tet(K), and multidrug transporter genes, acrAB, acrEF, and bcr, was examined. Tigecycline showed potent activity against all three Tet-expressing, tetracycline-resistant strains, with the MICs for the strains being equal to that for the host strain. In the Tet(B)-containing vesicle study, tigecycline did not significantly inhibit tetracycline efflux-coupled proton translocation and at 10 microM did not cause proton translocation. This suggests that tigecycline is not recognized by the Tet efflux transporter at a low concentration; therefore, it exhibits significant antibacterial activity. These properties can explain its potent activity against bacteria with a Tet efflux resistance determinant. Tigecycline induced the Tet(B) protein approximately four times more efficiently than tetracycline, as determined by Western blotting, indicating that it is at least recognized by a TetR repressor. The MICs for multidrug efflux proteins AcrAB and AcrEF were increased fourfold. Tigecycline inhibited active ethidium bromide efflux from intact E. coli cells overproducing AcrAB. Therefore, tigecycline is a possible substrate of AcrAB and its close homolog, AcrEF, which are resistance-modulation-division-type multicomponent efflux transporters.

Assembly of the MexAB-OprM multidrug efflux system of Pseudomonas aeruginosa: identification and characterization of mutations in mexA compromising MexA multimerization and interaction with MexB

The membrane fusion protein (MFP) component, MexA, of the MexAB-OprM multidrug efflux system of P. aeruginosa is proposed to link the inner (MexB) and outer (OprM) membrane components of this pump as a probable oligomer. A cross-linking approach confirmed the in vivo interaction of MexA and MexB, while a LexA-based assay for assessing protein-protein interaction similarly confirmed MexA multimerization. Mutations compromising the MexA contribution to antibiotic resistance but yielding wild-type levels of MexA were recovered and shown to map to two distinct regions within the N- and C-terminal halves of the protein. Most of the N-terminal mutations occurred at residues that are highly conserved in the MFP family (P68, G72, L91, A108, L110, and V129), consistent with these playing roles in a common feature of these proteins (e.g., oligomerization). In contrast, the majority of the C-terminal mutations occurred at residues poorly conserved in the MFP family (V264, N270, H279, V286, and G297), with many mapping to a region of MexA that corresponds to a region in the related MFP of Escherichia coli, AcrA, that is implicated in binding to its RND component, AcrB (C. A. Elkins and H. Nikaido, J. Bacteriol. 185:5349-5356, 2003). Given the noted specificity of MFP-RND interaction in this family of pumps, residues unique to MexA may well be important for and define the MexA interaction with its RND component, MexB. Still, all but one of the MexA mutations studied compromised MexA-MexB association, suggesting that native structure and/or proper assembly of the protein may be necessary for this.

Differential impact of MexB mutations on substrate selectivity of the MexAB-OprM multidrug efflux pump of Pseudomonas aeruginosa

The integral inner membrane resistance-nodulation-division (RND) components of three-component RND-membrane fusion protein-outer membrane factor multidrug efflux systems define the substrate selectivity of these efflux systems. To gain a better understanding of what regions of these proteins are important for substrate recognition, a plasmid-borne mexB gene encoding the RND component of the MexAB-OprM multidrug efflux system of Pseudomonas aeruginosa was mutagenized in vitro by using hydroxylamine and mutations compromising the MexB contribution to antibiotic resistance identified in a DeltamexB strain. Of 100 mutants that expressed wild-type levels of MexB and showed increased susceptibility to one or more of carbenicillin, chloramphenicol, nalidixic acid, and novobiocin, the mexB genes of a representative 46 were sequenced, and 19 unique single mutations were identified. While the majority of mutations occurred within the large periplasmic loops between transmembrane segment 1 (TMS-1) and TMS-2 and between TMS-7 and TMS-8 of MexB, mutations were seen in the TMSs and in other periplasmic as well as cytoplasmic loops. By threading the MexB amino acid sequence through the crystal structure of the homologous RND transporter from Escherichia coli, AcrB, a three-dimensional model of a MexB trimer was obtained and the mutations were mapped to it. Unexpectedly, most mutations mapped to regions of MexB predicted to be involved in trimerization or interaction with MexA rather than to regions expected to contribute to substrate recognition. Intragenic second-site suppressor mutations that restored the activity of the G220S mutant version of MexB, which was compromised for resistance to all tested MexAB-OprM antimicrobial substrates, were recovered and mapped to the apparently distal portion of MexB that is implicated in OprM interaction. As the G220S mutation likely impacted trimerization, it appears that either proper assembly of the MexB trimer is necessary for OprM interaction or OprM association with an unstable MexB trimer might stabilize it, thereby restoring activity.

Efflux-mediated multiresistance in Gram-negative bacteria

Multiresistance in Gram-negative pathogens, particularly Pseudomonas aeruginosa, Stenotrophomonas maltophilia, Acinetobacter spp. and the Enterobacteriaceae, is a significant problem in medicine today. While multiple mechanisms often contribute to multiresistance, a broadly distributed family of three-component multidrug efflux systems is an increasingly recognised determinant of both intrinsic and acquired multiresistance in these organisms. Homologues of these efflux systems are also readily identifiable in the genome sequences of a wide range of Gram-negative organisms, pathogens and non-pathogens alike, where they probably promote efflux-mediated resistance to multiple antimicrobials. Significantly, these systems often accommodate biocides, raising the spectre of biocide-mediated selection of multiresistance in Gram-negative pathogens. While there is some debate as to the natural function of these efflux systems, only some of which are inducible by their antimicrobial substrates, their contribution to resistance in a variety of pathogens nonetheless makes them reasonable targets for therapeutic intervention. Indeed, given the incredible chemical diversity of substrates accommodated by these efflux systems, it is likely that many novel or yet to be discovered antimicrobials will themselves be efflux substrates and, as such, efflux inhibitors may become an important component of Gram-negative antimicrobial therapy.

Oritavancin and Tigecycline: Investigational Antimicrobials for Multidrug-Resistant Bacteria

The advent of multidrug-resistant gram-positive aerobes such as Staphylococcus aureus, Streptococcus pneumoniae, and the enterococci, which are resistant to beta-lactams, vancomycin, and a host of other commonly used antimicrobials, has complicated our approach to antibiotic therapy. Despite marketing of the first oxazolidinone, linezolid, and the streptogramin combination, quinupristin-dalfopristin, an urgent need exists for more agents to combat these pathogens. Two such agents, the glycopeptide oritavancin (LY333328) and the glycylcycline tigecycline (GAR-936), are in phase III clinical trials. These agents, which require parenteral administration, exhibit substantial in vitro activity against a variety of gram-positive aerobes and anaerobes, including the multidrug-resistant organisms listed previously. Only tigecycline demonstrates useful activity against gram-negative organisms. Combination therapy of these agents with ampicillin or aminoglycosides frequently leads to synergistic in vitro activity against multidrug-resistant staphylococci and streptococci. These agents are also active in a variety of animal models of systemic and localized infections. Few published efficacy and tolerability data are available in humans. If controlled clinical trial data verify these agents' efficacy and tolerability, both drugs should become welcome additions to the available antimicrobials. However, restricting their use to the treatment of infections caused by bacteria resistant to other antimicrobials, especially multidrug-resistant staphylococci and streptococci, may prolong their clinical utility by retarding the development of resistance. Careful surveillance of bacterial sensitivity to these agents should be undertaken to assist clinicians in the decision whether or not to use these agents empirically to treat infections caused by suspected multidrug-resistant gram-positive pathogens.