Streptogramins for the treatment of infections caused by Gram-positive pathogens.

INTRODUCTION: Streptogramins (pristinamycin and quinupristin-dalfopristin) can be interesting options for the treatment of infections due to Gram-positive cocci, especially multidrug-resistant isolates. AREAS COVERED: This review provides an updated overview of structural and activity characteristics, mechanisms of action and resistance, pharmacokinetic/pharmacodynamic, and clinical use of streptogramins. EXPERT OPINION: The streptogramin antibiotics act by inhibition of the bacterial protein synthesis. They are composed of two chemically distinct compounds, namely type A and type B streptogramins, which exert a rapid bactericidal activity against a wide range of Gram-positive bacteria (including methicillin-resistant staphylococci and vancomycin-resistant enterococci). Several mechanisms of resistance have been identified in staphylococci and enterococci but the prevalence of streptogramin resistance among clinical isolates remains very low. Even if only a few randomized clinical trials have been conducted, the efficacy of pristinamycin has been largely demonstrated with an extensive use for 50 years in France and some African countries. Despite its effectiveness in the treatment of severe Gram-positive bacterial infections demonstrated in several studies and the low rate of reported resistance, the clinical use of quinupristin-dalfopristin has remained limited, mainly due to its poor tolerance. Altogether, streptogramins (especially pristinamycin) can be considered as potential alternatives for the treatment of Gram-positive infections.

Pristinamycin in the treatment of MSSA bone and joint infection.

OBJECTIVES: The aim of this study was to evaluate pristinamycin in the treatment of MSSA bone and joint infection (BJI). PATIENTS AND METHODS: A retrospective, single-centre cohort study (2001-11) investigated outcome in adults receiving pristinamycin for MSSA BJI and pristinamycin-related adverse events (AEs). RESULTS: One hundred and two MSSA BJIs were assessed in 98 patients [chronic infection, 33.3%; and orthopaedic device-related infection (ODI), 67.6%]. Surgery was performed in 77.5% of total cases, and in all but three ODIs, associated with antibiotic therapy of a median total duration of 29.2 weeks. Pristinamycin was prescribed as a part of the initial intensive treatment phase (29.4%) and/or included in final maintenance therapy (83.3%) at a dose of 47.6 (45.5-52.6) mg/kg/day for 9.3 (1.4-20.4) weeks. AEs occurred in 13.3% of patients, consisting of gastrointestinal disorder (76.9%) or allergic reaction (23.1%), leading to treatment interruption in 11 cases. AEs were related to daily dose (OR, 2.733 for each 10 additional mg/kg/day; P = 0.049). After a follow-up of 76.4 (29.6-146.9) weeks, the failure rate was 34.3%, associated with ODI (OR, 4.421; P = 0.006), particularly when the implant was retained (OR, 4.217; P = 0.007). In most patients, the pristinamycin companion drug was a fluoroquinolone (68.7%) or rifampicin (21.7%), without difference regarding outcome. CONCLUSIONS: Pristinamycin is an effective, well-tolerated alternative therapeutic option in MSSA BJI, on condition that a daily dosage of 50 mg/kg is respected.

Streptogramins - two are better than one!

Streptogramins are potent drugs against numerous highly resistant pathogens and therefore are used as antibiotics of last-resort human therapy. They consist of a mixture of two different types of chemical substances - the group A streptogramins, which are polyunsaturated macrolactones, and the group B streptogramins, representing cyclic hexadepsipeptides. Streptogramins are unique in their mode of action: each component alone exhibits a moderate bacteriostatic activity by binding to the bacterial 50S ribosomal subunit and thereby blocking translation, whereas the synergic combination of both substances is up to hundred fold more effective than the single compounds, resulting in a bactericidal activity. The streptogramin biosynthetic genes are organized as large antibiotic superclusters. These clusters harbour numerous regulatory genes, which encode different types of regulators that together form a complex hierarchical signalling system, which governs the regulation of streptogramin biosynthesis. Resistance is also regulated by this cascade. However, whereas resistance against streptogramins is quite well understood in diverse pathogenic organisms, only little is known about how the natural producer strains protect themselves against these toxic compounds. Here, we give an overview about the recent advances in streptogramin investigations with a main focus on the best-studied representatives, pristinamycin and virginiamycin. We concentrate on the biosynthesis of these compounds, their regulation and resistance determinants as well as their application in medicine and food industry.

Isolation of conditional expression mutants in Mycobacterium tuberculosis by transposon mutagenesis.

In Mycobacterium tuberculosis identification of essential genes has been hampered by the scarcity of suitable genetic tools for genome wide screenings. We constructed two Himar1 transposon derivatives in which the Streptomyces pristinamycin I-inducible ptr promoter was inserted at one transposon end in outward orientation. These transposons, Tn-pip/pptr (which harbours the promoter and its repressor pip gene) and Tn-pptr (which depends on a host expressing the pip gene), were inserted in the thermosensitive mycobacteriophage phAE87. After transduction into M. tuberculosis H37Rv, hygromycin resistant clones were selected in the presence of pristinamycin, screened for inducer dependent growth, and the transposon insertion point mapped by sequencing. Out of 3530 Hyg(R) mutants tested, we obtained 14 (0.4%) single insertion conditional mutants. In three (leuA, mazE6, rne) pptr was located upstream of genes whose function had been assessed by experimental evidence, whereas in seven the transposon targeted genes (ftsK, glf, infB, metC, pyrD, secY, and tuf) whose function had been assigned by similarity with homologous genes and four ORFs of unknown function (Rv0883c, Rv1478, Rv2050 and Rv2204c). These results validate our mutagenesis system and provide previously unavailable conditional expression mutants in genes of known, putative and unknown functions for genetic and physiological studies.

ermB-mediated erythromycin resistance in Streptococcus uberis from bovine mastitis.

The ermB gene was identified in 111 erythromycin resistant isolates of Streptococcus uberis from cases of bovine mastitis associated either with a constitutive (47/111) or an inducible (64/111) phenotype, as well as a phenotypic resistance to all macrolides tested. Resistance to lincosamides was identified in 14 other isolates of S. uberis from bovine mastitis cases and was mainly mediated by the linB gene; resistance conferred by a combination of two genes (linB-lnuD, ermB-linB) was also detected.

[Evolution of antibiotic-resistance Staphylococcus aureus in Saint Camille Medical Centre in Ouagadougou]. / Evolution de la résistance de Staphylococcus aureus aux antibiotiques au Centre Médical Saint Camille de Ouagadougou.

BACKGROUND: Monitoring the antibiotic resistance of microorganisms in a specific geographic area can be useful in developing new approaches to first-intention antibiotherapy. OBJECTIVE: The purpose of this study was to describe the evolution of resistance of Staphylococcus aureus to antibiotics routinely used at Saint Camille Medical Centre in Ouagadougou, Burkina Faso from 1996 to 2006. METHOD: Strains of S. aureus, isolated from various pathologic sources were tested to determine their susceptibility to antibiotics. Sensitivity tests were performed in accordance with the guidelines of the Antibiogram Committee of the French Society for Microbiology (version 2007). RESULTS: During the study period, 1160 staphylococci strains were isolated including 73.45% identified as S. aureus. Susceptibility tests demonstrated a significant increase in resistance to beta-lactam antibiotics. The proportion of strains showing resistance to ampicillin reached 58.29% in 2000. Resistance to these antibiotics regressed significantly from 2000 to 2006. Resistance to pristinamycin and erythromycin showed a tendency to increase while resistance to gentamicin and oleandomycin showed no statistically significant change. CONCLUSION: This study demonstrated that S. aureus was the most common Staphylococcus genus present at the center and that it was resistant to several antibiotics. Reducing use of beta-lactam probably accounted for the significant decline in resistance to this type of antibiotic. Care should also be given to the use of other antibiotics such as pristinamycin and erythromycin since resistance appears to be increasing.

Development of a repressible mycobacterial promoter system based on two transcriptional repressors.

Tightly regulated gene expression systems represent invaluable tools for studying gene function and for the validation of drug targets in bacteria. While several regulated bacterial promoters have been characterized, few of them have been successfully used in mycobacteria. In this article we describe the development of a novel repressible promoter system effective in both fast- and slow-growing mycobacteria based on two chromosomally encoded repressors, dependent on tetracycline (TetR) and pristinamycin (Pip), respectively. This uniqueness results in high versatility and stringency. Using this method we were able to obtain an ftsZ conditional mutant in Mycobacterium smegmatis and a fadD32 conditional mutant in Mycobacterium tuberculosis, confirming their essentiality for bacterial growth in vitro. This repressible promoter system could also be exploited to regulate gene expression during M. tuberculosis intracellular growth.

Pristinamycin-inducible gene regulation in mycobacteria.

In this work the Pip-inducible system, already used in eukaryotes, was tested in mycobacteria. This system is based on the Streptomyces coelicolor Pip repressor, the Streptomyces pristinaespiralis ptr promoter and the inducer pristinamycin I. By cloning in an integrative plasmid the ptr promoter upstream of the lacZ reporter gene and the pip gene under the control of a constitutive mycobacterial promoter, we demonstrated that the ptr promoter activity increased up to 50-fold in Mycobacterium smegmatis and up to 400-fold in Mycobacterium tuberculosis, in dependence on pristinamycin I concentration, and that the promoter was fully repressed in the absence of the inducer. Three mycobacterial genes were cloned under pptr-Pip control, both in sense and antisense direction; both proteins and antisense RNAs could be over-expressed, the antisenses causing a partial reduction of the amount of the targeted proteins. This system was used to obtain two M. tuberculosis conditional mutants in the fadD32 and pknB genes: the mutant strains grew only in the presence of the inducer pristinamycin I. Thus it showed to be an effective inducible system in mycobacteria.

A general strategy for the production of difficult-to-express inducer-dependent bacterial repressor proteins in Escherichia coli.

Inducer-dependent prokaryotic transcriptional repressor proteins that originally evolved to orchestrate the transcriptome with intracellular and extracellular metabolite pools, have become universal tools in synthetic biology, drug discovery, diagnostics and functional genomics. Production of the repressor proteins is often limited due to inhibiting effects on the production host and requires iterative process optimization for each individual repressor. At the example of the Streptomyces pristinaespiralis-derived streptogramin-dependent repressor PIP, the expression of which was shown to inhibit growth of Escherichia coli BL21*, we demonstrate that the addition of the PIP-specific streptogramin antibiotic pristinamycin I neutralizes the growth-inhibiting effect and results in >100-fold increased PIP titers. The yield of PIP was further increased 2.5-fold by the engineering of a new E. coli host suitable for the production of growth-inhibiting proteins encoded by an unfavorable codon usage. PIP produced in the presence of pristinamycin I was purified and was shown to retain the antibiotic-dependent binding to its operator pir as demonstrated by a fluorescence resonance energy transfer (FRET)-based approach. At the example of the macrolide-, tetracycline- and arsenic-dependent repressors MphR(A), TetR and ArsR, we further demonstrate that the production yields can be increased 2- to 3-fold by the addition of the cognate inducer molecules erythromycin, tetracycline and As(3+), respectively. Therefore, the addition of inducer molecules specific to the target repressor protein seems to be a general strategy to increase the yield of this interesting protein class.

Evolution of Streptomyces pristinaespiralis for resistance and production of pristinamycin by genome shuffling.

Improvement of pristinamycin production by Streptomyces pristinaespiralis was performed by using recursive protoplast fusion and selection for improved resistance to the product antibiotic in a genome shuffling format. A 100-microg/ml pristinamycin resistant recombinant, G 4-17, was obtained after four rounds of protoplast fusion, and its production of pristinamycin reached 0.89 g/l, which was increased by 89.4% and 145.9% in comparison with that of the highest parent strain M-156 and the original strain CGMCC 0957, respectively. The subculture experiments indicated that the hereditary character of high producing S. pristinaespiralis G 4-17 was stable. It is concluded that genome shuffling improves the production of pristinamycin by enhancing product-resistance in a stepwise manner. Pristinamycin fermentation experiments by recombinant G 4-17 were carried out in a 5-l fermentor, and its production of pristinamycin reached 0.90 g/l after 60 h of fermentation.

Analysis of pristinamycin-resistant Staphylococcus epidermidis isolates in the Tunisian Bone Marrow Transplant Center.

AIMS: We report the analysis of genetic determinants conferring resistance to pristinamycin in Staphylococcus epidermidis strains and epidemiology typing of these strains by pulsed-field gel electrophoresis. METHODS AND RESULTS: Staphylococcus epidermidis (346 isolates) were searched for strains with pristinamycin resistance. Pristinamycin-resistant strains (seven isolates) were isolated in five patients with haematological cancer in the Bone Marrow Transplant Centre of Tunisia in 2002. Resistance to pristinamycin was observed in 2% of isolates. The seven pristinamycin-resistant strains shared resistance to oxacillin (MIC = 8-512 microg ml(-1)), gentamicin (MIC = 16-512 microg ml(-1)), erythromycin (MIC > 1024 microg ml(-1)), lincomycin (MIC > 1024 microg ml(-1)), pristinamycin (MIC = 4-16 microg ml(-1)) and rifampin (MIC = 128-256 microg ml(-1)). erm genes were amplified: ermA from six strains and ermC from one. vga gene encoding streptogramins A resistance (pristinamycin résistance) was amplified from all strains and typed as vgaA by analysis after electrophoresis of restriction profiles of vga amplicons (two fragments with Sau3A of 164 and 378 bp; one fragment with EcoRI). Pulsed-field gel electrophoresis (PFGE) of SmaI chromosomal DNA digests of the seven S. epidermidis isolates divided them into two distinct pattern types: pulsed-field type A (classified from A1 to A6 subtypes) and type B. The six strains harbouring ermA genes belonged to the PFGE type A while the strain harbouring ermC genes belonged to the PFGE type B. We characterized an epidemic strain carrying the vgaA and ermA genes responsible for the outbreak. CONCLUSIONS: Two clones of pristinamycin-resistant S. epidermidis were isolated in our patients. One of them, isolated in all patients, had expanded over six months suggesting acquisition by cross-contamination. SIGNIFICANCE AND IMPACT OF THE STUDY: Increasing isolation of pristinamycin resistant S. epidermidis strains is an alarming indicator of nosocomial dissemination. The vector will be determined to establish a system of epidemiological surveillance.

Effect of antibiotic therapy on human fecal microbiota and the relation to the development of Clostridium difficile.

The gastrointestinal tract is a complex ecosystem. Recent studies have shown that the human fecal microbiota is composed of a consortium of microorganism. It is known that antibiotic treatment alters the microbiota, facilitating the proliferation of opportunists that may occupy ecological niches previously unavailable to them. It is therefore important to characterize resident microbiota to evaluate its latent ability to permit the development of pathogens such as Clostridium difficile. Using samples from 260 subjects enrolled in a previously published clinical study on antibiotic-associated diarrhea, we investigated the possible relationship between the fecal dominant resident microbiota and the subsequent development of C. difficile. We used molecular profiling of bacterial 16S rDNA coupled with partial least square (PLS) regression analysis. Fecal samples were collected on day 0 (D0) before antibiotic treatment and on day 14 (D14) after the beginning of the treatment. Fecal DNA was isolated, and V6-to-V8 regions of the 16S rDNA were amplified by polymerase chain reaction with general primers and analyzed by temporal temperature gradient gel electrophoresis (TTGE). Main bacteria profiles were compared on the basis of similarity (Pearson correlation coefficient). The characteristics of the microbiota were determined using PLS discriminant analysis model. Eighty-seven TTGE profiles on D0 have been analyzed. The banding pattern was complex in all cases. The subsequent onset of C. difficile was not revealed by any clustering of TTGE profiles, but was explained up to 46% by the corresponding PLS model. Furthermore, 6 zones out of the 438 dispatched from the TTGE profiles by the software happened to be specific for the group of patients who acquired C. difficile. The first approach in the molecular phylogenetic analysis showed related sequences to uncultured clones. As for the 87 TTGE profiles on D14, no clustering could be found either, but the subsequent onset of C. difficile was explained up to 74.5% by the corresponding PLS model, thus corroborating the results found on D0. The non exhaustive data of the microbiota we found should be taken as the first step to assess the hypothesis of permissive microbiota. The PLS model was used successfully to predict C. difficile development. We found that important criteria in terms of main bacteria could be markedly considered as predisposing factors for C. difficile development. Yet, the resident microbiota in case of antibiotic-associated diarrhea has still to be analyzed. Furthermore, these findings suggest that strategies reinforcing the ability of the fecal microbiota to resist to modifications would be of clinical relevance.

Correlation of agar dilution and VITEK2 system for detection of resistance to macrolides, lincosamides and pristinamycin among Staphylococcus aureus and Staphylococcus epidermidis: association with genotypes.

The performance of the VITEK2 system was evaluated against the agar dilution reference procedure for testing susceptibility of Staphylococcus aureus and Staphylococcus epidermidis to macrolides, lincosamides and streptogramins (MLS). Eighty clinical isolates were selected according to their resistance phenotype and genotype. Results for erythromycin and clindamycin showed 100% agreement; results for lincomycin showed agreement of 78%, with one very major error and 17 minor errors; and results for pristinamycin showed agreement of 46%, with one major error and 43 minor errors. Most isolates resistant to lincomycin and streptogramin A (L SgAr phenotype) were falsely susceptible to lincomycin, and intermediately-resistant or resistant to pristinamycin, with the VITEK2 system. No resistance gene was detected. Most (80%) isolates resistant constitutively to MLS (MLS(r)BC phenotype) were falsely intermediately-resistant to pristinamycin with the VITEK2 system. The erm(A) gene was more common than erm(C) in MLS(r)BC strains. Resistance to pristinamycin alone (SgA SgB PTr phenotype), or associated with either lincomycin resistance (L SgA SgB PTr phenotype) or constitutive MLS(B) resistance (MLS(BC) SgA PTr phenotype), was well-characterised without discordant results. Resistance to pristinamycin was always associated with resistance to streptogramin A, encoded by the vga(A), vga(B), vgb(A) and vat(A) genes in association with the erm(A) or erm(C) genes.

Susceptibility of methicillin-resistant Staphylococcus aureus to vancomycin, teicoplanin, linezolid, pristinamycin and other antibiotics.

BACKGROUND: Methicillin-resistant Staphylococcus aureus is a major nosocomial pathogen worldwide. Vancomycin is the traditional drug of choice, but decreasing susceptibility to vancomycin and other glycopeptides has been reported since 1996. OBJECTIVES: To test the in vitro activity of linezolid (oxazolidinone) and other antimicrobial agents against MRSA isolates recovered from hospitalized patients. METHODS: We tested 150 MRSA isolates recovered from hospitalized patients. The minimal inhibitory concentration of vancomycin, teicoplanin, pristinamycin (quinupristin-dalforistin) and linezolid was determined by the Etest method. Susceptibility to other antibiotics was tested by the disk diffusion method. RESULTS: All isolates were sensitive to vancomycin, teicoplanin, pristinamycin, and linezolid. The MIC90 was 2.0 microg/ml for vancomycin and teicoplanin (range 0.5-2.0 microg/ml and 0.125-2.0 microg/ml, respectively), and 0.5 microg/ml for pristinamycin and linezolid (range 0.125-0.75 microg/ml and 0.125-0.5 microg/ml, respectively). Of the other antibiotics, fusidic acid showed the best in vitro activity, with 96.7% susceptibility, associated with trimethoprim/sulfamethoxazole (85.8%) and minocycline (84%). Penicillin was associated with the lowest susceptibility (1.3%), associated with ofloxacin (3%) and erythromycin (14%). An increase in the minimal inhibitory concentration value of vancomycin was associated with a significant decrease in resistance to TMP-SMZ (P < 0.01) and an apparent increase in resistance to other antibiotics. CONCLUSION: The excellent in vitro activity of linezolid and its reported in vivo effectiveness renders it an important therapeutic alternative to vancomycin in the treatment of MRSA infection.

Phenotypic and molecular characterization of macrolide and streptogramin resistance in Streptococcus mitis from neutropenic patients.

OBJECTIVES: To determine the prevalence of macrolide and streptogramin resistance in Streptococcus mitis isolates from neutropenic patients and to identify mechanisms of macrolide and streptogramin resistance in resistant isolates. METHODS: MICs of erythromycin, spiramycin, lincomycin and pristinamycin were determined for S. mitis isolates. Macrolide-resistance genes were characterized by PCR and ribosomal mutations by sequencing. RESULTS: A total of 169 S. mitis isolates were recovered from 66 patients at the Tunisian Bone Marrow Transplant Centre. Of these, 120 (70%) were non-susceptible to erythromycin and one was resistant to pristinamycin; 48.5% of isolates had an MLSB phenotype with cross-resistance between erythromycin, spiramycin and lincomycin, 4% had a dissociated MLSB phenotype with resistance to erythromycin and spiramycin but apparent susceptibility to lincomycin and 47.5% displayed the M phenotype. Resistance determinants were characterized in 33 isolates. Ten of 14 isolates with the cross MLSB resistance contained an ermB-like gene and four a combination of ermB- and mefA-like genes. Four of the five isolates with a dissociated MLSB phenotype contained ermB-like and one a combination of ermB- and mefA-like genes. All the 14 isolates with an M phenotype contained mefA-like genes. The pristinamycin-resistant strain had G105 and A108 substitutions in the conserved C terminus of the L22 ribosomal protein. CONCLUSIONS: The prevalence of macrolide resistance is high in S. mitis from neutropenic patients and is due to the spread of ermB- or mefA-like genes alone or combined. Resistance to streptogramins is rare and in this case associated with ribosomal mutation.

An engineered epigenetic transgene switch in mammalian cells.

In multicellular systems cell identity is imprinted by epigenetic regulation circuits, which determine the global transcriptome of adult cells in a cell phenotype-specific manner. By combining two repressors, which control each other's expression, we have developed a mammalian epigenetic circuitry able to switch between two stable transgene expression states after transient administration of two alternate drugs. Engineered Chinese hamster ovary cells (CHO-K1) showed toggle switch-specific expression profiles of a human glycoprotein in culture, as well as after microencapsulation and implantation into mice. Switch dynamics and expression stability could be predicted with mathematical models. Epigenetic transgene control through toggle switches is an important tool for engineering artificial gene networks in mammalian cells.

Drug skin tests in cutaneous adverse drug reactions to pristinamycin: 29 cases with a study of cross-reactions between synergistins.

The present study was made to determine the value of drug skin tests in patients with cutaneous adverse drug reactions (CADRs) due to a synergistin (pristinamycin) and to determine the frequency of cross-reactions between synergistins. 29 patients were referred during the onset of the CADR due to pristinamycin: 18 with maculopapular rash, 9 erythrodermas, 1 angioedema and 1 Stevens-Johnson syndrome. They all had patch tests with pristinamycin and, in most cases, with other synergistins [virginiamycin and dalfopristin-quinupristin (DQ)], prick tests (10 cases) and intradermal tests (IDT) (5 cases). Skin tests with synergistins were positive in 27 cases, patch tests with pristinamycin in 20/29 cases (69%), prick tests with pristinamycin in 3/9 cases on immediate (1 case) or on delayed (2 cases) readings, and IDT with DQ in 4/5 cases. Cross-reactions between synergistins occurred in 9/22 with virginiamycin and in 7/8 cases with DQ. Skin tests with synergistins are useful in investigating CADR due to pristinamycin. Synergistins are composed of 2 chains (1 depsipeptide and 1 macrocyclic lactone) with many structural analogies between all synergistins. According to the chemical structures and our results, it seems advisable to avoid all synergistins in patients with CADR due to pristinamycin.

Microbiology of mandibular third molar pericoronitis: incidence of beta-lactamase-producing bacteria.

OBJECTIVES: The purpose of this study was to evaluate the predominant flora associated with pericoronitis in third molars and to investigate the presence of beta-lactamase-producing strains. STUDY DESIGN: The third molars in 26 adults were evaluated by cultures with nonselective media and with selective media containing amoxicillin, pristinamycin, spiramycin, metronidazole, and spiramycin plus metronidazole. RESULTS: In the majority of cases (19/26), the flora found in an anaerobic atmosphere predominated. Obligate anaerobes were present in 21 of the 26 samples. The bacteria most commonly detected were alpha-hemolytic streptococci (26/26) and the genera Prevotella (15/26), Veillonella (15/26), Bacteroides (9/26), and Capnocytophaga (9/26). Amoxicillin and pristinamycin were the most active in reducing the anaerobic cultivable counts. beta-Lactamase-producing strains were detected in 9 samples and were mostly bacteria of the genera Prevotella, Staphylococcus, and Bacteroides. CONCLUSIONS: These results highlight (1) the diversity of the microflora associated with pericoronitis and the importance of the anaerobic flora and (2) the existence of selection pressure related to the use of beta-lactams that may culminate in failure of prescribed penicillins.

Bidirectional expression units enable streptogramin-adjustable gene expression in mammalian cells.

Serious initiatives in gene therapy and tissue engineering require a sophisticated molecular toolbox combining DNA transfer technologies, human-compatible transcription control systems, as well as compact and robust expression configurations. We have designed several versatile bidirectional expression cassettes that enable coadjustable expression of two desired transgenes in response to clinically licensed antibiotics of the streptogramin class (pristinamycin, Pyostacin, Synercid). The bidirectional expression modules consist of a central operator (PIR) that is specific for the pristinamycin-dependent transactivator (PIT). Streptogramin-adjustable binding of PIT to PIR transactivates two divergently oriented promoters and initiates transcription of the desired transgenes. The bidirectional expression module can be equipped with different minimal promoters and configured for expression of (1) two functional effector genes, (2) one effector gene and a reporter gene, (3) PIT and an effector gene to form a highly compact one-vector expression arrangement. We have validated the streptogramin-adjustable bidirectional expression technology in different basic and autoregulated expression configurations in a variety of mammalian and human cell lines.