Tetracycline diffusion through phospholipid bilayers and binding to phospholipids

Bioavailability of tetracyclines is substantially increased by administration of cyclosporine A, a non-specific efflux-pump blocker

OBJECTIVES

To investigate how cyclosporine A, a nonspecific efflux-pump blocker, affects the plasma concentrations and oral bioavailability of tigecycline, oxytetracycline, chlortetracycline, doxycycline, minocycline, and tetracycline.

METHODS

Broiler chickens were used as an animal model. The tetracyclines (10 mg/kg BW) were administered intravenously, orally, and orally with cyclosporine A (50 mg/kg BW; administration: oral or intravenous). After administration, plasma samples were taken, and their concentrations of tetracyclines were measured using high-performance liquid chromatography coupled with tandem mass spectrometry. For pharmacokinetic analyses of mean plasma concentrations versus time, compartmental and non-compartmental analyses were used.

RESULTS

After oral administration of the tetracyclines, cyclosporine A administration (oral or intravenous) significantly (P < 0.05) increased the plasma concentrations, the bioavailability, the maximum plasma concentration, and the area under the curve of all the tetracyclines. Interestingly, the bioavailability of the tetracyclines was around two times higher after orally administering cyclosporine A than after intravenously administering it (P < 0.05).

CONCLUSIONS

Cyclosporine A administration increases the plasma concentrations of orally administered tetracyclines. Although cyclosporine A also inhibits renal and hepatic clearance, these results strongly suggest that efflux pumps in the intestinal epithelium are involved in the regulation of tetracycline absorption from the gastrointestinal tract.

Tetracycline accumulation in biofilms enhances the selection pressure on Escherichia coli for expression of antibiotic resistance

Microorganisms are present as either biofilm or planktonic species in natural and engineered environments. Little is known about the selection pressure emanating from exposure to sub-minimal inhibitory concentration of antibiotics on planktonic vs. biofilm bacteria. In this study, an E. coli bioreporter was used to develop biofilms on glass and high-density polyethylene (HDPE) surfaces, and compared with the corresponding planktonic bacteria in antibiotic resistance expression when exposed to a range of μg/L levels of tetracycline. The antibiotic resistance-associated fluorescence emissions from biofilm E. coli reached up to 1.6 times more than those from planktonic bacteria. The intensively developed biofilms on glass surfaces caused the embedded bacteria to experience higher selection pressure and express more antibiotic resistance than those on HDPE surfaces. The temporal pattern of fluorescence emissions from biofilm E. coli was consistent with the biofilm-developing processes during the experimental period. The increased expression of antibiotic resistance from biofilm bacteria could be attributed to the high affinity of tetracycline with extracellular polymeric substances (EPS). The enhanced accumulation of tetracycline in biofilms could exert higher selection pressure on the embedded bacteria. These results suggest that in many natural and engineered systems the higher antibiotic resistance in biofilm bacteria could be attributed partially to the retention antibiotics by the EPS in biofilms.

An in vitro and ex vivo wound infection model to test topical and systemic treatment with antibiotics

AIMS

This study aimed to develop a wound infection model that could be used to test antibiotic-loaded electrospun matrices for the topical treatment of infected skin and compare the effectiveness of this treatment to systemically applied antibiotics.

METHODS AND RESULTS

3D-printed flow chambers were made in which Staphylococcus aureus biofilms were grown either on a polycarbonate membrane or explanted porcine skin. The biofilms were then treated either topically, by placing antibiotic-loaded electrospun matrices on top of the biofilms, or systemically by the addition of antibiotics in the growth medium that flowed underneath the membrane or skin. The medium that was used was either a rich medium or an artificial wound fluid. The results showed that microbial viability in the biofilms was reduced to a greater extent with the topical electrospun matrices when compared to systemic treatment.

CONCLUSIONS

An ex vivo infection model was developed that is flexible and can be used to test both topical and systemic treatment of wound infections. It represents a significant improvement over previous in vitro models that we have used to test electrospun membranes.

SIGNIFICANCE AND IMPACT OF THE STUDY

The availability of a relatively simple wound infection model in which different delivery methods and dosage regimes can be tested is beneficial for the development of improved treatments for wound infections.

Efficient splicing-based RNA regulators for tetracycline-inducible gene expression in human cell culture and C. elegans

Synthetic riboswitches gain increasing interest for controlling transgene expression in diverse applications ranging from synthetic biology, functional genomics, and pharmaceutical target validation to potential therapeutic approaches. However, existing systems often lack the pharmaceutically suited ligands and dynamic responses needed for advanced applications. Here we present a series of synthetic riboswitches for controlling gene expression through the regulation of alternative splicing. Placing the 5′-splice site into a stem structure of a tetracycline-sensing aptamer allows us to regulate the accessibility of the splice site. In the presence of tetracycline, an exon with a premature termination codon is skipped and gene expression can occur, whereas in its absence the exon is included into the coding sequence, repressing functional protein expression. We were able to identify RNA switches controlling protein expression in human cells with high dynamic ranges and different levels of protein expression. We present minimalistic versions of this system that circumvent the need to insert an additional exon. Further, we demonstrate the robustness of our approach by transferring the devices into the important research model organism Caenorhabditis elegans, where high levels of functional protein with very low background expression could be achieved.

A convolutional neural network for the prediction and forward design of ribozyme-based gene-control elements

Ribozyme switches are a class of RNA-encoded genetic switch that support conditional regulation of gene expression across diverse organisms. An improved elucidation of the relationships between sequence, structure, and activity can improve our capacity for de novo rational design of ribozyme switches. Here, we generated data on the activity of hundreds of thousands of ribozyme sequences. Using automated structural analysis and machine learning, we leveraged these large data sets to develop predictive models that estimate the in vivo gene-regulatory activity of a ribozyme sequence. These models supported the de novo design of ribozyme libraries with low mean basal gene-regulatory activities and new ribozyme switches that exhibit changes in gene-regulatory activity in the presence of a target ligand, producing functional switches for four out of five aptamers. Our work examines how biases in the model and the data set that affect prediction accuracy can arise and demonstrates that machine learning can be applied to RNA sequences to predict gene-regulatory activity, providing the basis for design tools for functional RNAs.

Hysteretic Genetic Circuit for Detection of Proteasomal Degradation in Mammalian Cells

Synthetic hysteretic mammalian gene circuits generating sustained cellular responses to transient perturbations provide important tools to investigate complex cellular behaviors and reprogram cells for a variety of applications, ranging from protein production to cell fate decisions. The design rules of synthetic gene circuits with controlled hysteretic behaviors, however, remain uncharacterized. To identify the criteria for achieving predictable control of hysteresis, we built a genetic circuit for detection of proteasomal degradation (Hys-Deg). The Hys-Deg circuit is based on a tetracycline-controlled transactivator (tTA) variant engineered to interface with the ubiquitin proteasome system (UPS). The tTA variant activates its own expression, generating a positive feedback loop that is triggered by expression of another tTA gene that is constitutively regulated. Guided by predictive modeling, we characterized the hysteretic response of the Hys-Deg circuit. We demonstrated that control of the hysteretic response is achieved by modulating the ratio of expression of constitutive to inducible tTA. We also showed that the system can be finely tuned through dosage of the inducer tetracycline to calibrate the circuit for detection of the desired levels of UPS activation. This study establishes the design rules for building a hysteretic genetic circuit with an autoregulatory feedback loop and provides a synthetic memory module that could be easily integrated into regulatory gene networks to study and engineer complex cellular behaviors.

Investigation of chlorophyll degradation by tetracycline

Antibiotics represent a novel type of environment pollutants which modify chlorophyll content in plants. Spectroscopic methods were employed to investigate the effect of tetracycline on chlorophyll degradation. Changes in absorbance and fluorescence demonstrated that tetracycline reaction with chlorophyll results in the formation of pheophytin, which was confirmed by new bands typical of pheophytin which appeared in the absorbance spectrum. The rate of pheophytin formation depended on ratio tetracycline to chlorophyll concentration in solution. In solutions with chlorophyll concentration of C = 1 × 10^-5 M and tetracycline concentrations of C = 1 × 10^-3 M and C = 1 × 10^-2 M, pheophytin was formed after 28 h and 25 min, respectively. The obtained lifetime for pheophytin formed during chlorophyll reaction - with tetracycline hydrochloride was τ = 5.71 ± 0.02 ns and its value coincides, within the error limits, with the value obtained for pure pheophytin purchased from ChromaDex. The experiment demonstrated two mechanisms of chlorophyll degradation to pheophytin by tetracycline hydrochloride, i.e. 1) loss of Mg²⁺ ions from the chlorophyll molecule as a result of the presence of H⁺ ions in solution (i.e. as a result of medium acidification), and 2) removal of Mg²⁺ ions directly from chlorophyll by tetracycline which binds Mg²⁺ ions from the chlorophyll. We demonstrated that magnesium occurring in low concentrations attached to a tetracycline molecule in the BCD ring, and that the second ion of Mg2+ may attach to the A ring of tetracycline at higher Mg2+ concentrations. Two fluorescence bands appeared which indicated such magnesium attachments indeed occurred.

Tetracycline in Combination with Sodium Dioctylsulfosuccinate Show Increased Antimicrobial Activity in Resistant Microorganisms

We present evidence that sodium dioctylsulfosuccinate, at non inhibitory concentrations (1000 mg/L), is able to increase the antimicrobial activity of tetracycline in non susceptible bacterial and fungal strains. In culture inhibition tests, pretreatment with sodium dioctylsulfosuccinate caused a 52-fold decrease in the geometric mean MIC to tetracycline in 10 Candida albicans strains (p<0.01), a 165-fold decrease in the geometric mean MIC to tetracycline in 10 E. coli strains (p<0.001) and a significant decrease in the mean MIC of 3 strains of Candida krusei and Candida glabrata. In microbicidal tests, tetracycline in association with sodium dioctylsulfosuccinate killed 10(4) cfu tetracycline-resistant Candida albicans in 15 min and 10(4) CFU resistant E. coli in 3 min (p<0.001). Furthermore, in the N-acetyl-D-glucosamine test to calculate the number of hyphal cells, a combination of tetracycline (50 mg/L) (non inhibitory concentrations) plus sodium dioctylsulfosuccinate (25 mg/L) caused a 50-fold increase in the inhibition of hyphal cells in C. albicans (p<0.001); C. albicans cells treated with tetracycline plus sodium dioctylsolfosuccinate annulled the cell surface hydrophobicity (p<0.001). This increase in antimicrobial activity may be attributed to impairment and alteration of the membrane barrier within the microorganisms and a depletion of the thiol groups (p<0.001) critical to their efficiency.

Inducible expression of transmembrane proteins on bacterial magnetic particles in Magnetospirillum magneticum AMB-1

Bacterial magnetic particles (BacMPs) produced by the magnetotactic bacterium Magnetospirillum magneticum AMB-1 are used for a variety of biomedical applications. In particular, the lipid bilayer surrounding BacMPs has been reported to be amenable to the insertion of recombinant transmembrane proteins; however, the display of transmembrane proteins in BacMP membranes remains a technical challenge due to the cytotoxic effects of the proteins when they are overexpressed in bacterial cells. In this study, a tetracycline-inducible expression system was developed to display transmembrane proteins on BacMPs. The expression and localization of the target proteins were confirmed using luciferase and green fluorescent protein as reporter proteins. Gene expression was suppressed in the absence of anhydrotetracycline, and the level of protein expression could be controlled by modulating the concentration of the inducer molecule. This system was implemented to obtain the expression of the tetraspanin CD81. The truncated form of CD81 including the ligand binding site was successfully displayed at the surface of BacMPs by using Mms13 as an anchor protein and was shown to bind the hepatitis C virus envelope protein E2. These results suggest that the tetracycline-inducible expression system described here will be a useful tool for the expression and display of transmembrane proteins in the membranes of BacMPs.

Tetracycline regulated systems in functional oncogenomics

The increasing number of proteomic and DNA-microarray studies is continually providing a steady acquisition of data on the molecular abnormalities associated with human tumors. Rapid translation of this accumulating biological information into better diagnostics and more effective cancer therapeutics in the clinic depends on the use of robust function-testing strategies. Such strategies should allow identification of molecular lesions that are essential for the maintenance of the transformed phenotype and enable validation of potential drug-targets. The tetracycline regulated gene expression/ suppression systems (Tet-systems) developed and optimized by bioengineers over recent years seem to be very well suited for the function-testing purposes in cancer research. We review the history and latest improvements in Tet-technology in the context of functional oncogenomics.

Ultrasensitivity and noise propagation in a synthetic transcriptional cascade

The precise nature of information flow through a biological network, which is governed by factors such as response sensitivities and noise propagation, greatly affects the operation of biological systems. Quantitative analysis of these properties is often difficult in naturally occurring systems but can be greatly facilitated by studying simple synthetic networks. Here, we report the construction of synthetic transcriptional cascades comprising one, two, and three repression stages. These model systems enable us to analyze sensitivity and noise propagation as a function of network complexity. We demonstrate experimentally steady-state switching behavior that becomes sharper with longer cascades. The regulatory mechanisms that confer this ultrasensitive response both attenuate and amplify phenotypical variations depending on the system's input conditions. Although noise attenuation allows the cascade to act as a low-pass filter by rejecting short-lived perturbations in input conditions, noise amplification results in loss of synchrony among a cell population. The experimental results demonstrating the above network properties correlate well with simulations of a simple mathematical model of the system.

Gene regulation by tetracyclines

Gene regulation by tetracyclines has become a widely-used tool to study gene functions in pro- and eukaryotes. This regulatory system originates from Gram-negative bacteria, in which it fine-tunes expression of a tetracycline-specific export protein mediating resistance against this antibiotic. This review attempts to describe briefly the selective pressures governing the evolution of tetracycline regulation, which have led to the unique regulatory properties underlying its success in manifold applications. After discussing the basic mechanisms we will present the large variety of designed alterations of activities which have contributed to the still growing tool-box of components available for adjusting the regulatory properties to study gene functions in different organisms or tissues. Finally, we provide an overview of the various experimental setups available for pro- and eukaryotes, and touch upon some highlights discovered by the use of tetracycline-dependent gene regulation.

Gene regulation by tetracyclines. Constraints of resistance regulation in bacteria shape TetR for application in eukaryotes

The Tet repressor protein (TetR) regulates transcription of a family of tetracycline (tc) resistance determinants in Gram-negative bacteria. The resistance protein TetA, a membrane-spanning H+-[tc.M]+ antiporter, must be sensitively regulated because its expression is harmful in the absence of tc, yet it has to be expressed before the drugs' concentration reaches cytoplasmic levels inhibitory for protein synthesis. Consequently, TetR shows highly specific tetO binding to reduce basal expression and high affinity to tc to ensure sensitive induction. Tc can cross biological membranes by diffusion enabling this inducer to penetrate the majority of cells. These regulatory and pharmacological properties are the basis for application of TetR to selectively control the expression of single genes in lower and higher eukaryotes. TetR can be used for that purpose in some organisms without further modifications. In mammals and in a large variety of other organisms, however, eukaryotic transcriptional activator or repressor domains are fused to TetR to turn it into an efficient regulator. Mechanistic understanding and the ability to engineer and screen for mutants with specific properties allow tailoring of the DNA recognition specificity, the response to inducer tc and the dimerization specificity of TetR-based eukaryotic regulators. This review provides an overview of the TetR properties as they evolved in bacteria, the functional modifications necessary to transform it into a convenient, specific and efficient regulator for use in eukaryotes and how the interplay between structure--function studies in bacteria and specific requirements of particular applications in eukaryotes have made it a versatile and highly adaptable regulatory system.

Combining molecular modeling with experimental methodologies: mechanism of membrane permeation and accumulation of ofloxacin

The interaction between ofloxacin, as a model drug of the fluoroquinolone class, and biomembranes was examined as the possible initial step in a transmembrane diffusion process. Dipalmitoylphosphatidylcholine was used for the preparation of biomembrane models. The influence of environmental conditions and protonation on molecular physicochemical behavior, and hence on the membrane interaction, was investigated by differential scanning calorimetry (DSC). This technique has been shown to be very effective in the interpretation of interactions of drug microspeciations with biomembranes. These findings suggest that the interaction occurred owing to ionic and hydrophobic forces showing how the passage through the membrane is mainly favored in the pH interval 6-7.4. It was demonstrated that a pH gradient through model membranes may be responsible for a poorly homogeneous distribution of ofloxacin (or other related fluoroquinolones), which justifies the in vivo accumulation properties of this drug. DSC experiments, which are in agreement with computational data, also showed that the complexing capability of ofloxacin with regard to Mg(++) or Ca(++) may govern the drug entrance into bacterial cells before the DNA Girase inhibition and could ensure the formation of hydrophobic and more fluid phospholipid domains on the surface of the model membrane. These regions are more permeable with regard to various solutes, as well as ofloxacin, allowing a so-called 'self-promoted entrance pathway'. The combination of experimental methodologies with computational data allowed a further rationalization of the results and opened new perspectives into the mechanism of action of ofloxacin, namely its interaction with lipid bilayers and drug-divalent cation complex formation, which might be extended to the entire fluoroquinolone class. Ofloxacin accumulation within Escherichia coli ATCC 25922 was measured as a function of time. Also in this example, the environmental conditions influenced ofloxacin penetration and accumulation. The in vitro experiments, reported here, show that a suitable balance of hydrophilic and hydrophobic fluoroquinolone properties needs to occur for there to be increased drug permeation.

Permeation of tetracyclines through membranes of liposomes and Escherichia coli

Uptake of tetracycline (tc), 2-tetracyclinonitrile (CN-tc), and 9-(N, N-dimethylglycylamido)-6-demethyl-6-deoxytetracycline (DMG-DMDOT) by liposomes containing Tet repressor (TetR) and by Escherichia coli cells overexpressing TetR was examined. TetR specifically binds to tetracyclines, enhances their fluorescence and thereby allows selective detection of tetracyclines that have crossed the membranes. Analysis of the diffusion of tc and DMG-DMDOT into liposomes yielded permeation coefficients of (2.4 +/- 0.6) x 10-9 cm.s-1 and (3.3 +/- 0.8) x 10-9 cm.s-1, respectively. Similar coefficients were obtained for uptake of these tetracyclines by E. coli, indicating that diffusion through the cytoplasmic membrane is the rate-limiting step. The permeation coefficients translate into half-equilibration times of approximately 35 +/- 15 min and explain how efflux pumps can mediate resistance against tetracyclines. Furthermore, diffusion of CN-tc into liposomes was at least 400-fold slower than that of tc, indicating that the carboxamide group at position C2 is required for efficient permeation of tc through lipid membranes and thereby explaining the lack of antibiotic activity of CN-tc.

A recombinant Escherichia coli sensor strain for the detection of tetracyclines

A bioluminescent Escherichia coli K-12 strain for the specific detection of the tetracycline group of antibiotics is described. A sensor plasmid, containing five genes from bacterial luciferase operon of Photorhabdus luminescens inserted under the control of tetracycline-responsive elements of the transposon Tn10, was constructed. Usage of the full-length luciferase operon in the sensor resulted in tetracycline-dependent light production without additions, i.e., self-luminescent phenotype, since all the substrates were intrinsically produced by the recombinant organism. The time needed for optimal induction of light emission was 90 min. Maximal induction of approximately 100-fold over uninduced levels by using 20 ng of tetracycline, and picomole sensitivities for the seven different tetracyclines tested, were obtained without added Mg2+ ions. The higher the pH and the magnesium ion concentration in the assay medium the higher was the amount of membrane-impermeable tetracycline-Mg2+ chelate complex. In consequence, by adjusting the pH and the Mg2+ ion concentration, the sensitivity of the assay can be modified for different analytical purposes. Different non-tetracycline antibiotics did not cause induction of light emission.

Partition and distribution coefficients of solutes and drugs in brush border membrane vesicles

Partition and distribution coefficients (log P, log D) into rat small intestinal brush border membrane (BBM) were measured for a variety of ionizable and non-ionizable drugs and solutes using a novel technique. The log P values were compared with those determined with model solvents, octanol and propylene glycol dipelargonate (PGDP). Non-ionizable solutes with log P values up to 3.0 showed that octanol was a better model for partition into the BBM than PGDP. With one exception, BBM partition coefficients of greater than 3 were not observed, even for solutes with log P values in model solvents that were greater than 5. Liposomes prepared from BBM lipids, or synthetic lipid mixtures of similar composition to BBM, demonstrated similar trends in partition coefficients to the intact BBM. Two cationic drugs, Atenolol and Xamoterol were investigated for partition into BBM lipid liposomes. An apparent enhancement of log D with respect to octanol was attributed to a "surfactant-like" orientation in the membrane and an interaction of the ionized drug with anionic phospholipid head groups. The anionic drug Proxicromil shows the expected decrease in log D with increasing pH, at low NaCl concentrations. Changes in electrophoretic mobility of liposomes after incorporation of Proxicromil into them were consistent with the negative charge of the ionized drug being at the membrane surface. It was concluded that Proxicromil also associates with membranes in a "surfactant-like" orientation and that increased extraction with increasing NaCl concentrations is a result of ionic strength effects. Partition of solutes into BBM vesicles is more complex than into organic solvents and probably represents an important step in overall intestinal permeation of solutes.

Lack of evidence for a saturable tetracycline transport system in Staphylococcus aureus

Previous studies on tetracycline transport into Staphylococcus aureus identified a high-affinity, saturable uptake system for the antibiotic (Km, 4.76 microM) (B.L. Hutchings, Biochim. Biophys. Acta 174:734-738, 1969). However, the earlier results could not be confirmed using conditions that permitted energy-dependent, concentrative uptake of tetracycline. Kinetic artifacts introduced by inappropriate washing procedures may explain the previous results.

Fully automated high-performance liquid chromatographic analysis of whole blood and plasma samples using on-line dialysis as sample preparation. Determination of oxytetracycline in bovine and salmon whole blood and plasma

A fully automated technique for high-performance liquid chromatographic analysis of whole blood and plasma is described. Samples are automatically injected into a dialyser where proteins and blood cells are removed. The dialysates are concentrated on a small column prior to analysis. This technique is used for the determination of oxytetracycline in whole blood and plasma. After dialysis oxytetracycline and the internal standard, tetracycline, are retained on a polystyrene enrichment column and subsequently separated on a polystyrene analytical column by ion-pair chromatography. Using ultraviolet detection 50 ng/ml can be detected. Validation showed good within-day and between-day accuracy and precision. Different oxytetracycline concentrations were found in plasma and whole blood. This difference varied between the species.

Delta pH-dependent accumulation of tetracycline in Escherichia coli

The effects of ionophores on tetracycline accumulation in Escherichia coli cells were investigated in the presence of polymyxin B nonapeptide. Accumulation was inhibited by nigericin but not by valinomycin. Tetracycline accumulation was stimulated by decreasing the pH of the medium and inhibited by the addition of magnesium ions. These results indicated that tetracycline enters cells through diffusion as a protonated form (TH2) and is accumulated as a membrane-impermeable magnesium-tetracycline chelate complex (THMg+). This noncarrier diffusion hypothesis was confirmed by the fact that tetracycline accumulated in protein-free liposomes through an artificially imposed pH difference.

Overproduction of transposon Tn10-encoded tetracycline resistance protein results in cell death and loss of membrane potential

High-level expression of the Tn10 tetracycline resistance protein TetA in Escherichia coli caused partial collapse of the membrane potential, arrest of growth, and killing of the cells. Since alpha-methylglucoside transport was not affected, the overproduced TetA protein may cause not destruction of membrane structure but rather unrestricted translocation of protons and/or ions across the membrane.

Preparation and use of liposomes in the treatment of microbial infections

The potential application of liposomes to drug delivery has been apparent since 1965, when these phospholipid vesicles were first described by Bangham. Since then, experiments on animals have shown that liposome encapsulation can dramatically alter the distribution of drugs in the body and their rate of clearance. These pharmacokinetic differences, as well as other less well-understood effects, can result in reduced toxicity and enhanced efficacy of the encapsulated drug. The vast majority of studies on the therapeutic use of liposomes have involved the delivery of drugs used in cancer chemotherapy and metabolic storage diseases, but there is now more literature on the use of liposomes for the delivery of antimicrobial drugs and immunomodulating agents. This review briefly discusses the general properties of liposomes and the rationale for their use in antimicrobial drug delivery and immunomodulation, as well as the encapsulation of specific agents and the effect of encapsulation on the treatment of infectious diseases.

Molecular mechanisms involved in the transport of antibiotics into bacteria

Many clinically useful antibacterial drugs have intracellular target sites. Therefore, in order to reach their targets, these compounds must be able to cross bacterial outer and cytoplasmic membranes. Considerable information is available on the mechanisms by which antibiotics cross bacterial membranes and, in many cases, it is now possible to define the molecular basis of their uptake. Passage of drugs across the outer membrane of Gram-negative bacteria can occur by diffusion through porin channels (e.g. beta-lactams and tetracyclines), by facilitated diffusion using specific carriers (e.g. albomycin), or by self-promoted uptake (e.g. aminoglycosides and polymyxins). Transfer of antibiotics across the bacterial cytoplasmic membrane is usually mediated by active, carrier-mediated, transport systems normally operating to transport essential solutes into the cell. For example, the antibiotic streptozotocin bears sufficient structural resemblance to N-acetyl-D-glucosamine to be transported by the phosphoenolpyruvate:phosphotransferase system, and D-cycloserine is recognized by the D-alanine, proton motive force dependent transport system. However, in some cases (e.g. tetracycline) although carrier-mediated transport is implied by the observation that drug uptake is energy dependent, the nature of the membrane carrier(s) responsible is unknown. Knowledge acquired from studies on bacterial peptide transport has been successfully used to deliver (or smuggle) amino acid mimetics disguised as peptides into the bacterial cell. These amino acid mimetics, although often poorly transported in their own right, are frequently potent inhibitors of bacterial peptidoglycan or lipopolysaccharide synthesis once they have gained access to the interior of the cell.

Tetracycline uptake by susceptible Escherichia coli cells

Experiments measuring the initial uptake of commercial (3H) tetracycline exhibit two distinct kinetic phases: a rapid phase followed by a slow phase. (3H) tetracycline purified by chromatography on a Dowex 50WX2 column exhibited only monophasic rapid uptake when tested with susceptible Escherichia coli cells. Cyanide inhibited the uptake of purified (3H) tetracycline only partially while transport of proline and maltose was entirely abolished. Energy independent accumulation of tetracycline may be accounted for by binding to cellular constituents. Uptake of tetracycline--as measured by inhibition of beta-galactosidase synthesis--was strongly affected by a shift in temperature from 37 degrees C to 21 degrees C while carrier-mediated transport systems revealed only minor reductions. Taken together with the non-saturability of tetracycline uptake and the evidence for diffusion of tetracycline through phospholipid bilayers [Argast and Beck (1984) Antimicrob Agents Chemother 26:263-265] these data support the hypothesis that tetracycline enters the cytoplasm by diffusion.