Pharmacodynamic effects of subinhibitory antibiotic concentrations

The Mechanism of Tigecycline Resistance in Acinetobacter baumannii Revealed by Proteomic and Genomic Analysis

The mechanism of tigecycline resistance in A. baumannii remains largely unclear. In this study, we selected a tigecycline-resistant and a tigecycline-susceptible strain from a tigecycline-susceptible and a resistant strain, respectively. Proteomic and genomic analyses were performed to elucidate the variations associated with tigecycline resistance. Our study showed proteins associated with efflux pump, biofilm formation, iron acquisition, stress response, and metabolic ability are upregulated in tigecycline resistant strains, and efflux pump should be the key mechanism for tigecycline resistance. By genomic analysis, we found several changes in the genome that can explain the increased level of efflux pump, including the loss of the global negative regulator hns in the plasmid and the disruption of the hns gene and acrR gene on the chromosome by the insertion of IS5. Collectively, we not only revealed the phenomenon that the efflux pump is mainly responsible for tigecycline resistance, but also highlighted the mechanism at the genomic level, which will help in understanding the resistance mechanism in detail and provide clues for the treatment of clinical multiple drug-resistant A. baumannii.

Design, Synthesis and Biological Evaluation of Novel Pleuromutilin Derivatives Containing 6-Chloro-1-R-1H-pyrazolo[3,4-d]pyrimidine-4-amino Side Chain

Two series of pleuromutilin derivatives were designed and synthesized as inhibitors against Staphylococcus aureus (S. aureus). 6-chloro-4-amino-1-R-1H-pyrazolo[3,4-d]pyrimidine or 4-(6-chloro-1-R-1H-pyrazolo[3,4-d]pyrimidine-4-yl)amino-phenylthiol were connected to pleuromutilin. A diverse array of substituents was introduced at the N-1 position of the pyrazole ring. The in vitro antibacterial activities of these semisynthetic derivatives were evaluated against two standard strains, Methicillin-resistant Staphylococcus aureus (MRSA) ATCC 43300, Staphylococcus aureus (S. aureus), ATCC 29213 and two clinical S. aureus strains (144, AD3) using the broth dilution method. Compounds 12c, 19c and 22c (MIC = 0.25 μg/mL) manifested good in vitro antibacterial ability against MRSA which was similar to that of tiamulin (MIC = 0.5 μg/mL). Among them, compound 22c killed MRSA in a time-dependent manner and performed faster bactericidal kinetics than tiamulin in time-kill curves. In addition, compound 22c exhibited longer PAE than tiamulin, and showed no significant inhibition on the cell viability of RAW 264.7, Caco-2 and 16-HBE cells at high doses (≤8 μg/mL). The neutropenic murine thigh infection model study revealed that compound 22c displayed more effective in vivo bactericidal activity than tiamulin in reducing MRSA load. The molecular docking studies indicated that compound 22c was successfully localized inside the binding pocket of 50S ribosomal, and four hydrogen bonds played important roles in the binding of them.

Subinhibitory antibiotic concentrations promote the horizontal transfer of plasmid-borne resistance genes from Klebsiellae pneumoniae to Escherichia coli

Horizontal gene transfer plays an important role in the spread of antibiotic resistance, in which plasmid-mediated conjugation transfer is the most important mechanism. While sub-minimal inhibitory concentrations (sub-MIC) of antibiotics could promote conjugation frequency, the mechanism by which sub-MIC levels of antibiotics affect conjugation frequency is not clear. Here, we used Klebsiella pneumoniae SW1780 carrying the multi-drug resistance plasmid pSW1780-KPC as the donor strain, to investigate the effects of sub-MICs of meropenem (MEM), ciprofloxacin (CIP), cefotaxime (CTX), and amikacin (AK) on conjugational transfer of pSW1780-KPC from SW1780 to Escherichia coli J53. Our results showed that the transfer frequencies increased significantly by treating SW1780 strain with sub-MIC levels of MEM, CIP, CTX and AK. Transfer frequencies at sub-MIC conditions in a Galleria mellonella were significantly higher than in vitro. To investigate gene expression and metabolic effects, RT-qPCR and LC-MS-based metabolome sequencing were performed. Transcript levels of T4SS genes virB1, virB2, virB4, virB8, and conjugation-related genes traB, traK, traE, and traL were significantly upregulated by exposure to sub-MICs of MEM, CIP, CTX, and AK. Metabolome sequencing revealed nine differentially regulated metabolites. Our findings are an early warning for a wide assessment of the roles of sub-MIC levels of antibiotics in the spread of antibiotic resistance.

Oxidative Stress Response in Pseudomonas aeruginosa

Pseudomonas aeruginosa is a Gram-negative environmental and human opportunistic pathogen highly adapted to many different environmental conditions. It can cause a wide range of serious infections, including wounds, lungs, the urinary tract, and systemic infections. The high versatility and pathogenicity of this bacterium is attributed to its genomic complexity, the expression of several virulence factors, and its intrinsic resistance to various antimicrobials. However, to thrive and establish infection, P. aeruginosa must overcome several barriers. One of these barriers is the presence of oxidizing agents (e.g., hydrogen peroxide, superoxide, and hypochlorous acid) produced by the host immune system or that are commonly used as disinfectants in a variety of different environments including hospitals. These agents damage several cellular molecules and can cause cell death. Therefore, bacteria adapt to these harsh conditions by altering gene expression and eliciting several stress responses to survive under oxidative stress. Here, we used PubMed to evaluate the current knowledge on the oxidative stress responses adopted by P. aeruginosa. We will describe the genes that are often differently expressed under oxidative stress conditions, the pathways and proteins employed to sense and respond to oxidative stress, and how these changes in gene expression influence pathogenicity and the virulence of P. aeruginosa. Understanding these responses and changes in gene expression is critical to controlling bacterial pathogenicity and developing new therapeutic agents.

Clinical Biofilm Ring Test® Reveals the Potential Role of β-Lactams in the Induction of Biofilm Formation by P. aeruginosa in Cystic Fibrosis Patients

Biofilms are characterized by high tolerance to antimicrobials. However, conventional antibiograms are performed on planktonic microorganisms. Through the clinical Biofilm Ring Test^® (cBRT), initially aimed to measure the adhesion propensity of bacteria, we discerned a variable distribution of biofilm-producer strains among P. aeruginosa samples isolated from expectorations of cystic fibrosis (CF) patients. Despite a majority of spontaneous adherent isolates, few strains remained planktonic after 5 h of incubation. Their analysis by an adapted protocol of the cBRT revealed an induction of the biofilm early formation by sub-inhibitory doses of β-lactams. Microscopic observations of bacterial cultures stained with Syto 9/Propidium Iodide (PI) confirmed the ability of antimicrobials to increase either the bacterial biomass or the biovolume occupied by induced sessile cells. Finally, the cBRT and its derivatives enabled to highlight in a few hours the potential inducer property of antibiotics on bacterial adhesion. This phenomenon should be considered carefully in the context of CF since patients are constantly under fluctuating antimicrobial treatments. To conclude, assays derived from the Biofilm Ring Test^® (BRT) device, not only define efficient doses preventing biofilm formation, but could be useful for the antimicrobial selection in CF, to avoid inducer molecules of the early biofilm initiation.

A synthetic peptide sensitizes multi-drug resistant Pseudomonas aeruginosa to antibiotics for more than two hours and permeabilizes its envelope for twenty hours

BACKGROUND

Pseudomonas aeruginosa is a Gram-negative pathogen that frequently causes life-threatening infections in immunocompromised patients. We previously showed that subinhibitory concentrations of short synthetic peptides permeabilize P. aeruginosa and enhance the lethal action of co-administered antibiotics.

METHODS

Long-term permeabilization caused by exposure of multidrug-resistant P. aeruginosa strains to peptide P4-9 was investigated by measuring the uptake of several antibiotics and fluorescent probes and by using confocal imaging and atomic force microscopy.

RESULTS

We demonstrated that P4-9, a 13-amino acid peptide, induces a growth delay (i.e. post-antibiotic effect) of 1.3 h on a multidrug-resistant P. aeruginosa clinical isolate. Remarkably, when an independently P4-9-treated culture was allowed to grow in the absence of the peptide, cells remained sensitive to subinhibitory concentrations of antibiotics such as ceftazidime, fosfomycin and erythromycin for at least 2 h. We designated this persistent sensitization to antibiotics occurring in the absence of the sensitizing agent as Post-Antibiotic Effect associated Permeabilization (PAEP). Using atomic force microscopy, we showed that exposure to P4-9 induces profound alterations on the bacterial surface and that treated cells need at least 2 h of growth to repair those lesions. During PAEP, P. aeruginosa mutants overexpressing either the efflux pump MexAB-OprM system or the AmpC β-lactamase were rendered sensitive to antibiotics that are known substrates of those mechanisms of resistance. Finally, we showed for the first time that the descendants of bacteria surviving exposure to a membrane disturbing peptide retain a significant level of permeability to hydrophobic compounds, including propidium iodide, even after 20 h of growth in the absence of the peptide.

CONCLUSIONS

The phenomenon of long-term sensitization to antibiotics shown here may have important therapeutic implications for a combined peptide-antibiotic treatment because the peptide would not need to be present to exert its antibiotic enhancing activity as long as the target organism retains sensitization to the antibiotic.

Ciprofloxacin, amoxicillin, and aminoglycosides stimulate genetic and phenotypic changes in uropathogenic Escherichia coli strains

Antibiotic therapy and its consequences in bacterial and human aspects are widely investigated. Despite this, the emergence of new multidrug resistant bacteria is still a current problem. The scope of our work included the observation of changes among uropathogenic Escherichia coli strains after the treatment with a subinhibitory concentration of different antibiotics. The sensitive strains with or without virulence factors were incubated with amoxicillin, ciprofloxacin, gentamycin, or tobramycin. After each passage, the E. coli derivatives were compared to their wild types based on their susceptibility profiles, virulence genes, biofilm formations and the fingerprint profiles of PCR products amplified with using the (N)(6)(CGG)(4) primer. It turned out that antibiotics caused significant changes in the repertoire of bacterial virulence and biofilm formation, corresponding to acquired cross-resistance. The genomic changes among the studied bacteria were reflected in the changed profiles of the CGG-PCR products. In conclusion, the inappropriate application of antibiotics may cause a rapid rise of Multidrug Resistant (MDR) strains and give bacteria a chance to modulate their own pathogenicity. This phenomenon has been easily observed among uropathogenic E. coli strains and it is one of the main reasons for recurrent infections of the urinary tract.

Antimicrobials Are a Photodynamic Inactivation Adjuvant for the Eradication of Extensively Drug-Resistant Acinetobacter baumannii

The worldwide emergence of extensively drug resistant (XDR) Acinetobacter baumannii has reduced the number of antimicrobials that exert high bactericidal activity against this pathogen. This is the reason why many scientists are focusing on investigations concerning novel non-antibiotic strategies such as antimicrobial photodynamic inactivation (aPDI) or the use of antimicrobial blue light (aBL). Therefore, the aim of the current study was to screen for antimicrobial synergies of routinely used antibiotics and phototherapies, including both aPDI involving exogenously administered photosensitizing molecules, namely, rose bengal, and aBL, involving excitation of endogenously produced photoactive compounds. The synergy testing was performed in accordance with antimicrobial susceptibility testing (AST) standards, including various methodological approaches, i.e., antibiotic diffusion tests, checkerboard assays, CFU counting and the evaluation of postantibiotic effects (PAEs). We report that combining antimicrobials and aPDI/aBL treatment led to a new strategy that overcomes drug resistance in XDR A. baumannii, rendering this pathogen susceptible to various categories of antibiotics. Sublethal aPDI/aBL treatment in the presence of sub-MIC levels of antimicrobials effectively killed A. baumannii expressing drug resistance to studied antibiotics when treated with only antibiotic therapy. The susceptibility of XDR A. baumannii to a range of antibiotics was enhanced following sublethal aPDI/aBL. Furthermore, 3′-(p-aminophenyl) fluorescein (APF) testing indicated that significantly increased reactive oxygen species production upon combined treatment could explain the observed synergistic activity. This result represents a conclusive example of the synergistic activity between photodynamic inactivation and clinically used antimicrobials leading to effective eradication of XDR A. baumannii isolates and indicates a potent novel therapeutic approach.

Long Noncoding RNA SSR42 Controls Staphylococcus aureus Alpha-Toxin Transcription in Response to Environmental Stimuli

Staphylococcus aureus is a human pathogen causing a variety of diseases by versatile expression of a large set of virulence factors that most prominently features the cytotoxic and hemolytic pore-forming alpha-toxin. Expression of alpha-toxin is regulated by an intricate network of transcription factors. These include two-component systems sensing quorum and environmental signals as well as regulators reacting to the nutritional status of the pathogen. We previously identified the repressor of surface proteins (Rsp) as a virulence regulator. Acute cytotoxicity and hemolysis are strongly decreased in rsp mutants, which are characterized by decreased transcription of toxin genes as well as loss of transcription of a 1,232-nucleotide (nt)-long noncoding RNA (ncRNA), SSR42. Here, we show that SSR42 is the effector of Rsp in transcription regulation of the alpha-toxin gene, hla SSR42 transcription is enhanced after exposure of S. aureus to subinhibitory concentrations of oxacillin which thus leads to an SSR42-dependent increase in hemolysis. Aside from Rsp, SSR42 transcription is under the control of additional global regulators, such as CodY, AgrA, CcpE, and σB, but is positioned upstream of the two-component system SaeRS in the regulatory cascade leading to alpha-toxin production. Thus, alpha-toxin expression depends on two long ncRNAs, SSR42 and RNAIII, which control production of the cytolytic toxin on the transcriptional and translational levels, respectively, with SSR42 as an important regulator of SaeRS-dependent S. aureus toxin production in response to environmental and metabolic signals.IMPORTANCEStaphylococcus aureus is a major cause of life-threatening infections. The bacterium expresses alpha-toxin, a hemolysin and cytotoxin responsible for many of the pathologies of S. aureus Alpha-toxin production is enhanced by subinhibitory concentrations of antibiotics. Here, we show that this process is dependent on the long noncoding RNA, SSR42. Further, SSR42 itself is regulated by several global regulators, thereby integrating environmental and nutritional signals that modulate hemolysis of the pathogen.

Stress-Induced, Highly Efficient, Donor Cell-Dependent Cell-to-Cell Natural Transformation in Bacillus subtilis

Horizontal gene transfer (HGT) is a driving force for bacterial evolution that occurs via conjugation, transduction, and transformation. Whereas conjugation and transduction depend on nonbacterial vehicles, transformation is considered a naturally occurring process in which naked DNA molecules are taken up by a competent recipient cell. Here, we report that HGT occurred between two Bacillus subtilis strains cocultured on a minimum medium agar plate for 10 h. This process was almost completely resistant to DNase treatment and appeared to require close proximity between cells. The deletion of comK in the recipient completely abolished gene transfer, indicating that the process involved transformation. This process was also highly efficient, reaching 1.75 × 10⁶ transformants/μg DNA compared to 5.3 × 10³ and 1.86 × 10⁵ transformants/μg DNA for DNA-to-cell transformation by the same agar method and the standard two-step procedure, respectively. Interestingly, when three distantly localized chromosomal markers were selected simultaneously, the efficiency of cell-to-cell transformation still reached 6.26 × 10⁴ transformants/μg DNA, whereas no transformants were obtained when free DNA was used as the donor. Stresses, such as starvation and exposure to antibiotics, further enhanced transformation efficiency by affecting the donor cells, suggesting that stress served as an important signal for promoting this type of HGT. Taken together, our results defined a bona fide process of cell-to-cell natural transformation (CTCNT) in B. subtilis and related species. This finding reveals the previously unrecognized role of donor cells in bacterial natural transformation and improves our understanding of how HGT drives bacterial evolution at a mechanistic level.IMPORTANCE Because DNA is easily prepared, studies of bacterial natural genetic transformation traditionally focus on recipient cells. However, such laboratory artifacts cannot explain how this process occurs in nature. In most cases, competence is only transient and involves approximately 20 to 50 genes, and it is unreasonable for bacteria to spend so many genetic resources on unpredictable and uncertain environmental DNA. Here, we characterized a donor cell-dependent CTCNT process in B. subtilis and related species that was almost completely resistant to DNase treatment and was more efficient than classical natural transformation using naked DNA as a donor, i.e., DNA-to-cell transformation, suggesting that DNA donor cells were also important in the transformation process in natural environments.

Combined Antimicrobial Activity of Photodynamic Inactivation and Antimicrobials-State of the Art

Antimicrobial photodynamic inactivation (aPDI) is a promising tool for the eradication of life-threatening pathogens with different profiles of resistance. This study presents the state-of-the-art published studies that have been dedicated to analyzing the bactericidal effects of combining aPDI and routinely applied antibiotics in in vitro (using biofilm and planktonic cultures) and in vivo experiments. Furthermore, the current paper reviews the methodology used to obtain the published data that describes the synergy between these antimicrobial approaches. The authors are convinced that even though the combined efficacy of aPDI and antimicrobials could be investigated with the wide range of methods, the use of a unified experimental methodology that is in agreement with antimicrobial susceptibility testing (AST) is required to investigate possible synergistic cooperation between aPDI and antimicrobials. Conclusions concerning the possible synergistic activity between the two treatments can be drawn only when appropriate assays are employed. It must be noticed that some of the described papers were just aimed at determination if combined treatments exert enhanced antibacterial outcome, without following the standard methodology to evaluate the synergistic effect, but in most of them (18 out of 27) authors indicated the existence of synergy between described antibacterial approaches. In general, the increase in bacterial inactivation was observed when both therapies were used in combination.

Mupirocin at Subinhibitory Concentrations Induces Biofilm Formation in Staphylococcus aureus

OBJECTIVES

Mupirocin is a useful antibiotic against superficial skin infections. We compared the impact of mupirocin with a cephalosporin, a fluoroquinolone, an aminoglycoside, and a macrolide on planktonic cell growth and biofilm formation of methicillin-susceptible Staphylococcus aureus (MSSA) and methicillin-resistant S. aureus (MRSA).

MATERIALS AND METHODS

Minimum inhibitory concentration (MIC) of mupirocin was determined against S. aureus strains used in this study. Biofilm formation of S. aureus strains exposed to mupirocin was quantified by crystal violet staining assay. Moreover, biofilm structure and viability of the biofilm cells were visualized by Live/Dead staining assay. Biofilm-related gene expression was investigated by quantitative real-time PCR.

RESULTS

MRSA USA300 clone was resistant to mupirocin with MIC of 1,024 mg/L, while MRSA ATCC-43300 and MSSA ATCC-29213 were susceptible with MICs of 0.03 mg/L. Planktonic cell growth of the S. aureus strains was inhibited by mupirocin in a dose-dependent manner. However, some of the low concentrations of mupirocin less than the MICs promoted biofilm formation. Confocal laser scanning microscopy of the biofilm structures and cell viabilities showed established biofilms of slightly higher cell density in the mupirocin treated groups, especially in the MRSA USA300 clone. Gene expression of RNAIII in planktonic cells and biofilms of MRSA USA300 clone showed the highest upregulation after initial exposure to sub-MIC of mupirocin followed by downregulation, whereas the other antibiotics showed various fluctuations.

CONCLUSION

The results showed that subinhibitory concentrations of mupirocin promoted biofilm formation of S. aureus, in particular the MRSA USA300 clone.

Antibiotic-Induced Biofilm Formation

Surface-attached colonies of bacteria known as biofilms play a major role in the pathogenesis of device-related infections. Biofilm colonies are notorious for their resistance to suprainhibitory concentrations of antibiotics. Numerous studies have shown that subminimal inhibitory concentrations of some antibiotics can act as agonists of bacterial biofilm formation in vitro, a process that may have clinical relevance. This article reviews studies demonstrating that low-dose antibiotics induce bacterial biofilm formation. These studies have provided important information about the regulation of biofilm formation and the signaling pathways involved in global gene regulation in response to cell stressors. It is still unclear whether antibiotic-induced biofilm formation contributes to the inconsistent success of antimicrobial therapy for device infections.

Antibiotic Persistence as a Metabolic Adaptation: Stress, Metabolism, the Host, and New Directions

Persistence is a phenomenon during which a small fraction of a total bacterial population survives treatment with high concentrations of antibiotics for an extended period of time. In conjunction with biofilms, antibiotic persisters represent a major cause of recalcitrant and recurring infections, resulting in significant morbidity and mortality. In this review, we discuss the clinical significance of persister cells and the central role of bacterial metabolism in their formation, specifically with respect to carbon catabolite repression, sugar metabolism, and growth regulation. Additionally, we will examine persister formation as an evolutionary strategy used to tolerate extended periods of stress and discuss some of the response mechanisms implicated in their formation. To date, the vast majority of the mechanistic research examining persistence has been conducted in artificial in vitro environments that are unlikely to be representative of host conditions. Throughout this review, we contextualize the existing body of literature by discussing how in vivo conditions may create ecological niches that facilitate the development of persistence. Lastly, we identify how the development of next-generation sequencing and other “big data” tools may enable researchers to examine persistence mechanisms within the host to expand our understanding of their clinical importance.

Underestimated Collateral Effects of Antibiotic Therapy in Prosthesis-Associated Bacterial Infections

Antibiotic treatment of infections associated with the use of indwelling medical devices in ageing and/or severely ill patients represents a significant healthcare problem due to the difficulty of treating such infections and to the various collateral effects that may be observed following the often aggressive therapy We summarize some effects of antibiotics on the expression of virulence factors of the microorganisms which cause such infections. These effects, particularly those resulting in a stimulation of bacterial virulence, might be usefully included among the other well-known collateral effects of antibiotic therapy

Inhibitory effect of resin composite containing S-PRG filler on Streptococcus mutans glucose metabolism

OBJECTIVES

Resin composites containing surface pre-reacted glass-ionomer (S-PRG) fillers have been reported to inhibit Streptococcus mutans growth on their surfaces, and their inhibitory effects were attributed to BO₃^3- and F^- ions. The aim of this study was to evaluate S. mutans acid production through glucose metabolism on resin composite containing S-PRG fillers and assess inhibitory effects of BO₃^3- and F^- on S. mutans metabolic activities.

METHODS

The pH change through S. mutans acid production on experimental resin composite was periodically measured after the addition of glucose. Inhibitory effects of BO₃^3- or F^- solutions on S. mutans metabolism were evaluated by XTT assays and measurement of the acid production rate.

RESULTS

The pH of experimental resin containing S-PRG fillers was significantly higher than that of control resin containing silica fillers (p < 0.05). OD₄₅₀ values by XTT assays and S. mutans acid production rates significantly decreased in the presence of BO₃^3- and F^- compared with the absence of these ions (p < 0.05).

CONCLUSIONS

pH reduction by S. mutans acid production was inhibited on resin composite containing S-PRG fillers. Moreover, S. mutans glucose metabolism and acid production were inhibited in the presence of low concentrations of BO₃^3- or F^-.

CLINICAL SIGNIFICANCE

BO₃^3- or F^- released from resin composite containing S-PRG fillers exhibits inhibitory effects on S. mutans metabolism at concentrations lower than those which inhibit bacterial growth.

Antibacterial Activity of 7-Epiclusianone and Its Novel Copper Metal Complex on Streptococcus spp. Isolated from Bovine Mastitis and Their Cytotoxicity in MAC-T Cells

Mastitis is an inflammation of mammary gland parenchyma that adversely affects bovine health and dairy production worldwide despite significant efforts to eradicate it. The aim of this work was to characterize the antimicrobial activity of 7-epiclusianone (7-epi), a compound extracted from the Rheedia brasiliensis fruit, its complex with copper against Streptococcus spp. isolated from bovine mastitis, and to assess their cytotoxicity to bovine mammary alveolar cells (MAC-T). The complex 7-epiclusianone-Cu (7-epi-Cu) was an amorphous green solid with optical activity. Its vibrational spectrum in the infrared region showed absorption bands in the high-frequency region, as well as bands that can be attributed to the unconjugated and conjugated stretching of the free ligand. The complex was anhydrous. One of the tested bacterial strains was not sensitive to the compounds, while the other three had MIC values of 7.8 µg mL^-1 and minimum bactericidal concentration (MBC) values between 15.6 and 31.3 µg mL^-1. These two compounds are bacteriostatic, did not cause damage to the cell wall and, at sub-inhibitory concentrations, did not induce bacterial adhesion. The compounds were not cytotoxic. Based on these results, 7-epi and 7-epi-Cu exhibited desirable antimicrobial properties and could potentially be used in bovine mastitis treatment.

Bergenin Plays an Anti-Inflammatory Role via the Modulation of MAPK and NF-κB Signaling Pathways in a Mouse Model of LPS-Induced Mastitis

Mastitis is a major disease in humans and other animals and is characterized by mammary gland inflammation. It is a major disease of the dairy industry. Bergenin is an active constituent of the plants of genus Bergenia. Research indicates that bergenin has multiple biological activities, including anti-inflammatory and immunomodulatory properties. The objective of this study was to evaluate the protective effects and mechanism of bergenin on the mammary glands during lipopolysaccharide (LPS)-induced mastitis. In this study, mice were treated with LPS to induce mammary gland mastitis as a model for the disease. Bergenin treatment was initiated after LPS stimulation for 24 h. The results indicated that bergenin attenuated inflammatory cell infiltration and decreased the concentration of NO, TNF-α, IL-1β, and IL-6, which were increased in LPS-induced mouse mastitis. Furthermore, bergenin downregulated the phosphorylation of nuclear factor-kappaB (NF-κB) and mitogen-activated protein kinases (MAPK) signaling pathway proteins in mammary glands with mastitis. In conclusion, bergenin reduced the expression of NO, TNF-α, IL-1β, and IL-6 proinflammatory cytokines by inhibiting the activation of the NF-κB and MAPKs signaling pathways, and it may represent a novel treatment strategy for mastitis.

Effects of Subinhibitory Concentrations of Ceftaroline on Methicillin-Resistant Staphylococcus aureus (MRSA) Biofilms

Ceftaroline (CPT) is a novel cephalosporin with in vitro activity against Staphylococcus aureus. Ceftaroline exhibits a level of binding affinity for PBPs in S. aureus including PBP2a of methicillin-resistant S. aureus (MRSA). The aims of this study were to investigate the morphological, physiological and molecular responses of MRSA clinical strains and MRSA biofilms to sub-MICs (1/4 and 1/16 MIC) of ceftaroline by using transmission, scanning and confocal microscopy. We have also used quantitative Real-Time PCR to study the effect of sub-MICs of ceftaroline on the expression of the staphylococcal icaA, agrA, sarA and sasF genes in MRSA biofilms. In one set of experiments, ceftaroline was able to inhibit biofilm formation in all strains tested at MIC, however, a strain dependent behavior in presence of sub-MICs of ceftaroline was shown. In a second set of experiments, destruction of preformed biofilms by addition of ceftaroline was evaluated. Ceftaroline was able to inhibit biofilm formation at MIC in all strains tested but not at the sub-MICs. Destruction of preformed biofilms was strain dependent because the biofilm formed by a matrix-producing strain was resistant to a challenge with ceftaroline at MIC, whereas in other strains the biofilm was sensitive. At sub-MICs, the impact of ceftaroline on expression of virulence genes was strain-dependent at 1/4 MIC and no correlation between ceftaroline-enhanced biofilm formation and gene regulation was established at 1/16 MIC. Our findings suggest that sub-MICs of ceftaroline enhance bacterial attachment and biofilm formation by some, but not all, MRSA strains and, therefore, stress the importance of maintaining effective bactericidal concentrations of ceftaroline to fight biofilm-MRSA related infections.

Effects of antibiotics on biofilm and unattached cells of a clinical Staphylococcus aureus isolate from bone and joint infection

Treatment of orthopaedic infections remains challenging owing to the inability of antibiotics to eradicate biofilms and prevent their regrowth. The present study characterized the effects of 12 antibiotics on in vitro biofilm formed by a representative strain of meticillin-susceptible Staphylococcus aureus (MSSA) isolated from a bone infection. Determination of the minimum biofilm eradication concentrations indicated that in vitro eradication of 24 h-old biofilms required concentrations up to 51,200 times higher than MICs. The influence of the same panel of antibiotics was also investigated on biofilm formation at concentrations including the breakpoints, by numbering viable cells in the suspensions (individual cells) and the biofilm biomass. Except for fusidic acid, the presence of antibiotics during the initial steps of biofilm formation resulted in significant decreases in the number of sessile viable bacteria at the highest concentrations tested. Ceftarolin, daptomycin, fosfomycin, gentamicin, ofloxacin, rifampicin and vancomycin were the most effective drugs. Confocal microscopy analysis indicated that daptomycin was more efficient at bacteria lysis than gentamicin and vancomycin. However, viable individual cells were still detectable in the assays performed with ceftarolin, fosfomycin, ofloxacin, rifampicin and vancomycin at concentrations for which no sessile cells were detected. Although none of the molecules tested was effective at classical therapeutic concentrations against 24 h-old MSSA biofilms, all except fusidic acid were able to impair biofilm formation at concentrations near the breakpoints. However, presence of viable individual unattached cells could imply a significant risk of microbial dissemination and increased risk of infections.

In vitro evaluation of synergistic activity between ciprofloxacin and broad snouted caiman serum against Escherichia coli

The in vitro synergistic activity between ciprofloxacin and serum of broad snouted caiman on Escherichia coli was studied. The estimated MIC value of ciprofloxacin was 0.0188 µg/ml, and two assays of kill curve during 5 hours were performed: the first one in a standard culture medium and the second one in the presence of caiman serum. Different concentrations of ciprofloxacin were tested. Ciprofloxacin showed higher values of bacterial elimination rate in the presence of caiman serum in all concentrations tested. The combined activity of sub-inhibitory concentrations of ciprofloxacin and the humoral immune factors present in caiman serum determined an increase in the bacterial elimination observed in this assay. We suggest that the antibacterial activity of complement and natural antibodies present in caiman serum, which can bind to both Gram-negative and Gram-positive bacteria and acting through the classical complement pathway, can inhibit bacterial growth of Escherichia coli by lysis.

Biofilm formation on biotic and abiotic surfaces in the presence of antimicrobials by Escherichia coli Isolates from cases of bovine mastitis

Escherichia coli is a highly adaptive microorganism, and its ability to form biofilms under certain conditions can be critical for antimicrobial resistance. The adhesion of four E. coli isolates from bovine mastitis to bovine mammary alveolar (MAC-T) cells, biofilm production on a polystyrene surface, and the expression profiles of the genes fliC, csgA, fimA, and luxS in the presence of enrofloxacin, gentamicin, co-trimoxazole, and ampicillin at half of the MIC were investigated. Increased adhesion of E. coli isolates in the presence of antimicrobials was not observed; however, increased internalization of some isolates was observed by confocal microscopy. All of the antimicrobials induced the formation of biofilms by at least one isolate, whereas enrofloxacin and co-trimoxazole decreased biofilm formation by at least one isolate. Quantitative PCR analysis revealed that all four genes were differentially expressed when bacteria were exposed to subinhibitory concentrations of antimicrobials, with expression altered on the order of 1.5- to 22-fold. However, it was not possible to associate gene expression with induction or reduction of biofilm formation in the presence of the antimicrobials. Taken together, the results demonstrate that antimicrobials could induce biofilm formation by some isolates, in addition to inducing MAC-T cell invasion, a situation that might occur in vivo, potentially resulting in a bacterial reservoir in the udder, which might explain some cases of persistent mastitis in herds.

Influence of sub-inhibitory antibiotics and flow condition on Staphylococcus aureus ATCC 6538 biofilm development and biofilm growth rate: BioTimer assay as a study model

Staphylococcus biofilm exhibits high antibiotic resistance and therapeutic doses of antibiotics are often sub-inhibitory. Whereas data are available on the effect of sub-inhibitory antibiotics on matrix formation, little is known on their influence on biofilm population. Here, using BioTimer Assay (BTA), a method developed to quantify biofilm population, the influence of sub-inhibitory gentamicin, ofloxacin and azithromycin on Staphylococcus aureus ATCC 6538 biofilm population in flow with respect to static condition was assessed. Antibiotics and flow condition increased biofilm population even if at different extent, depending on the antibiotic molecule. The greatest bacterial population was found in biofilm developed under flow condition in the presence of azithromycin. A significant increase in biofilm matrix was recorded for biofilm developed in the presence of antibiotics in flow with respect to static condition. The growth rates (GRs) of 24-h biofilm developed under the influence of antibiotics and flow condition were also evaluated using BTA and a specific mathematical model. Antibiotics and flow condition affected the GRs of 24-h biofilm even if at different extent. The lowest GR value was recorded for biofilm developed under flow condition in the presence of ofloxacin. Although further studies are needed, our data indicate that antibiotics and flow condition influenced biofilm development by increasing both bacterial population and matrix formation and affected the GRs of the developed biofilm. To the best of our knowledge, BTA is unique in allowing the calculation of the GRs of biofilm and it may be considered to be a useful study model to evaluate the activity of antibiofilm molecules.

Effects of intracanal irrigant MTAD Combined with nisin at sub-minimum inhibitory concentration levels on Enterococcus faecalis growth and the expression of pathogenic genes

Exposure to antibiotics is considered to be the major driver in the selection of antibiotic-resistant bacteria and may induce diverse biological responses in bacteria. MTAD is a common intracanal irrigant, but its bactericidal activity remains to be improved. Previous studies have indicated that the antimicrobial peptide nisin can significantly improve the bactericidal activity of MTAD against Enterococcus faecalis. However, the effects of MTAD and its modification at sub-minimum inhibitory concentration (sub-MIC) levels on Enterococcus faecalis growth and the expression of pathogenic genes still need to be explored. In this study, the results of post-antibiotic effects (PAE) and post-antibiotic sub-MIC effects (PASME) showed that MTADN (nisin in combination with MTAD) had the best post-antibiotic effect. E. faecalis after challenge with MTAD was less sensitive to alkaline solutions compared with MTAN (nisin in place of doxycycline in MTAD) and MTADN. E. faecalis induced with sub-MIC of MTAD generated resistance to the higher concentration, but induction of E. faecalis with MTAN did not cause resistance to higher concentrations. Furthermore, real-time polymerase chain reaction (RT-PCR) showed that the stress caused by sub-MIC exposure to MTAD, MTAN, or MTADN resulted in up- or down-regulation of nine stress genes and four virulence-associated genes in E. faecalis and resulted in different stress states. These findings suggested that nisin improved the post-antibacterial effect of MTAD at sub-MIC levels and has considerable potential for use as a modification of MTAD.

Postantibiotic effect and postantibiotic sub-minimum inhibitory concentration effect of valnemulin against Staphylococcus aureus isolates from swine and chickens

The postantibiotic effect (PAE) and postantibiotic sub-minimum inhibitory concentration (MIC) effect (PA-SME) of valnemulin against Staphylococcus aureus were investigated in vitro using a spectrophotometric technique and classic viable count method. A standard curve was constructed by regression analysis of the number of colonies and the corresponding optical density (OD) at 630 nm of the inoculum. After exposure to valnemulin at different concentrations for an hour, the antibiotic was removed by centrifuging and washing. The PA-SMEs were measured after initial exposure to valnemulin at 4 × the MIC, and then, valnemulin was added to reach corresponding desired concentrations in the resuspended culture. Samples were collected hourly until the culture became turbid. The results were calculated by converting the OD values into the counts of bacteria in accordance with the curve. The MIC of valnemulin against eight strains was identically 0.125 μg ml(-1) . The mean PAEs were 2.12 h (1 × MIC) and 5.06 h (4 × MIC), and the mean PA-SMEs were 6.85 h (0.1 × MIC), 9.12 h (0.2 × MIC) and 10.8 h (0.3 × MIC). The results showed that the strains with identical MICs exhibited different PAEs and PA-SMEs. Valnemulin produced prolonged PAE and PA-SME periods for Staph. aureus, supporting a longer dosing interval while formulating a daily administration dosage.

SIGNIFICANCE AND IMPACT OF THE STUDY

In this study, valnemulin demonstrated prolonged postantibiotic effects and postantibiotic sub-MIC effects on strains of Staphylococcus aureus. The strains with identical MICs of valnemulin exhibited different PAEs and PA-SMEs. Staphylococcus aureus isolated from different species has little impact on the postantibiotic effect of valnemulin. The result suggests a longer dosing interval while formulating a daily administration dosage, and it may play a valuable role of valnemulin in treating Staph. aureus infections in animals.

Induction of MRSA Biofilm by Low-Dose β-Lactam Antibiotics: Specificity, Prevalence and Dose-Response Effects

Methicillin-resistant Staphylococcus aureus (MRSA) is a leading cause of hospital- and community-associated infections. The formation of adherent clusters of cells known as biofilms is an important virulence factor in MRSA pathogenesis. Previous studies showed that subminimal inhibitory (sub-MIC) concentrations of methicillin induce biofilm formation in the community-associated MRSA strain LAC. In this study we measured the ability sub-MIC concentrations of eight other β-lactam antibiotics and six non-β-lactam antibiotics to induce LAC biofilm. All eight β-lactam antibiotics, but none of the non-β-lactam antibiotics, induced LAC biofilm. The dose-response effects of the eight β-lactam antibiotics on LAC biofilm varied from biphasic and bimodal to near-linear. We also found that sub-MIC methicillin induced biofilm in 33 out of 39 additional MRSA clinical isolates, which also exhibited biphasic, bimodal and linear dose-response curves. The amount of biofilm formation induced by sub-MIC methicillin was inversely proportional to the susceptibility of each strain to methicillin. Our results demonstrate that induction of biofilm by sub-MIC antibiotics is a common phenotype among MRSA clinical strains and is specific for β-lactam antibiotics. These findings may have relevance to the use of β-lactam antibiotics in clinical and agricultural settings.

Effects of Antibiotics onPseudomonas aeruginosaNK125502 andPseudomonas fluorescensMF0 Biofilm Formation on Immobilized Fibronectin

The effect of subminimal inhibitory concentration (1/2 MIC) of antibiotics on the biofilm formation on immobilized fibronectin by Pseudomonas was investigated by examining the reference strains NK125502 P. aeruginosa and MF0 P. fluorescens in a microtiter plates assay. When the antibiotics were added during bacterial growth and biofilm development, gentamicin was the only antimicrobial agent tested which decreased significantly the biofilm formation by the two strains. Cefsulodin and chloramphenicol also decreased the P. aeruginosa biofilm development (P<0.01), whereas polymyxin B inhibited biofilm formation by P. fluorescens (p<0.05). When the antibiotics were only present during bacterial growth and not during biofilm development, gentamicin was the only antibiotic tested to decrease significantly the biofilm formation by P. aeruginosa for incubation times of 20 and 72h (P<0.01), whereas P. fluorescens was not affected. This persistent inhibition of P. aeruginosa biofilm formation may be interesting in intermittent antibiotherapy treatments.

When the most potent combination of antibiotics selects for the greatest bacterial load: the smile-frown transition

Conventional wisdom holds that the best way to treat infection with antibiotics is to 'hit early and hit hard'. A favoured strategy is to deploy two antibiotics that produce a stronger effect in combination than if either drug were used alone. But are such synergistic combinations necessarily optimal? We combine mathematical modelling, evolution experiments, whole genome sequencing and genetic manipulation of a resistance mechanism to demonstrate that deploying synergistic antibiotics can, in practice, be the worst strategy if bacterial clearance is not achieved after the first treatment phase. As treatment proceeds, it is only to be expected that the strength of antibiotic synergy will diminish as the frequency of drug-resistant bacteria increases. Indeed, antibiotic efficacy decays exponentially in our five-day evolution experiments. However, as the theory of competitive release predicts, drug-resistant bacteria replicate fastest when their drug-susceptible competitors are eliminated by overly-aggressive treatment. Here, synergy exerts such strong selection for resistance that an antagonism consistently emerges by day 1 and the initially most aggressive treatment produces the greatest bacterial load, a fortiori greater than if just one drug were given. Whole genome sequencing reveals that such rapid evolution is the result of the amplification of a genomic region containing four drug-resistance mechanisms, including the acrAB efflux operon. When this operon is deleted in genetically manipulated mutants and the evolution experiment repeated, antagonism fails to emerge in five days and antibiotic synergy is maintained for longer. We therefore conclude that unless super-inhibitory doses are achieved and maintained until the pathogen is successfully cleared, synergistic antibiotics can have the opposite effect to that intended by helping to increase pathogen load where, and when, the drugs are found at sub-inhibitory concentrations.

Low levels of β-lactam antibiotics induce extracellular DNA release and biofilm formation in Staphylococcus aureus

Subminimal inhibitory concentrations of antibiotics have been shown to induce bacterial biofilm formation. Few studies have investigated antibiotic-induced biofilm formation in Staphylococcus aureus, an important human pathogen. Our goal was to measure S. aureus biofilm formation in the presence of low levels of β-lactam antibiotics. Fifteen phylogenetically diverse methicillin-resistant Staphylococcus aureus (MRSA) and methicillin-sensitive S. aureus (MSSA) strains were employed. Methicillin, ampicillin, amoxicillin, and cloxacillin were added to cultures at concentrations ranging from 0× to 1× MIC. Biofilm formation was measured in 96-well microtiter plates using a crystal violet binding assay. Autoaggregation was measured using a visual test tube settling assay. Extracellular DNA was quantitated using agarose gel electrophoresis. All four antibiotics induced biofilm formation in some strains. The amount of biofilm induction was as high as 10-fold and was inversely proportional to the amount of biofilm produced by the strain in the absence of antibiotics. MRSA strains of lineages USA300, USA400, and USA500 exhibited the highest levels of methicillin-induced biofilm induction. Biofilm formation induced by low-level methicillin was inhibited by DNase. Low-level methicillin also induced DNase-sensitive autoaggregation and extracellular DNA release. The biofilm induction phenotype was absent in a strain deficient in autolysin (atl). Our findings demonstrate that subminimal inhibitory concentrations of β-lactam antibiotics significantly induce autolysin-dependent extracellular DNA release and biofilm formation in some strains of S. aureus.

The widespread use of antibiotics as growth promoters in agriculture may expose bacteria to low levels of the drugs. The aim of this study was to investigate the effects of low levels of antibiotics on bacterial autoaggregation and biofilm formation, two processes that have been shown to foster genetic exchange and antibiotic resistance. We found that low levels of β-lactam antibiotics, a class commonly used in both clinical and agricultural settings, caused significant autoaggregation and biofilm formation by the important human pathogen Staphylococcus aureus. Both processes were dependent on cell lysis and release of DNA into the environment. The effect was most pronounced among multidrug-resistant strains known as methicillin-resistant S. aureus (MRSA). These results may shed light on the recalcitrance of some bacterial infections to antibiotic treatment in clinical settings and the evolution of antibiotic-resistant bacteria in agricultural settings.

Post-antibiotic effect of orbifloxacin against Escherichia coli and Pseudomonas aeruginosa isolates from dogs

Orbifloxacin is a fluoroquinolone drug used widely in companion animal medicine. In this study, we firstly determined post-antibiotic effects (PAEs) and post-antibiotic sub-minimum inhibitory concentrations (MIC) effects (PA-SMEs) of orbifloxacin for two strains each of Escherichia coli and Pseudomonas aeruginosa from dogs, and these parameters were compared with those of enrofloxacin. At twice the MIC, the PAEs of orbifloxacin ranged from -0.28-0.93 h (mean, 0.29 h) for E. coli and -0.18-1.18 h (mean, 0.37 h) for P. aeruginosa. These parameters were not significantly different for E. coli and shorter for P. aeruginosa, compared to enrofloxacin (P < 0.05). Continued exposure to 0.1, 0.2, and 0.3 the MIC of orbifloxacin resulted in average PA-SMEs of 0.55, 1.11, and 2.03 h, respectively, for E. coli, and 1.04, 1.40, and 2.47 h, respectively, for P. aeruginosa. These PA-SMEs, which had no significant differences with those of enrofloxacin, were significantly longer than the corresponding PAEs (P < 0.05). These results suggest that the PA-SME of orbifloxacin for E. coli and P. aeruginosa can be meaningfully prolonged by increase of sub-MICs.

Predicting in vitro antibacterial efficacy across experimental designs with a semimechanistic pharmacokinetic-pharmacodynamic model

We have previously described a general semimechanistic pharmacokinetic-pharmacodynamic (PKPD) model that successfully characterized the time course of antibacterial effects seen in bacterial cultures when exposed to static concentrations of five antibacterial agents of different classes. In this PKPD model, the total bacterial population was divided into two subpopulations, one growing drug-susceptible population and one resting drug-insensitive population. The drug effect was included as an increase in the killing rate of the drug-susceptible bacteria with a maximum-effect (E(max)) model. The aim of the present study was to evaluate the ability of this PKPD model to describe and predict data from in vitro experiments with dynamic concentration-time profiles. Dynamic time-kill curve experiments were performed by using an in vitro kinetic system, where cultures of Streptococcus pyogenes were exposed to benzylpenicillin, cefuroxime, erythromycin, moxifloxacin, or vancomycin using different starting concentrations (2 and 16 times the MIC) and elimination conditions (human half-life, reduced half-life, and constant concentrations). The PKPD model was applied, and the observations for the static as well as dynamic experiments were compared to model predictions based on parameter estimation using (i) static data, (ii) dynamic data, and (iii) combined static and dynamic data. Differences in experimental settings between static and dynamic experiments did not affect the growth kinetics of the bacteria significantly. With parameter reestimation, the structure of our previously proposed PKPD model could well characterize the bacterial growth and killing kinetics when exposed to dynamic concentrations with different elimination rates of all five investigated antibiotics. Furthermore, the model with parameter estimates based on data from only the static time-kill curve experiments could predict the majority of the time-kill curves from the dynamic experiments reasonably well. Adding data from dynamic experiments in the estimation improved the model fit for cefuroxime and vancomycin, indicating some differences in sensitivity to experimental conditions among the antibiotics studied.

Use of pharmacodynamic principles to inform β-lactam dosing: "S" does not always mean success

Dose optimization is one of the key strategies for enhancing antimicrobial stewardship. There have been tremendous strides in our understanding of antibiotic exposure-response relationships over the past 25 years. For many antibiotics, the "pharmacodynamic" or the exposure variable associated with outcome has been identified. With advances in mathematical modeling, it is possible to apply our understanding of antimicrobial pharmacodynamics (PD) into clinical practice and design empirical regimens that have a high probability of achieving the PD target linked to effect. By optimizing antibiotic doses to achieve PD targets predictive of efficacy, clinicians can improve care and minimize drug toxicity. For β-lactams, the PD parameter most predictive of maximal bactericidal activity is the duration of time free drug concentrations remain above the minimum inhibitory concentration (MIC) during the dosing interval (fT > MIC). Unfortunately, the conventional intermittent β-lactam dosing schemes often used in practice have suboptimal PD profiles. Prolonging the infusion time of β-lactams is one method to maximize the probability of achieving concentrations in excess of the MIC for the majority of the dosing interval, especially against pathogens with elevated MIC values. Prolonged infusions of intravenous β-lactams are not only associated with improved probability of target attainment (PTA) profiles but offer possible cost savings and greater potential for reducing emergence of resistance relative to intermittent infusions.

Molecular mechanisms of compounds affecting bacterial biofilm formation and dispersal

Bacteria can switch between planktonic forms (single cells) and biofilms, i.e., bacterial communities growing on solid surfaces and embedded in a matrix of extracellular polymeric substance. Biofilm formation by pathogenic bacteria often results in lower susceptibility to antibiotic treatments and in the development of chronic infections; thus, biofilm formation can be considered an important virulence factor. In recent years, much attention has been directed towards understanding the biology of biofilms and towards searching for inhibitors of biofilm development and of biofilm-related cellular processes. In this report, we review selected examples of target-based screening for anti-biofilm agents: We focus on inhibitors of quorum sensing, possibly the most characterized target for molecules with anti-biofilm activity, and on compounds interfering with the metabolism of the signal molecule cyclic di-GMP metabolism and on inhibitors of DNA and nucleotide biosynthesis, which represent a novel and promising class of biofilm inhibitors. Finally, we discuss the activation of biofilm dispersal as a novel mode of action for anti-biofilm compounds.

[Beta-lactam antibiotics]

Beta-lactam drugs, whose mechanism of action is inhibition of the last stage of bacterial cell wall synthesis, are the largest family of antimicrobial agents and the most widely used in current clinical practice. These drugs have a slow, time-dependent bactericidal action, generally good distribution in the body, and low toxicity. Modifications of the original molecule have led to new compounds with a greater antimicrobial spectrum and activity; nonetheless, the use and efficacy of beta-lactams is limited in some clinical settings, owing to the increasing emergence of antimicrobial resistance. Despite this problem, penicillin remains the treatment of choice in a large number of infections, cephalosporins have a wide range of indications, carbapenems are used in nosocomially-acquired infection and infection caused by multiresistant microorganisms, and beta-lactam inhibitors restore the spectrum of activity of their companion penicillins (aminopenicillins, ureidopenicillins) when resistance is caused by beta lactamase production.

Tigecycline: first of a new class of antimicrobial agents

Tigecycline is the first commercially available member of the glycylcyclines, a new class of antimicrobial agents. The glycylcyclines are derivatives of the tetracycline antibiotics, with structural modifications that allow for potent gram-positive, gram-negative, and anaerobic activity, including certain multidrug-resistant strains. The enhanced activity can be attributed to stronger binding affinity and enhanced protection against several mechanisms of resistance that affect other antibiotic classes such as tetracyclines. Tigecycline exhibits generally bacteriostatic action by reversibly binding to the 30S ribosomal subunit and inhibiting protein translation. In vitro activity has been demonstrated against multidrug-resistant gram-positive pathogens including methicillin-resistant and glycopeptide-intermediate and -resistant Staphylococcus aureus, as well as vancomycin-resistant enterococci. Multidrug-resistant gram-negative pathogens, such as Acinetobacter baumannii and extended-spectrum beta-lactamase-producing Klebsiella pneumoniae and Escherichia coli, are typically highly susceptible to tigecycline. The drug also has displayed significant activity against many clinically important anaerobic organisms. This agent demonstrates a predictable pharmacokinetic profile and minimal drug interactions, and is generally well tolerated, with nausea being the most common adverse event. It was approved in June 2005 for the treatment of complicated skin and skin structure infections (SSSIs) and complicated intraabdominal infections. Currently, a limited number of broad-spectrum antimicrobials are available to combat multidrug-resistant organisms. The addition of new agents is essential to limiting the spread of these pathogens and improving outcomes in patients with these types of infections. Tigecycline has demonstrated promising results in initial in vitro and clinical studies for SSSIs and complicated intraabdominal infections; however, further clinical experience will clarify its role as a broad-spectrum agent.

7-O-malonyl macrolactin A, a new macrolactin antibiotic from Bacillus subtilis active against methicillin-resistant Staphylococcus aureus, vancomycin-resistant enterococci, and a small-colony variant of Burkholderia cepacia

We report here the discovery, isolation, and chemical and preliminary biological characterization of a new antibiotic compound, 7-O-malonyl macrolactin A (MMA), produced by a Bacillus subtilis soil isolate. MMA is a bacteriostatic antibiotic that inhibits a number of multidrug-resistant gram-positive bacterial pathogens, including methicillin-resistant Staphylococcus aureus, vancomycin-resistant enterococci, and a small-colony variant of Burkholderia cepacia. MMA-treated staphylococci and enterococci were pseudomulticellular and exhibited multiple asymmetric initiation points of septum formation, indicating that MMA may inhibit a cell division function.

Rate of elimination of Wolbachia pipientis by doxycycline in vitro increases following drug withdrawal

Wolbachia pipientis is an obligate intracellular bacterium within the family Anaplasmataceae that infects many terrestrial arthropods and arthropod-transmitted nematodes (filariae). Several filarial species are major human pathogens, and antibiotics with activity against Wolbachia offer a promising new therapeutic approach, since the adult worms are relatively refractory to conventional anthelmintics but depend on Wolbachia for reproduction and viability. In a natural filarial parasite of cattle, Onchocerca ochengi, intermittent chemotherapy is adulticidal whereas the equivalent dose administered as a continuous treatment is not. To investigate this further and to aid the design of efficacious regimens for human therapy, we used Wolbachia-infected Aedes albopictus mosquito cells in vitro. Here, we describe for the first time the accelerated depletion of bacteria after antibiotic withdrawal relative to the rate of elimination in the continuous presence of the drug. Mosquito cells were incubated with doxycycline while changes in 16S (bacterial) and 18S (host) rRNA and rRNA genes were determined by quantitative PCR assays. In cultures treated for 7 or 14 days followed by 7 days of drug withdrawal, the Wolbachia-to-Aedes rRNA ratio declined by approximately 6 log, whereas immediately after 14 or 21 days of continuous treatment, the reduction was only approximately 4 log (P < 0.05). However, low levels of 16S rRNA remained after 21 days of treatment, irrespective of whether doxycycline was withdrawn. Application of similar methodology to related intracellular bacteria may reveal that this posttreatment effect is not restricted to Wolbachia and could have wider implications for the design of intermittent regimens for antibiotic chemotherapy.

Microbiologic surrogate end points in clinical trials of infectious diseases: example of acute otitis media trials

Clinical outcomes that measure how patients feel, function, or survive are the most important and relevant outcomes of therapy in clinical trials and in clinical practice. Surrogate end points, which do not directly measure clinical benefit to the patient, may function as substitutes for clinical end points in clinical trials. Such surrogates are attractive as they may allow measurement of outcomes earlier in time or with a smaller sample size than with clinical outcomes. Microbiologic biomarkers, such as culture results at a specific time after start of therapy, or pharmacodynamic analyses of the effect of drugs on organisms often are proposed as surrogate end points in clinical trials of therapies for infectious diseases. However, evaluation of biomarkers as surrogate end points poses distinct challenges, and only a few biomarkers have been useful replacements for clinical end points. Evaluation of biomarkers as potential surrogate end points first requires an understanding of the differences among measurements of the cause of a disease, risk factors for outcome, and measurements of treatment effects. We will discuss the definitions of clinical and surrogate end points and the reasons why surrogate end points may not predict the true clinical benefit of therapies. We will use the example of the biomarker of microbiologic outcomes from tympanocenteses performed during therapy as the sole measure of clinical effectiveness in clinical trials of acute otitis media to illustrate the challenges in evaluating biomarkers as surrogate end points.

Preclinical evaluation of novel antibacterial agents by microbiological and molecular techniques

The defining property of an antibacterial agent is its ability to selectively interfere with bacterial growth and/or survival. Consequently, a considerable and crucial part of the preclinical evaluation of any novel antibacterial drug involves judging and characterising its effects on bacteria in vitro. These critical stages in drug development are sometimes made to appear somewhat trivial, sandwiched as they are between the highly demanding antibacterial discovery process and the formidable task of demonstrating safety and efficacy in vivo. However, careful biological evaluation in vitro is key to quantifying and understanding the basis of the antibacterial activity, providing preliminary indications and evaluations of therapeutic potential, assessing the likelihood for the development of bacterial resistance, guiding chemical refinement and assisting subsequent stages of the appraisal of any new antibacterial drug. This review covers concepts in, and strategies for, the in vitro microbiological and molecular evaluation of antibacterial drug candidates.

Pharmacokinetics and pharmacodynamics of cefpirome in subcutaneous adipose tissue of septic patients

The objective of the present study was to evaluate whether cefpirome, a member of the latest class of broad-spectrum cephalosporins, sufficiently penetrates subcutaneous adipose tissue in septic patients. After the administration of the drug at 2 g, tissue cefpirome concentrations in septic patients (n = 11) and healthy controls (n = 7) were determined over a period of 4 h by means of microdialysis. To assess the antibacterial effect of cefpirome at the target site, the measured pharmacokinetic profiles were simulated in vitro with select strains of Staphylococcus aureus and Pseudomonas aeruginosa. The tissue penetration of cefpirome was significantly impaired in septic patients compared with that in healthy subjects. For subcutaneous adipose tissue, the area under the concentration-versus-time curve values from 0 to 240 min were 13.11 +/- 5.20 g . min/liter in healthy subjects and 6.90 +/- 2.56 g . min/liter in septic patients (P < 0.05). Effective bacterial growth inhibition was observed in all in vitro simulations. This was attributed to the significantly prolonged half-life in tissue (P < 0.05), which kept the tissue cefpirome levels above the MICs for relevant pathogens for extended periods in the septic group. By consideration of a dosing interval of 8 h, the values for the time above MIC (T > MIC) in tissue were greater than 60% for pathogens for which the MIC was Collapse

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MESH Headings

• Adipose Tissue/metabolism
• Adult
• Area Under Curve
• Cephalosporins/adverse effects
• Cephalosporins/pharmacokinetics
• Chromatography, Micellar Electrokinetic Capillary
• Colony Count, Microbial
• Female
• Humans
• Male
• Microbial Sensitivity Tests
• Microdialysis
• Pseudomonas aeruginosa/drug effects
• Sepsis/metabolism
• Staphylococcus aureus/drug effects
• Cefpirome

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Affiliation(s)

Robert Sauermann Department of Clinical Pharmacology, Division of Clinical Pharmacokinetics, Medical University of Vienna, Waehringer Guertel 18-20, 1090 Vienna, Austria
Georg Delle-Karth
Claudia Marsik
Ilka Steiner
Markus Zeitlinger
Bernhard X Mayer-Helm
Apostolos Georgopoulos
Markus Müller
Christian Joukhadar

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Collaborators Collapse

Antimicrobial therapy in critically ill patients: a review of pathophysiological conditions responsible for altered disposition and pharmacokinetic variability

Antimicrobials are among the most important and commonly prescribed drugs in the management of critically ill patients. Selecting the appropriate antimicrobial at the commencement of therapy, both in terms of spectrum of activity and dose and frequency of administration according to concentration or time dependency, is mandatory in this setting. Despite appropriate standard dosage regimens, failure of the antimicrobial treatment may occur because of the inability of the antimicrobial to achieve adequate concentrations at the infection site through alterations in its pharmacokinetics due to underlying pathophysiological conditions. According to the intrinsic chemicophysical properties of antimicrobials, hydrophilic antimicrobials (beta-lactams, aminoglycosides, glycopeptides) have to be considered at much higher risk of inter- and intraindividual pharmacokinetic variations than lipophilic antimicrobials (macrolides, fluoroquinolones, tetracyclines, chloramphenicol, rifampicin [rifampin]) in critically ill patients, with significant frequent fluctuations of plasma concentrations that may require significant dosage adjustments. For example, underexposure may occur because of increased volume of distribution (as a result of oedema in sepsis and trauma, pleural effusion, ascites, mediastinitis, fluid therapy or indwelling post-surgical drainage) and/or enhanced renal clearance (as a result of burns, drug abuse, hyperdynamic conditions during sepsis, acute leukaemia or use of haemodynamically active drugs). On the other hand, overexposure may occur because of a drop in renal clearance caused by renal impairment. Care with all these factors whenever choosing an antimicrobial may substantially improve the outcome of antimicrobial therapy in critically ill patients. However, since these situations may often coexist in the same patient and pharmacokinetic variability may be unpredictable, the antimicrobial policy may further benefit from real-time application of therapeutic drug monitoring, since this practice, by tailoring exposure to the individual patient, may consequently be helpful both in improving the outcome of antimicrobial therapy and in containing the spread of resistance in the hospital setting.

Pharmacodynamics of piperacillin in severely ill patients evaluated by using a PK/PD model

Penetration of antiinfective drugs into soft tissues is essential for antimicrobial killing at the target site, but is substantially lower in severely ill patients compared with healthy subjects. The present study was conducted to assess the antimicrobial effect of piperacillin in severely ill patients. Strains of Staphylococcus aureus and Pseudomonas aeruginosa were exposed in vitro to concentrations of piperacillin, simulating the pharmacokinetic profiles measured in soft tissue of patients and healthy subjects. The simulation for patients resulted in effective killing, whereas bacterial regrowth was detected for healthy subjects. Our in vitro simulation showed that bacterial killing may be effective in severely ill patients despite relatively low concentrations of piperacillin at the target site. This finding is due to impaired renal function and subsequently prolonged tissue and plasma half-lives of piperacillin in intensive care patients.

Exposure Time-Dependent Bactericidal Activities of Amoxicillin Against Actinobacillus pleuropneumoniae; an In Vitro and In Vivo Pharmacodynamic Model

The pharmacodynamic effects of amoxicillin against Actinobacillus pleuropneumoniae at exposure concentration above and below minimum inhibitory concentration (MIC) were evaluated in both in vitro and in vivo. In vitro, the growth and morphological change of A. pleuropneumoniae in culture medium was observed. In vivo, the efficacy of amoxicillin on experimentally induced A. pleuropneumoniae infection in disease-free pigs was evaluated. Fifteen pigs were divided into three groups (n = 5 per group). After the onset of clinical respiratory disease symptoms, 6 h post-infection, amoxicillin sustained-release injectable formulation was injected intramuscularly at 7.5 mg/kg/day (group I) and 15 mg/kg/day (group II). Then the serum concentration of amoxicillin was measured. An untreated infected group served as controls. In each amoxicillin administration group, if symptoms were not absent after 48 h, the pig was injected with the amoxicillin sustained-release injectable formulation again using the same dosage. In vitro, the growth of A. pleuropneumoniae inhibited by amoxicillin exposure at the concentration above the MIC (1.28 x MIC), and the inhibition time was in directly proportion to the time of amoxicillin exposure. Moreover, all the cells were lysed. Whereas the bacterial growth inhibition at the amoxicillin exposure concentration below the MIC (0.25 x MIC) was not done, and the shape of cells were normal or long filamentous. In vivo, the group I clinical and pathological score was higher than the group II, and the group I weight gain was significantly less than the group II. Performance with respect to weight gain corresponded with clinical signs. The infected control group was severely affected with an 80% (4/5) mortality rate 24-96 h post-challenge. The duration of time above MIC (T > MIC) of serum amoxicillin concentration in the group I was less than group II. The present studies suggest that amoxicillin has exposure time-dependent bactericidal activity against A. pleuropneumoniae.

Postantibiotic effects of daptomycin against 14 staphylococcal and pneumococcal clinical isolates

Daptomycin mean staphylococcal postantibiotic effects (PAEs) were 1.1 to 6.2 h, with a mean of 2.5 h. The mean pneumococcal PAEs were 1.7 h, ranging between 1.0 and 2.5 h. The staphylococcal and pneumococcal postantibiotic sub-MIC effects at 0.4 times the MIC ranged from 3.0 to >12.0 h and 1.9 to >12.0 h, respectively.

Effects of subminimal inhibitory concentrations of amoxicillin against Actinobacillus pleuropneumoniae

The bactericidal effects of amoxicillin at below minimum inhibitory concentration (MIC) against Actinobacillus pleuropneumoniae NB001 were studied in vitro and in vivo. In vivo, the efficacy of amoxicillin on experimentally induced A. pleuropneumoniae infection in disease-free pigs was evaluated. Nine pigs were divided into three groups and all three groups were housed in the same room. Group I pigs were given long-acting amoxicillin injection 22 h prior to A. pleuropneumoniae challenge. Group II pigs were also A. pleuropneumoniae challenged but not given long-acting amoxicillin. Group III pigs were not treated. In vitro, A. pleuropneumoniae growth was suppressed in porcine blood with amoxicillin at below MIC. In vivo, clinical signs of disease were absent or mild in group I during 50 h post-challenge, and serum amoxicillin concentration was already less than MIC from 15 h post-challenge. Infected group II controls were severely affected by the infection, and mortality reached 100% within 50 h post-challenge. All non-treated pigs in group III became infected with NB001 from infected control pigs, and they displayed severe clinical signs of disease within 24 h post-challenge of groups I and II, and died within 50 h post-challenge of groups I and II.

Postantibiotic effect and delay of regrowth in strains carrying mutations that save proteins or RNA

The postantibiotic effect (PAE) values found for proteinase-defective (Lon(-)) Escherichia coli and RNase-defective E. coli exposed to antibiotics were reduced (31 to 60% and 35 to 50%, respectively) in comparison with the control (AB1157), and in the recA13 mutant these values were about 0.4 h with all drugs. Nalidixic acid, under anaerobic conditions, induced no PAE (0 to 0.1 h) in AB1157. A delay in regrowth (0.2 to 0.26 h) was noted with dnaA46(Ts), gyrA43(Ts), and gyrB41(Ts) mutants cultured for 2 h at 43 degrees C. These findings suggest that when proteins and RNA are saved, the cell rapidly resumes the original growth rate.