Role of ciprofloxacin in its synergistic effect with fosfomycin on drug-resistant strains of Pseudomonas aeruginosa

In vitro killing of multidrug/extensively drug-resistant Pseudomonas aeruginosa by fosfomycin alone or in combination with antipseudomonal antibiotics

Pseudomonas aeruginosa is a leading cause of nosocomial infections. Given the constant rise in resistance, adequate therapy is increasingly demanding. Fosfomycin recently became an appealing treatment option of bacterial infections due to multidrug-resistant bacteria (MDR). So far, fosfomycin synergy with other antibiotics has been assessed in studies, but only a limited number focused on MDR P. aeruginosa and on the effect of these combinations on the duration of the postantibiotic effect (PAE). We investigated synergy of fosfomycin with an array of antipseudomonal antibiotics using gradient diffusion strip cross method and time-kill method, and their effect on the duration of PAE against 51 variously resistant P. aeruginosa isolates. The highest rate of synergy was observed for combination with ceftazidime (23.4%) and gentamicin (19.1%). The PAE of antibiotic combinations was superior to that of the drugs alone. Our findings indicate that fosfomycin combination therapy may be a valuable treatment alternative.

Fosfomycin and nitrofurantoin: classic antibiotics and perspectives

Antibiotics are essential molecules for the treatment and prophylaxis of many infectious diseases. However, drugs that combat microbial infections can become a human health threat due to their high and often indiscriminate consumption, considered one of the factors of antimicrobial resistance (AMR) emergence. The AMR crisis, the decrease in new drug development by the pharmaceutical industry, and reduced economic incentives for research have all reduced the options for treating infections, and new strategies are necessary, including the return of some traditional but "forgotten" antibiotics. However, prescriptions for these older drugs including nitrofurantoin and oral fosfomycin, have been based on the results of pioneer studies, and the limited knowledge generated 50-70 years ago may not be enough. To avoid harming patients and further increasing multidrug resistance, systematic evaluation is required, mainly for the drugs prescribed for community-acquired infections, such as urinary tract infections (UTI). Therefore, this review has the objective of reporting the use of two classic drugs from the nitrofuran and phosphonic acid classes for UTI control nowadays. Furthermore, we also explore new approaches used for these antibiotics, including new combination regimes for spectral amplification, and the prospects for reducing bacterial resistance in the fight against bacteria responsible for UTI.

Material properties of interfacial films of mucoid and nonmucoid Pseudomonas aeruginosa isolates

Chronic lung infection with bacterial biofilms is a leading cause of death in cystic fibrosis (CF) patients. Pseudomonas aeruginosa, one of the many species colonizing the lung airways, can undergo pathoadaptation, leading to a mucoid phenotype with interesting material properties. We hypothesize that the surface properties and extracellular materials of mucoid P. aeruginosa cells greatly influence the mechanical behavior of their films at fluid interfaces. In this study, we investigate the interfacial properties of films formed by nonmucoid (PANT) and mucoid (PASL) strains of P. aeruginosa isolated from CF patients. We use pendant drop elastometry to analyze the interfacial response of the films formed by PANT and PASL at the hexadecane-water interface. The dynamic rheological analyses of the films highlight the distinctive signature of the mucoid strains at fluid interfaces. The mucoid films exhibit greater relaxation following a compressive strain than a tensile one, while a full hysteresis response is achieved by the nonmucoid films; this indicates that the material properties of the PANT films are conserved under both compression and tension. The wrinkling and shape analyses of the interfacial bacterial films elucidate that the mucoid strain exhibits remarkable viscoelastic properties, enabling the remodeling of the living films and dissipation of the compressive stress. The comparative analysis of the material properties of mucoid and nonmucoid P. aeruginosa cells indicates that mucoid switch can play an important role in protecting the bacteria from interfacial stresses. Further characterization of interfacial bacterial films will provide new insights into the development of methods for controlling interfacial films of bacteria.

Fosfomycin as Partner Drug for Systemic Infection Management. A Systematic Review of Its Synergistic Properties from In Vitro and In Vivo Studies

Fosfomycin is being increasingly prescribed for multidrug-resistant bacterial infections. In patients with systemic involvement, intravenous fosfomycin is usually administered as a partner drug, as part of an antibiotic regimen. Hence, the knowledge of fosfomycin pharmacodynamic interactions (synergistic, additive, indifferent and antagonistic effect) is fundamental for a proper clinical management of severe bacterial infections. We performed a systematic review to point out fosfomycin’s synergistic properties, when administered with other antibiotics, in order to help clinicians to maximize drug efficacy optimizing its use in clinical practice. Interactions were more frequently additive or indifferent (65.4%). Synergism accounted for 33.7% of total interactions, while antagonism occurred sporadically (0.9%). Clinically significant synergistic interactions were mostly distributed in combination with penicillins (51%), carbapenems (43%), chloramphenicol (39%) and cephalosporins (33%) in Enterobactaerales; with linezolid (74%), tetracyclines (72%) and daptomycin (56%) in Staphylococcus aureus; with chloramphenicol (53%), aminoglycosides (43%) and cephalosporins (36%) against Pseudomonas aeruginosa; with daptomycin (97%) in Enterococcus spp. and with sulbactam (75%) and penicillins (60%) and in Acinetobacter spp. fosfomycin-based antibiotic associations benefit from increase in the bactericidal effect and prevention of antimicrobial resistances. Taken together, the presence of synergistic interactions and the nearly total absence of antagonisms, make fosfomycin a good partner drug in clinical practice.

In Vitro Activity of Fosfomycin in Double and Triple Combinations with Imipenem, Ciprofloxacin and Tobramycin Against Multidrug-Resistant Escherichia coli

The rates of urinary tract infection with multidrug-resistant (MDR) Escherichia coli have dramatically increased and the treatment of these infections with single and double antibiotic combinations became limited or ineffective. The present study aimed at finding effective antibiotic combinations against MDR uropathogenic E. coli. Antibiotic susceptibility testing of uropathogenic E. coli isolates (n = 29) showed that all the examined isolates were found to be MDR. The interaction of double and triple combinations of fosfomycin (FOS) with imipenem (IPM), ciprofloxacin (CIP) and tobramycin (TOB) against selected isolates (n = 8) by checkerboard method showed that all the examined combinations exhibited synergistic effects (FIC index < 1) against tested isolate. However, 1/8, 5/8 and 6/8 of the isolates remained resistant to the constituent antibiotics in FOS/IPM, FOS/CIP and FOS/TOB combinations, respectively. Notably, the triple combinations (FOS/IPM/CIP, FOS/IPM/TOB and FOS/CIP/TOB) increased the synergism against all selected isolates at MIC levels lower than the susceptible breakpoints. Furthermore, time-kill analysis demonstrated that FOS/IPM combination exhibited synergistic and bactericidal effects with UTI-9. However, the combination had no effect on UTI-13. The highest synergistic and bactericidal effects against both representative isolates were achieved by FOS/IPM/CIP, FOS/IPM/TOB and FOS/CIP/TOB combinations after 2 h of post-treatment and lasted up to 24 h. Therefore, we report here that the combinations of FOS with IPM, CIP and TOB could be beneficial against MDR uropathogenic E. coli at least in vitro. The effectiveness of these antibiotics increased in combination with FOS compared to individual antibiotics acting alone.

Synergistic Activity of Fosfomycin, Ciprofloxacin, and Gentamicin Against Escherichia coli and Pseudomonas aeruginosa Biofilms

Gram-negative (GN) rods cause about 10% periprosthetic joint infection (PJI) and represent an increasing challenge due to emergence of antimicrobial resistance. Escherichia coli and Pseudomonas aeruginosa are among the most common cause of GN-PJI and ciprofloxacin is the first-line antibiotic. Due to emergence of fluoroquinolone resistance, we evaluated in vitro the activity of fosfomycin, ciprofloxacin, and gentamicin, alone and in combinations, against E. coli and P. aeruginosa biofilms. Conventional microbiological tests and isothermal microcalorimetry were applied to investigate the anti-biofilm activity of the selected antibiotics against standard laboratory strains as well as clinical strains isolated from patients with prosthetic joint associated infections. The biofilm susceptibility to each antibiotic varied widely among strains, while fosfomycin presented a poor anti-biofilm activity against P. aeruginosa. Synergism of two-pair antibiotic combinations was observed against different clinical strains from both species. Highest synergism was found for the fosfomycin/gentamicin combination against the biofilm of E. coli strains (75%), including a gentamicin-resistant but fosfomycin-susceptible strain, whereas the gentamicin/ciprofloxacin combination presented synergism with higher frequency against the biofilm of P. aeruginosa strains (71.4%). A hypothetical bacteriolysis effect of gentamicin could explain why combinations with this antibiotic seem to be particularly effective. Still, the underlying mechanism of the synergistic effect on biofilms is unknown. In conclusion, combinatorial antibiotic application has shown to be more effective against biofilms compared to monotherapy. Further in vivo and clinical studies are essential to define the potential treatment regimen based on our results.

In vitro and in vivo activity of ciprofloxacin/fosfomycin combination therapy against ciprofloxacin-resistant Shigella flexneri isolates

Objective: Ciprofloxacin resistance (CIP^R) for Shigella isolates is becoming more prevalent. This study systematically investigated the antibacterial activity of ciprofloxacin (CIP)/fosfomycin (FOS) combination in vitro and in vivo against CIP^R S. flexneri isolates. Method: Eighty CIP^R S. flexneri isolates were selected for synergy studies by the microtiter plate checkerboard assay. Two S. flexneri isolates (GN120471, CIP^RFOS^R; GN120454, CIP^RFOS^S) were used to investigate the efficacy of the CIP/FOS combination by the time-kill methodology. Clinically relevant concentrations (CIP, 0.5, 1, or 2.5 μg/mL; FOS, 30, 150, or 300 μg/mL) were combined, and the colony counts were conducted at 3, 5, 8, and 24 hours. The in vivo activity of the CIP/FOS combination was assessed using a Galleria mellonella larvae model. Results: In checkerboard assays, 31 strains (38.75%) showed synergy for the CIP/FOS combination. For the isolate GN120471, monotherapy with CIP or FOS at all concentrations produced little or no bacterial killing, while the CIP/FOS combination produced enhanced bacterial killing with FOS concentrations of 150 and 300 μg/mL, especially when combined with CIP at 2.5 μg/mL. For the isolate GN120454, the CIP/FOS combination at all concentrations produced more rapid and extensive killing (up to 5log₁₀ colony forming units (CFU)/mL with many combinations) than with either antibiotic alone. Mortality at 96 hours was around 80% at approximately 10⁴ CFU/larva for GN120471 and GN120454. When CIP at 2.5 μg/mL was combined with FOS at 150 μg/mL for the bactericidal activity in vivo, the survival rates for CIP/FOS combination against GN120471-infected and GN120454-infected larvae were significantly higher than that of CIP (68.75% vs 25%, P=0.013; 81.25% vs 37.5%, P=0.012, respectively). Conclusion: Against CIP^R S. flexneri isolates, the CIP/FOS combination induced synergy, and increased bacterial killing in vitro and in a simple invertebrate model of infection.

Emergence of antibiotic resistance Pseudomonas aeruginosa in intensive care unit; a critical review

The emergence of antibiotic resistant bacteria in the healthcare is a serious concern. In the Healthcare premises precisely intensive care unit are major sources of microbial diversity. Recent findings have demonstrated not only microbial diversity but also drug resistant microbes largely habitat in ICU. Pseudomonas aeruginosa found as a part of normal intestinal flora and a significant pathogen responsible for wide range of ICU acquired infection in critically ill patients. Nosocomial infection associated with this organism including gastrointestinal infection, urinary tract infections and blood stream infection. Infection caused by this organism are difficult to treat because of the presence of its innate resistance to many antibiotics (β-lactam and penem group of antibiotics), and its ability to acquire further resistance mechanism to multiple class of antibiotics, including Beta-lactams, aminoglycosides and fluoroquinolones. In the molecular evolution microbes adopted several mechanism to maintain genomic plasticity. The tool microbe use for its survival is mainly biofilm formation, quorum sensing, and horizontal gene transfer and enzyme promiscuity. Such genomic plasticity provide an ideal habitat to grow and survive in hearse environment mainly antibiotics pressure. This review focus on infection caused by Pseudomonas aeruginosa, its mechanisms of resistance and available treatment options. The present study provides a systemic review on major source of Pseudomonas aeruginosa in ICU. Further, study also emphasizes virulence gene/s associated with Pseudomonas aeruginosa genome for extended drug resistance. Study gives detailed overview of antibiotic drug resistance mechanism.

Fosfomycin: the characteristics, activity, and use in critical care

Fosfomycin (C₃H₇O₄P) is a phosphonic acid derivative representing an epoxide class of antibiotics. The drug is a re-emerging bactericidal antibiotic with a wide range of actions against several Gram-positive and Gram-negative bacteria. Among the existing antibacterial agents, fosfomycin has the lowest molecular weight (138 Da), which is not structurally associated with other classes of antibiotics. In intensive care unit (ICU) patients, severe soft tissue infections (STIs) may lead to serious life-threatening problems, and therefore, appropriate antibiotic therapy and often intensive care management (ICM) coupled with surgical intervention are necessary. Fosfomycin is an antibiotic primarily utilized for the treatment of STIs in ICUs. Recently, fosfomycin has attracted renewed interest for the treatment of serious systemic infections caused by multidrug-resistant Enterobacteriaceae. In some countries, intravenous fosfomycin has been prescribed for various serious systemic infections, such as acute osteomyelitis, nosocomial lower respiratory tract infections, complicated urinary tract infections, bacterial meningitis, and bacteremia. Administration of intravenous fosfomycin can result in a sufficient concentration of the drug at different body regions. Dose modification is not required in hepatic deficiency because fosfomycin is not subjected to enterohepatic circulation.

Fosfomycin: Pharmacological, Clinical and Future Perspectives

Fosfomycin is a bactericidal, low-molecular weight, broad-spectrum antibiotic, with putative activity against several bacteria, including multidrug-resistant Gram-negative bacteria, by irreversibly inhibiting an early stage in cell wall synthesis. Evidence suggests that fosfomycin has a synergistic effect when used in combination with other antimicrobial agents that act via a different mechanism of action, thereby allowing for reduced dosages and lower toxicity. Fosfomycin does not bind to plasma proteins and is cleared via the kidneys. Due to its extensive tissue penetration, fosfomycin may be indicated for infections of the CNS, soft tissues, bone, lungs, and abscesses. The oral bioavailability of fosfomycin tromethamine is <50%; therefore, oral administration of fosfomycin tromethamine is approved only as a 3-gram one-time dose for treating urinary tract infections. However, based on published PK parameters, PK/PD simulations have been performed for several multiple-dose regimens, which might lead to the future use of fosfomycin for treating complicated infections with multidrug-resistant bacteria. Because essential pharmacological information and knowledge regarding mechanisms of resistance are currently limited and/or controversial, further studies are urgently needed, and fosfomycin monotherapy should be avoided.

Clinically relevant concentrations of fosfomycin combined with polymyxin B, tobramycin or ciprofloxacin enhance bacterial killing of Pseudomonas aeruginosa, but do not suppress the emergence of fosfomycin resistance

OBJECTIVES

Fosfomycin resistance occurs rapidly with monotherapy. This study systematically investigated bacterial killing and emergence of fosfomycin resistance with fosfomycin combinations against Pseudomonas aeruginosa.

METHODS

Four clinical isolates and a reference strain of P. aeruginosa were employed. Combinations of fosfomycin plus polymyxin B, tobramycin or ciprofloxacin were examined over 24 h using time-kill studies (inocula ∼10(6) cfu/mL) incorporating clinically relevant concentrations (fosfomycin, 30, 150 or 300 mg/L; polymyxin B, 0.5, 1 or 2 mg/L; tobramycin, 0.5, 1.5 or 4 mg/L; ciprofloxacin, 0.5, 1 or 2.5 mg/L). Microbiological response was examined by log changes and population analysis profiles.

RESULTS

Against susceptible isolates, monotherapy produced varying degrees of initial killing followed by rapid regrowth. Fosfomycin plus polymyxin B or tobramycin produced greater initial killing (up to ∼4 log10 cfu/mL) with many concentrations compared with monotherapy against fosfomycin-susceptible (FOF(S)) isolates. With these combinations, synergy or additivity was observed in 54 (67%) and 49 (60%) of 81 cases (nine combinations across three isolates at three timepoints) for polymyxin B and tobramycin, respectively. Substantial improvements in killing were absent against fosfomycin-resistant (FOF(R)) isolates. For fosfomycin/ciprofloxacin combinations, synergy or additivity was observed against FOF(R) isolates in 33 of 54 (61%) cases (nine combinations across two isolates at three timepoints), while improvements in killing were largely absent against FOF(S) isolates. No combination prevented emergence of fosfomycin resistance.

CONCLUSIONS

Against P. aeruginosa, fosfomycin in combination with polymyxin B or tobramycin (FOF(S) isolates) or ciprofloxacin (FOF(R) isolates) increased bacterial killing, but did not suppress emergence of fosfomycin resistance.

[In vitro activity of fosfomycin, alone or in combination, against clinical isolates of carbapenem resistant Pseudomonas aeruginosa]

INTRODUCTION

The increase in microorganisms showing patterns of multi-drug resistance or even pan-drug resistance is of growing concern. Fosfomycin (FO) is well known to be active against a wide variety of microorganisms, including highly resistant strains of Pseudomonas aeruginosa (P. aeruginosa), and can also synergistically act with other molecules.

METHODS

This study examines the in vitro activity shown by FO against 120 strains of carbapenem-resistant P. aeruginosa using an agar dilution and a gradient diffusion test. Possible synergistic effects of the combinations of FO/amikacin and FO/ciprofloxacin were also examined using E-test and time-kill techniques.

RESULTS

According to the epidemiological cut-off value (ECOFF) issued by the European Committee on Antimicrobial Susceptibility Testing (EUCAST), our results indicate that over three-quarters of the strains tested would be susceptible to FO treatment, especially if combined with another antimicrobial. The FO/ciprofloxacin combination had a synergistic effect on 40% of the clinical isolates, while for FO/amikacin this effect was only observed in 12% of the isolates.

CONCLUSION

The appearance of carbapenem-resistant P. aeruginosa strains requires the evaluation by combination therapy. This report suggests that the FO/ciprofloxacin combination can be useful, showing a synergistic effect in 40% of the isolates.

Unexpected challenges in treating multidrug-resistant Gram-negative bacteria: resistance to ceftazidime-avibactam in archived isolates of Pseudomonas aeruginosa

Pseudomonas aeruginosa is a notoriously difficult-to-treat pathogen that is a common cause of severe nosocomial infections. Investigating a collection of β-lactam-resistant P. aeruginosa clinical isolates from a decade ago, we uncovered resistance to ceftazidime-avibactam, a novel β-lactam/β-lactamase inhibitor combination. The isolates were systematically analyzed through a variety of genetic, biochemical, genomic, and microbiological methods to understand how resistance manifests to a unique drug combination that is not yet clinically released. We discovered that avibactam was able to inactivate different AmpC β-lactamase enzymes and that blaPDC regulatory elements and penicillin-binding protein differences did not contribute in a major way to resistance. By using carefully selected combinations of antimicrobial agents, we deduced that the greatest barrier to ceftazidime-avibactam is membrane permeability and drug efflux. To overcome the constellation of resistance determinants, we show that a combination of antimicrobial agents (ceftazidime/avibactam/fosfomycin) targeting multiple cell wall synthetic pathways can restore susceptibility. In P. aeruginosa, efflux, as a general mechanism of resistance, may pose the greatest challenge to future antibiotic development. Our unexpected findings create concern that even the development of antimicrobial agents targeted for the treatment of multidrug-resistant bacteria may encounter clinically important resistance. Antibiotic therapy in the future must consider these factors.

In vitro synergism of fosfomycin and clarithromycin antimicrobials against methicillin-resistant Staphylococcus pseudintermedius

BACKGROUND

Bacterial biofilms are of tremendous concern for clinicians, as they can compromise the ability of the immune system and antimicrobial therapy to resolve chronic and recurrent infections. Novel antimicrobial therapies or combinations targeted against biofilm establishment and growth subsequently represent a promising new option for the treatment of chronic infectious diseases. In this study, we treated bacterial biofilms produced by methicillin-resistant Staphylococcus pseudintermedius (MRSP) with a combination of fosfomycin and clarithromycin. We selected these agents, because they prevent biofilm formation and induce antimicrobial synergism that may also target other staphylococci.

RESULTS

We determined that the combination of fosfomycin and clarithromycin better impairs S. pseudintermedius biofilm formation compared to treatment with either therapy alone (P < 0.05). Morphological examination of these biofilms via scanning electron microscopy demonstrated that fosfomycin alone does impact biofilm formation on orthopaedic implants. However, this activity is enhanced in the presence of clarithromycin. We propose that the bacteriostatic activity of clarithromycin is accentuated when fosfoymcin is present, as it may allow better penetration into the biofilm matrix, allowing fosfomycin access to sessile bacteria near the surface of attachment.

CONCLUSIONS

Here, we demonstrate that the combination of fosfomycin and clarithromycin may be a useful therapy that could improve the clinical outcomes of treating antimicrobial resistant MRSP biofilms.

Blocking peptidoglycan recycling in Pseudomonas aeruginosa attenuates intrinsic resistance to fosfomycin

Gram-negative bacteria recycle as much as half of their cell wall per generation. Here we show that interference with cell wall recycling in Pseudomonas aeruginosa strains results in four- to eight-fold increased susceptibility to the antibiotic fosfomycin, pushing the minimal inhibitory concentration for strains PA14 and PA01 to therapeutically appropriate values of 2-4 and 8-16 mg/L, respectively. A newly discovered metabolic pathway that connects cell wall recycling with peptidoglycan de novo biosynthesis is responsible for the high intrinsic resistance of P. aeruginosa to fosfomycin. The pathway comprises an anomeric cell wall amino sugar kinase (AmgK) and an uridylyl transferase (MurU), which together convert N-acetylmuramic acid (MurNAc) through MurNAc α-1-phosphate to uridine diphosphate (UDP)-MurNAc, thereby bypassing the fosfomycin-sensitive de novo synthesis of UDP-MurNAc. Thus, inhibition of peptidoglycan recycling can be applied as a new strategy for the combinatory therapy against multidrug-resistant P. aeruginosa strains.

Fosfomycin for the treatment of resistant gram-negative bacterial infections. Insights from the Society of Infectious Diseases Pharmacists

The antimicrobial agent fosfomycin was discovered in 1969, at a time when bacteria had not yet developed extended-spectrum β-lactamases or carbapenemases. Decades later, it is not uncommon for gram-negative organisms to be multidrug-resistant and even pan-resistant to available antibiotic regimens, leaving clinicians with few therapeutic alternatives. Because fosfomycin has been shown to retain activity against these virulent pathogens, there is renewed interest in its use as a therapeutic agent. Fosfomycin formulations including fosfomycin disodium and the newer tromethamine salt are less toxic than other alternatives and are attractive options for resistant gram-negative and gram-positive infections. Oral fosfomycin tromethamine is approved for urinary tract infections in the United States, and an intravenous formulation is also available outside of the United States for systemic disease. The bactericidal action of fosfomycin occurs at an earlier step in cell wall synthesis than that of β-lactam antibiotics. From an in vitro standpoint, fosfomycin generally has high activity against ESBL- and carbapenemase-producing Enterobacteriaceae; multidrug-resistant Pseudomonas aeruginosa susceptibility appears to be more dependent on the local antibiogram. Fosfomycin formulations have a large volume of distribution, penetrate biofilms, and concentrate in the urine. Both oral and intravenous fosfomycin formulations are effective for a wide range of gram-negative infections and disease severities; however, clinical studies are limited. Fosfomycin formulations are well-tolerated, and mild gastrointestinal distress is the most common adverse effect. The primary limitations of fosfomycin are the lack of established regimens for complicated infections and the lack of availability of the intravenous formulation in the United States. Further study of this promising agent seems warranted in the current climate of antibiotic resistance.

In vitro activity of antimicrobial combinations against multidrug-resistant Pseudomonas aeruginosa

INTRODUCTION

Pseudomonas aeruginosa isolates related to nosocomial infections are often resistant to multiple antibacterial agents. In this study, antimicrobial combinations were evaluated to detect in vitro synergy against clinical isolates of P. aeruginosa.

METHODS

Four clinical P. aeruginosa isolates were selected at random among other isolates from inpatients treated at the public University hospital in Ribeirão Preto, SP, Brazil. Two isolates were susceptible to imipenem (IPM-S) and several other antimicrobials, while the other two isolates were imipenem and multidrug resistant (IPM-R). The checkerboard method was used to assess the interactions between antimicrobials.

RESULTS

Combinations of imipenem or other anti-Pseudomonas drugs with complementary antibiotics, such as aminoglycosides, fosfomycin and rifampin, reached synergy rates of 20.8%, 50%, 62.5% and 50% for the two IPM-S and two IPM-R Pseudomonas isolates, respectively. Imipenem, piperacillin-tazobactam and ceftazidime yielded a greater synergy rate than cefepime or ciprofloxacin. Synergist combinations were more commonly observed when the complementary drug was tobramycin (65%) or fosfomycin (57%).

CONCLUSIONS

Some antibacterial combinations led to significant reductions of the minimum inhibitory concentrations of both drugs, suggesting that they could be clinically applied to control infections caused by multidrug-resistant P. aeruginosa.

In vitro activity of fosfomycin in combination with imipenem, meropenem, colistin and tigecycline against OXA 48-positive Klebsiella pneumoniae strains

Carbapenem resistance due to OXA-48 enzymes in Klebsiella pneumoniae is increasing particularly in the Middle Eastern and European regions. Treatment options are limited. The aim of this study was to evaluate the in vitro synergistic activity of fosfomycin in combination with imipenem, meropenem, colistin and tigecycline against OXA-48 producing K. pneumoniae strains. Twelve carbapenem-resistant OXA-48 producing K. pneumoniae isolates were enrolled in this study. Synergistic activity of fosfomycin combined with imipenem, meropenem, colistin, and tigecycline was assessed by chequerboard method. The combination of fosfomycin was synergistic with imipenem, meropenem and tigecycline with the ratios of 42%, 33%, and 33%, respectively. Whilst the combination of fosfomycin with colistin was fully antagonistic against all of the strains, there was no statistically significant difference between the in vitro synergistic activities of fosfomycin in combination with imipenem, meropenem and tigecycline combinations (P > 0.05). Fosfomycin in combination with other agents can be preferred against multidrug resistant K. pneumoniae strains.

Requirement of lipid II biosynthesis for cell division in cell wall-less Wolbachia, endobacteria of arthropods and filarial nematodes

Obligate Wolbachia endobacteria have a reduced genome and retained genes are hypothesized to be crucial for survival. Although intracellular bacteria do not need a stress-bearing peptidoglycan cell wall, Wolbachia encode proteins necessary to synthesize the peptidoglycan precursor lipid II. The activity of the enzymes catalyzing the last two steps of this pathway was previously shown, and Wolbachia are sensitive to inhibition of lipid II synthesis. A puzzling characteristic of Wolbachia is the lack of genes for l-amino acid racemases essential for lipid II synthesis. Transcription analysis showed the expression of a possible alternative racemase metC, and recombinant Wolbachia MetC indeed had racemase activity that may substitute for the absent l-Ala racemase. However, enzymes needed to form mature peptidoglycan are absent and the function of Wolbachia lipid II is unknown. Inhibition of lipid II biosynthesis resulted in enlargement of Wolbachia cells and redistribution of Wolbachia peptidoglycan-associated lipoprotein, demonstrating that lipid II is required for coordinated cell division and may interact with the lipoprotein. We conclude that lipid II is essential for Wolbachia cell division and that this function is potentially conserved in the Gram-negative bacteria.

Thickening of the cell wall in macrolide-resistant Staphylococcus aureus

Macrolides are widely used at low dosage for long-term therapy of chronic sinusitis. Twenty clinical macrolide-resistant Staphylococcus aureus strains were morphologically compared with 10 clinical macrolide-sensitive strains. PCR amplification was performed to determine the presence of four known macrolide resistance genes. Transmission electron microscopy revealed significantly thicker cell walls in clinical macrolide-resistant strains. Even though the ultrastructural characteristics were shared by all macrolide-resistant strains, they were not associated with the presence or absence of the known macrolide-resistance genes. We also demonstrated that macrolide-resistant mutant strains derived in vitro from a macrolide-sensitive parent strain had thickened cell walls and did not harbor the known macrolide-resistance genes. These results, therefore, revealed that macrolide-resistant S. aureus strains have thickened cell walls as a common ultrastructural characteristic and that cell wall thickening is likely mediated by an unknown gene which is unrelated to any known macrolide resistance gene.

Antimicrobial effects of Burow's solution on Staphylococcus aureus and Pseudomonas aeruginosa

Burow's solution has been shown to be effective against chronic suppurative otitis media and otitis externa. We demonstrated that Burow's solution had antibacterial effects against Staphylococcus aureus and Pseudomonas aeruginosa, inducing ultrastructural changes in these bacteria in vitro. S. aureus strain 209P and P. aeruginosa strain IID1130 were treated with 13% Burow's solution. Viable cell counts were determined to measure bactericidal effects. Ultrastructural changes in cells of both strains were examined by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Viable cell counting revealed that S. aureus cells treated with Burow's solution were killed within 30 min. The viable cell count of P. aeruginosa was reduced by 1 × 10(7) colony-forming units/ml (CFU/ml) after a 60-min treatment. SEM examination of S. aureus revealed blebbing on the surface of bacterial cells, whereas TEM revealed undulating deformation of the bacterial cell wall, diluted cytoplasm, and cell membrane detachment. SEM observations of P. aeruginosa revealed a more apparent undulating deformation of the bacterial cell surface. TEM observations also revealed deformations in the bacterial cell wall and diluted cytoplasm in both bacteria. These findings show that Burow's solution is active against S. aureus and P. aeruginosa, resulting in damage to the cell wall.

Synergy of fosfomycin with other antibiotics for Gram-positive and Gram-negative bacteria

BACKGROUND

The alarming increase in drug resistance and decreased production of new antibiotics necessitate the evaluation of combinations of existing antibiotics. Fosfomycin shows no cross-resistance to other antibiotic classes. Thus, its combination with other antibiotics may potentially show synergy against resistant bacteria.

OBJECTIVE

To evaluate the available published evidence regarding the in vitro synergistic activity of fosfomycin with other antibiotic agents against Gram-positive and Gram-negative bacteria.

METHODS

PubMed and the Cochrane Library were searched.

RESULTS

Forty-one studies, including 34 (82.9%) conducted/published before 2000, were eligible for inclusion. The relatively limited number of isolates examined and the considerable heterogeneity of the retrieved studies regarding the definitions of synergy and the methodologies used hamper conclusive remarks for specific combinations of fosfomycin with other antibiotics. Yet, in the 27 studies providing data for Gram-positive strains (16 for Staphylococcus aureus, 3 for coagulase-negative staphylococci, 5 for Streptococcus pneumoniae, and 3 for Enterococcus spp.), fosfomycin showed synergy against methicillin-resistant Staphylococcus aureus when combined with cefamandole, cephazolin, ceftriaxone, ciprofloxacin, imipenem, and rifampicin. Data regarding Gram-negative strains reported from 15 studies (12 exclusively for P. aeruginosa, 2 exclusively for Enterobacteriaceae, 1 for both, and 1 for Acinetobacter baumannii) suggested that fosfomycin showed an estimable synergistic effect with gentamicin, amikacin, ceftazidime, cefepime, ciprofloxacin, levofloxacin, and aztreonam against P. aeruginosa.

CONCLUSIONS

The synergistic combination of fosfomycin with other antibiotics may be a useful alternative treatment option for Gram-negative and Gram-positive infections. Additional studies using more stringent definitions of synergy, and studies reporting on the clinical efficacy of fosfomycin combinations in the current era of high antimicrobial resistance are needed.

Fosfomycin: an old, new friend?

Fosfomycin (FOM) is an antibiotic which has varying application indications across the globe. European, Japanese, South African and Brazilian usage practices are much broader, involving multiple formulations of FOM than the currently limited application of FOM in the United States, where uncomplicated urinary tract infection represents the only indication for FOM-tromethamine. Based on early difficulty in determining FOMs genuine in vitro activity, there was initial skepticism about its efficacy and application range. However, in the mid 1970s, correctly executed experiments coupled with an improved understanding of microbiological concepts opened the door for broader use of FOM. During the following 40 years FOM was evaluated in pre-clinical and clinical trials in a wide range of applications and in a multitude of settings. The gathering of pharmacokinetic and pharmacodynamic data was incorporated into large scale studies in which FOM efficacy was further explored and proven. Among European nations, intravenous FOM-disodium for patients presenting with soft tissue infections, sepsis or deep seated infectious processes has become well accepted over the last two decades. The recent emergence of bacterial strains, which impede and encumber pharmacotherapy, namely, MRSA, ESBL and MSSA, lends itself to the idea of reviving long-standing, sensibly used antimicrobial agents like FOM. This review provides a comprehensive conspectus on FOM's history, mode of action, tissue penetration characteristics, resistance, antibacterial activity, combination partners and clinical uses among other facets of interest.