The pharmacodynamics of minocycline alone and in combination with rifampicin against Staphylococcus aureus studied in an in vitro pharmacokinetic model of infection.

BACKGROUND: Tetracyclines are widely used as oral therapy of MRSA infection, however, the pharmacodynamic underpinning is absent. OBJECTIVES: We employed an in vitro pharmacokinetic model to study the pharmacodynamics of minocycline alone and in combination with rifampicin. METHODS: An exposure-ranging design was used to establish fAUC/MIC targets for static, -1 log drop and -2 log drop effects against Staphylococcus aureus for minocycline and in combination with rifampicin. We then simulated 7-10 day human dosing of minocycline and the combination. RESULTS: The minocycline fAUC/MIC for 24 h static effect and -1 log drop in bacterial load were 12.5 ±â€Š7.1 and 23.3 ±â€Š12.4. fAUC/MIC targets for static and -1 log drop were greater at 48 and 72 h. The addition of simulated free rifampicin associated with dosing 300 mg q12h reduced the 24 h minocycline fAUC/MICs. Simulations performed over 7-10 days exposure indicated that for minocycline standard human doses there was a 1-3 log reduction in viable count and no changes in population profiles. Addition of rifampicin resulted in larger reductions in staphylococcal load but emergence of resistance to rifampicin. There was no resistance to minocycline. CONCLUSIONS: An fAUC/MIC minocycline target of 12-36 is appropriate for S. aureus. Addition of rifampicin decreases bacterial load but results in emergence of resistance to rifampicin. Unusually, there was no emergence of resistance to minocycline.

In vitro pharmacodynamics of omadacycline against Escherichia coli and Acinetobacter baumannii.

BACKGROUND: The pharmacodynamics of omadacycline have been extensively studied against Gram-positive pathogens but less information is available for Gram-negative pathogens. We describe the pre-clinical pharmacodynamics of omadacycline against Escherichia coli and Acinetobacter baumannii. METHODS: An in vitro dilutional pharmacokinetic model was used. Exposure experiments with fAUC/MIC ratios ranging from 0 to 1200 were performed using five strains of E. coli and five strains of A. baumannii. Reduction in bacterial load and changes in population profiles were measured. RESULTS: The fAUC/MIC targets against E. coli for 24 h static and -1 log reduction in load were 25.3 ±â€Š17.2 and 42.7 ±â€Š32.5, respectively. For A. baumannii the fAUC/MIC for 24 h static effect was 108.1 ±â€Š38.6. Changes in population profiles were observed for E. coli at fAUC/MIC ratios of ≤200 and for A. baumannii up to 1200. MICs were increased 2-32 fold. CONCLUSIONS: fAUC/MIC targets for A. baumannii are greater than for E.coli and changes in population profiles more likely. E. coli fAUC/MIC targets align with in vivo data and will be useful in determining omadacycline dosing for this pathogen.

An in vitro biofilm model of Staphylococcus aureus infection of bone.

Chronic osteomyelitis is difficult to treat, with biofilm growth and the diffusion barrier to antibiotics presented by bone contributory factors. The aim of this study was to develop and evaluate an in vitro model of osteomyelitis. A bioluminescent strain of Staphylococcus aureus was grown in bone blocks made from bovine femur. Light output was insufficient for detection of bacterial cells within bone by 24 h and viable counting of crushed bone blocks was used to determine bacterial survival. Challenge of 72 h biofilms with gentamicin and daptomycin for 24 h demonstrated that only concentrations of 10 times the clinical peak serum target levels (100 mg l-1 gentamicin and 1000 mg l-1 daptomycin) resulted in significant reductions in cell viability compared to controls. Once daily dosing over 7 days resulted in ≥3 log reductions in cell numbers by 48 h. Thereafter no significant reduction was achieved, although emergence of resistance was suppressed. Determination of antibiotic concentration in bone blocks over 7 days indicated that neither agent was able to consistently reach levels in bone of >10% of the original dose. The model was, therefore, able to demonstrate the challenges posed by biofilm growth on and within bone. SIGNIFICANCE AND IMPACT OF THE STUDY: The majority of studies of antibiotic efficacy in the treatment of chronic osteomyelitis are carried out in animals. We developed an in vitro model of Staphylococcus aureus infection of bone to evaluate the ability of antibiotics to eradicate mature biofilms on surfaces analogous to necrotic bone. The results demonstrated the difficulties which occur in osteomyelitis treatment, with only very high concentrations of antibiotic able to penetrate the bone sufficiently to reduce bacterial survival whilst still failing to eradicate biofilms. This model could be of use in initial screening of novel compounds intended for use in the treatment of osteomyelitis.

Viral infections in pregnancy: advice for healthcare workers.

BACKGROUND: Healthcare workers (HCWs) have the potential for increased exposure to infectious disease resulting from the provision of patient care. Pregnancy can confer specific problems in some infections for the mother and her unborn child. AIMS: To discuss the viral infections encountered in the UK that constitute a particular risk to the pregnant HCW: human immunodeficiency virus, hepatitis B virus, hepatitis C virus, varicella-zoster virus, herpes simplex virus, human parvovirus B19, cytomegalovirus, rubella, measles, enteroviruses, mumps and influenza. Evidence for nosocomial transmission, clinical aspects specific to pregnancy, and recommendations to protect the pregnant HCW at work are included. METHODS: Medline, EMBASE and Pubmed were searched using a list of keywords specific to each viral infection, including 'nosocomial', 'occupational' and 'healthcare workers'. References from the bibliographies of articles identified were reviewed for relevant material. FINDINGS: The evidence for increased risk in the healthcare setting for many of these infections, outside of outbreaks, is weak, possibly because of the application of standard protective infection control measures or because risk of community exposure is greater. The pregnant HCW should be advised on protective behaviour in both settings. Potential interventions include vaccination and reducing the likelihood of exposure through universal precautions, infection control and redeployment. CONCLUSION: Protection of the pregnant HCW is the responsibility of the individual, antenatal care provider and employer, and is made possible through awareness of the risks and potential interventions both before and after exposure. If exposure occurs or if the HCW develops an infective illness, urgent specialist advice is required.

Comparative antibacterial effects of moxifloxacin and levofloxacin on Streptococcus pneumoniae strains with defined mechanisms of resistance: impact of bacterial inoculum.

OBJECTIVES: We aim to further define the impact of the mechanism of fluoroquinolone resistance and inoculum load on the pharmacodynamic effects of levofloxacin and moxifloxacin on Streptococcus pneumoniae. METHODS: The antibacterial effects of and emergence of resistance (EoR) to moxifloxacin (400 mg once daily) or levofloxacin (750 mg once daily or 500 mg twice daily) were compared using five S. pneumoniae strains containing no known resistance mechanisms, efflux resistance mechanisms, a parC mutation or parC and gyrA mutations, at high (10(8) cfu/mL) and low (10(6) cfu/mL) inocula. An in vitro pharmacokinetic model was used and simulations were performed over 96 h. After drug exposure, isolates were tested for the presence of efflux pumps and mutations in the quinolone resistance-determining regions. RESULTS: A high inoculum diminished the antibacterial effect of moxifloxacin and levofloxacin. Levofloxacin at both dosages produced EoR with all strains. Levofloxacin regimens with AUC/MIC ratios <100 produced EoR. Moxifloxacin produced EoR with the parC strain only. CONCLUSIONS: Levofloxacin dosing regimens with low AUC/MIC ratios select for efflux pump overexpression, leading to fluoroquinolone resistance. Levofloxacin dosing may select for gyrA mutations, inducing moxifloxacin resistance. These data confirm that a fluoroquinolone AUC/MIC ratio of >100 is required for prevention of EoR.

EUCAST expert rules in antimicrobial susceptibility testing.

EUCAST expert rules have been developed to assist clinical microbiologists and describe actions to be taken in response to specific antimicrobial susceptibility test results. They include recommendations on reporting, such as inferring susceptibility to other agents from results with one, suppression of results that may be inappropriate, and editing of results from susceptible to intermediate or resistant or from intermediate to resistant on the basis of an inferred resistance mechanism. They are based on current clinical and/or microbiological evidence. EUCAST expert rules also include intrinsic resistance phenotypes and exceptional resistance phenotypes, which have not yet been reported or are very rare. The applicability of EUCAST expert rules depends on the MIC breakpoints used to define the rules. Setting appropriate clinical breakpoints, based on treating patients and not on the detection of resistance mechanisms, may lead to modification of some expert rules in the future.

Lack of upward creep of glycopeptide MICs for methicillin-resistant Staphylococcus aureus (MRSA) isolated in the UK and Ireland 2001-07.

OBJECTIVES: There have been several reports of upward creep in vancomycin MICs for Staphylococcus aureus [predominantly methicillin-resistant S. aureus (MRSA)] over recent years, but only in single centres or using contemporaneous results. We aimed to test the hypothesis of MIC creep in a multicentre study, testing all the isolates concurrently. METHODS: Nineteen laboratories in the UK and Ireland contributed isolates from blood to the BSAC Bacteraemia Resistance Surveillance Programme every year between 2001 and 2007. MICs for 271 MRSA from these sites were re-measured at a single central laboratory during a single week by the BSAC agar dilution method, but with √2-fold instead of conventional 2-fold dilutions. Re-test results were compared with the original results obtained each year at the same central laboratory. RESULTS: The re-test results were much less variable than the original results and avoided the confounding of experimental variation with year of collection. They demonstrated statistically significant but very slow downward trends in MICs of vancomycin and teicoplanin, at 0.027 and 0.055 doubling dilutions/year, respectively. The original results had suggested more rapid trends in MICs, upward for vancomycin and downward for teicoplanin. The proportion of EMRSA-16 fell from 21% to 9% over the study period, while EMRSA-15 rose from 76% to 85%. CONCLUSIONS: Historical data can give a misleading impression of trends in MIC values because of experimental variation between tests conducted at different times. There was no upward creep in glycopeptide MICs for MRSA in the UK and Ireland between 2001 and 2007.

The role of pharmacokinetics/pharmacodynamics in setting clinical MIC breakpoints: the EUCAST approach.

Clinical breakpoints are used in clinical microbiology laboratories to categorize microorganisms as clinically susceptible (S), intermediate (I) or resistant (R) dependent on the quantitative antimicrobial susceptibility as indicated by the MIC value determined in a well-defined standard test system. The laboratory report, with the designations of S, I or R for each antimicrobial agent, provides guidance to clinicians with respect to the potential use of agents in the treatment of patients, and clinical breakpoints should therefore distinguish between patients that are likely or unlikely to respond to antimicrobial treatment. In Europe, clinical breakpoints are set by the European Committee on Antimicrobial Susceptibility Testing (EUCAST), following a defined procedure. This includes evaluation of efficacy in experimental settings and clinical studies to derive pharmacodynamic targets such as the fAUC/MIC ratio or %fT > MIC required for efficacy, the pharmacokinetic properties of the agent, Monte Carlo simulations to estimate exposures of the antimicrobial agent in the target patient population and commonly used dosing regimens. The probability of target attainment is subsequently determined for a range of pharmacodynamic targets and the results from the Monte Carlo simulations. The breakpoints derived are subsequently evaluated with respect to the wild-type population of the target microorganisms, specific resistance mechanisms and other relevant data. In this paper, we provide an overview of the EUCAST process and considerations for setting pharmacokinetic/pharmacodynamic breakpoints. These are the breakpoints that in the EUCAST breakpoint tables are referred to as 'non-species-related breakpoints'.

Role of early intravenous to oral antibiotic switch therapy in the management of prosthetic hip infection treated with one- or two-stage replacement.

OBJECTIVES: To determine outcomes for an antibiotic regimen using early switch to oral antibiotics for treatment of infected total hip replacement (THR) treated by either a one-stage or two-stage procedure. METHODS: Cases of infected THR were identified from the microbiology records on all orthopaedic infections in a 24 month period. Diagnosis was made by microbiological culture of theatre specimens and findings at the time of surgery. A standard approach of 10-14 days intravenous (iv) antibiotic followed by a switch to oral antibiotics either for 6-8 weeks until second-stage re-implantation or for up to 3 months following a one-stage procedure was used. The exact date of oral switch and antibiotic duration was determined by clinical resolution and C-reactive protein (CRP). Outcome was recorded as no microbiological or clinical evidence of relapse of infection or relapse after completing the antibiotic course. Follow-up duration for all cases at the time of study was 24-36 months after completion of antibiotic treatment. RESULTS: In 24 months, 19 patients underwent two-stage THR for infection, of which 17 were treated with oral antibiotics after a median of 14 days initial iv antibiotics. None relapsed. Four patients underwent one-stage THR and had 12-20 days iv then 6-26 weeks oral antibiotics with no relapse. CONCLUSIONS: Early oral antibiotic switch therapy was effective in patients treated at the Avon Orthopaedic Centre with infected THR and plays an important role in enabling patients to return to independence after revision surgery and avoid complications of prolonged iv access.

Use of intravenous co-trimoxazole to treat bacterial infection: analysis of 50 treatment episodes.

Co-trimoxazole is infrequently prescribed in the UK due to concerns regarding adverse events. However, it has a low association with Clostridium difficile-associated disease (CDAD) and may represent an alternative to higher-risk agents. This retrospective study examines the efficacy and safety of intravenous co-trimoxazole in treatment of bacterial infection in a UK inpatient population of 50 inpatients. Outcome was determined to be successful in 58% of treatment episodes; in hospital-acquired pneumonia the response rate was 52%. Two treatment episodes were terminated due to adverse events: these resolved on discontinuation of co-trimoxazole. No significant change in renal function, evaluated by serum creatinine, was observed during therapy; blood platelet count was not affected by treatment with intravenous co-trimoxazole. One patient developed CDAD within one month of cessation of therapy. Intravenous co-trimoxazole was clinically effective with a low rate of adverse events and may represent a useful alternative antimicrobial agent in UK institutions with high rates of CDAD.

Analyses of teicoplanin concentrations from 1994 to 2006 from a UK assay service.

Objectives An analysis of the trough serum concentrations sent to the UK Antimicrobial Reference Laboratory for teicoplanin therapeutic drug monitoring (TDM). Methods All trough concentrations over a 13 year period were analysed and the percentages were calculated for the following: <10 mg/L (a sub-optimal concentration for all); ≥10-<20 mg/L (the target used for ordinary Gram-positive infections); ≥20-<60 mg/L (the target for all severe staphylococcal infections including endocarditis); and ≥60 mg/L (the concentration associated with toxicity). Results The percentage of patients with concentrations of <10 mg/L decreased each year to 13% in 2006. Almost 40% of the samples each year were in the ≥10-<20 mg/L range. In 1996, the percentage of samples in the ≥20-<60 mg/L range reached a study high of ∼70%. That percentage then fell to 30% and increased slowly to 50% at the end of the study. Fewer than 5% of the samples were ≥60 mg/L. Conclusions Our study shows that there is a need to increase the initial dose or extend the number of days that the loading dose is used in a significant number of patients. With such a wide optimal range and a low potential for toxicity, it is unclear why optimal therapy is not achieved in a higher percentage of patients.

Development, evaluation and application of an isocratic high-performance liquid chromatography (HPLC) assay for the simultaneous determination of aciclovir and its metabolite 9-carboxymethoxymethylguanine in human serum and cerebrospinal fluid.

9-Carboxymethoxymethylguanine (CMMG), the main metabolite of aciclovir (ACV), is a putative neurotoxin. Measurement of CMMG in body fluids may aid patient management. We describe the development, validation and application of a high-performance liquid chromatography (HPLC) method for the simultaneous determination of ACV and CMMG in human serum and cerebrospinal fluid (CSF). Recovery was between 94% and 100% at all concentrations both from serum (range 0-20 mg/L) and CSF (0-5 mg/L). The intra-assay precision (coefficient of variation (CV)) was <2% and the inter-assay precision (CV) was <5%. The limits of detection and quantification were 0.1 and 0.25 mg/L, respectively, in both body fluids. Significant interference from endogenous material or from drugs in clinical samples was not seen. CMMG was detected in most of the 55 clinical samples containing ACV, but little correlation was found between the levels of the drug and its metabolite.

European Committee on Antimicrobial Susceptibility Testing (EUCAST) Technical Notes on antimicrobial susceptibility testing.

The main objectives of the European Committee on Antimicrobial Susceptibility Testing (EUCAST) are to harmonise breakpoints for antimicrobial agents in Europe, and to act as the breakpoint committee for the European Medicines Agency (EMEA) during the registration of new antimicrobial agents. Detailed EUCAST procedures for harmonising and setting breakpoints for antimicrobial agents are available on the EUCAST website. Beginning with the current issue, a series of EUCAST Technical Notes will be published in CMI, based on the rationale documents produced by EUCAST for each of the antimicrobial agents studied, with the aim of highlighting important background information underlying decisions on breakpoints made by EUCAST.

Expression of tcaA and mprF and glycopeptide resistance in clinical glycopeptide-intermediate Staphylococcus aureus (GISA) and heteroGISA strains.

Two genes recently associated with glycopeptide intermediate resistance in Staphylococcus aureus (GISA) are mprF and tcaA, with inactivation causing shifts in vancomycin resistance. This study reveals that expression levels of both genes are similar in groups of clinical GISA, heteroGISA and glycopeptide susceptible strains, suggesting no association with clinical isolates.