Alasdair P. MacGowan

Continuous Infusion of β-Lactam Antibiotics

There are considerable laboratory data and information from animal and continuous culture in vitro models to support continuous infusion therapy for penicillins and cephalosporins, but, as yet, the only existing clinical data relate to cephalosporins.Penicillins do not exert concentration-dependent killing in the therapeutic range but have a post-antibiotic effect (PAE) against Gram-positive cocci but not Gram-negative rods. Animal models indicate the time (T) during which the serum concentrations exceed the minimum inhibitory concentration (MIC) of the pathogen [T > MIC] determines outcomes. Pharmacokinetic studies in humans indicate that continuous infusion with penicillins is possible but there are no clinical data on efficacy.Cephalosporins have similar pharmacodynamic properties to penicillins; T > MIC determines outcome. Data related to ceftazidime indicate that the drug concentration at steady-state (Css) should exceed the pathogen MIC by >1-fold and perhaps by 4- to 5-fold or more. Human pharmacokinetics of ceftazidime administered by continuous infusion to a wide variety of patient groups indicates that Css of>20 mg/L can easily be achieved using conventional daily doses. Clinical data indicate increased effectiveness of a continuous regimen in neutropenic patients with Gram-negative infection. Furthermore cefuroxime administration by continuous infusion has resulted in lower doses and shorter course durations.Little is known of the pharmacodynamics of monobactams and there are few clinical data on continuous infusion therapy.Carbapenems have different pharmacodynamics to other β-lactams as they have concentration-dependent killing and a PAE with both Gram-positive and Gram-negative bacteria. While T > MIC has a role in determining outcomes, the proportion of the dosing interval for which serum drug concentrations should exceed the pathogen MIC is less than for other β-lactams. In vitro models have shown that continuous infusion is effective, as is less frequent dosing. There are few data on continuous infusion of carbapenems but some patients have been treated with once-daily dosing.Clinically, continuous infusion therapy with penicillins and cephalosporins should be considered in patients infected with susceptible Gram-negative rods not responding to conventional therapy. As an approximation, the same total daily dose should be given but a bolus intravenous injection should be give at the start of continuous infusion to ensure Css is reached rapidly. The Css may be difficult to predict and determination of serum drug concentrations may be indicated. Ideally, the Css should be calculated based on the MIC of the potential pathogen and may be higher or lower than the Css achieved by a conventional daily dose.: There are considerable laboratory data and information from animal and continuous culture in vitro models to support continuous infusion therapy for penicillins and cephalosporins, but, as yet, the only existing clinical data relate to cephalosporins. Penicillins do not exert concentration-dependent killing in the therapeutic range but have a post-antibiotic effect (PAE) against Gram-positive cocci but not Gram-negative rods. Animal models indicate the time (T) during which the serum concentrations exceed the minimum inhibitory concentration (MIC) of the pathogen [T > MIC] determines outcomes. Pharmacokinetic studies in humans indicate that continuous infusion with penicillins is possible but there are no clinical data on efficacy. Cephalosporins have similar pharmacodynamic properties to penicillins; T > MIC determines outcome. Data related to ceftazidime indicate that the drug concentration at steady-state (Css) should exceed the pathogen MIC by > 1-fold and perhaps by 4- to 5-fold or more. Human pharmacokinetics of ceftazidime administered by continuous infusion to a wide variety of patient groups indicates that Css of > 20 mg/L can easily be achieved using conventional daily doses. Clinical data indicate increased effectiveness of a continuous regimen in neutropenic patients with Gram-negative infection. Furthermore cefuroxime administration by continuous infusion has resulted in lower doses and shorter course durations. Little is known of the pharmacodynamics of monobactams and there are few clinical data on continuous infusion therapy. Carbapenems have different pharmacodynamics to other beta-lactams as they have concentration-dependent killing and a PAE with both Gram-positive and Gram-negative bacteria. While T > MIC has a role in determining outcomes, the proportion of the dosing interval for which serum drug concentrations should exceed the pathogen MIC is less than for other beta-lactams. In vitro models have shown that continuous infusion is effective, as is less frequent dosing. There are few data on continuous infusion of carbapenems but some patients have been treated with once-daily dosing. Clinically, continuous infusion therapy with penicillins and cephalosporins should be considered in patients infected with susceptible Gram-negative rods not responding to conventional therapy. As an approximation, the same total daily dose should be given but a bolus intravenous injection should be give at the start of continuous infusion to ensure Css is reached rapidly. The Css may be difficult to predict and determination of serum drug concentrations may be indicated. Ideally, the Css should be calculated based on the MIC of the potential pathogen and may be higher or lower than the Css achieved by a conventional daily dose.

Clinical implications of antimicrobial resistance for therapy

The last decade has seen a significant improvement in published evidence to show the clinical predictive value of phenotypic susceptibility testing with categorization of pathogens as susceptible or resistant based on clinical breakpoints. Most of the published data are based on retrospective or prospective observational clinical studies of patients treated with appropriate [pathogen(s)-susceptible] or inappropriate [pathogen(s)-resistant] chemotherapy. Appropriate therapy has been shown to improve outcomes in infections occurring in hospitals, such as bloodstream infection (BSI) and pneumonia in the intensive care unit. Infections due to specific pathogens such as extended-spectrum beta-lactamase-producing Enterobacteriaceae, Pseudomonas aeruginosa and Staphylococcus aureus also respond better to appropriate than inappropriate antibiotics. The situation with vancomycin-resistant enterococci is less clear, perhaps due to the increased importance of patient confounders. Streptococcus pneumoniae when causing acute pneumonia with or without BSI is a well-known exception to the predictive value of laboratory-defined resistance. Antibiotic resistance also impacts on outcomes in the community where the evidence is best for urinary tract infection. The clinical studies are compatible with the current pharmacokinetic/pharmacodynamic paradigm used to explain and predict antibacterial effects and therefore have a sound basis in antimicrobial science. These data underline the importance of well-constructed epidemiological studies to determine the prevalence of antimicrobial resistance in clinical practice and the central place of laboratory-based susceptibility testing in dictating antimicrobial therapy and so optimizing patient outcomes.

Moxifloxacin (Bay 12-8039): a new methoxy quinolone antibacterial.

Moxifloxacin (Bay 12-8039) is a new 8 methoxy quinolone antibacterial. The MIC90 values are < or = 0.25 mg/l for Streptococcus pneumoniae (irrespective of penicillin susceptibility), Haemophilus influenzae (beta-lactamase positive or negative), Morexella catarrhalis, Bordetella pertussis, Legionella sp., Mycoplasma pneumoniae, Clamydia pneumoniae, Mycobacterium tuberculosis, methicillin-sensitive Staphylococcus aureus, beta-haemolytic streptococci (macrolide-sensitive or -resistant), Listeria sp., most Enterobacteriaceae, Salmonella sp., Shigella sp., Neisseria gonorrhoeae, N. menigitidis, Pasteurella spp., Vibrio spp. and Yersinia enterocolitica. For Mycobacterium intracellularae, methicillin-resistant S. aureus (MRSA), ciprofloxacin-resistant S. aureus, Citrobacter freundii, Providencia sp., Serratia sp., P. aeruginosa and other non-fermentive Gram-negative rods, MIC90s are in the range 0.5-4 mg/l. For anaerobic bacteria species, MIC90s are also in the range 0.25-4 mg/l. Moxifloxacin is bactericidal at concentrations 2- to 4-fold higher than the MIC and is rapidly bactericidal against most common pathogen groups at concentrations achieved in serum with a 400 mg dose that is between 0.5-4 mg/l. There is a post-antibiotic effect against Gram-positive and -negative bacteria. Resistant mutants are at present difficult to select in the laboratory but in general, moxifloxacin has poorer activity against strains resistant to ciprofloxacin compared to those which are susceptible. Animal and laboratory pharmacodynamic models indicate that the MIC and area under the serum concentration time curve predict outcome. Various animal models mainly of respiratory tract infection indicate equivalent or superior results compared to existing or other developmental agents. Human pharmacokinetics in healthy volunteers indicate linear pharmacokinetics over the dose range 50-800 mg/day. A single dose of 400 mg produces a maximum serum concentration of 2.5-4.5 mg/l, half-life of 11-15 h, AUC of 25-40 mg x h/l and volume of distribution of 2.5-3.5 L/kg. Protein binding is about 50% and two metabolites have been identified (M-1 and M-2). Bioavailability is > 85% and a minority of clearance is via the kidneys. No dose modification is required in renal impairment. Extra vascular penetration, where studied, is comparable to that of other quinolones. At present undergoing clinical trials, with a focus on respiratory tract infection, it is likely that moxifloxacin will provide effective therapy for pathogens with MICs of < or = 0.25-0.5 mg/l. The safety profile in a large number of human subjects is awaited.

Evidence for Reduction in Breakpoints Used To Determine Vancomycin Susceptibility in Staphylococcus aureus

Questions regarding the prevalence and clinical significance of both vancomycin-intermediate Staphylococcus aureus (VISA) and, more importantly, heterogeneous VISA (hVISA) are difficult to address, mainly due to the lack of accurate methods and guidelines for the identification of hVISA. VISA strains have been isolated from many areas of the world but are few in number (<30); however, hVISA strains appear more common. Current susceptibility techniques, along with CSLI (S/I/R-4/8-16/32) and EUCAST (S/R-4/8) recommended breakpoints, are generally inadequate for the identification of hVISA (10, 11). n nAlthough glycopeptides remain the recommended therapy for patients with methicillin-resistant Staphylococcus aureus infection, treatment failure is common, with 10 to 20% in cases of endocarditis and 40% in lower respiratory tract infections (3, 6). Treatment failure cause is not fully understood; however, clinical failures have been reported for patients with VISA (8, 1) and, more recently, for hVISA infection (2, 12, 13). Evidence suggests that hVISA infections are clinically relevant, with patients with hVISA bacteremia more likely to have high-bacterial-load infections, vancomycin treatment failure, bacteremia for >7 days, and a significantly higher mortality (63%) than those patients infected with vancomycin-susceptible methicillin-resistant S. aureus (VSSA) bacteremia (12%) (4, 5). More recently, treatment failure in patients with infections caused by strains with MICs within the susceptibility range (≤2 mg/liter) has been reported (9). Also, current CDC guidelines for testing S. aureus with vancomycin recommend that strains exhibiting MICs of ≤2 mg/liter plus growth on vancomycin screening plates are possibly VISA (www.cdc.gov/ncidod/hip/vanco/vanco.htm). These data suggest that current susceptibility breakpoints for glycopeptides require review. n nWe used an international collection of VSSA, VISA, and hVISA strains with CSLI agar dilution techniques to determine vancomycin MICs (7). Each data set was compared with a view to recalculating a vancomycin breakpoint. VISA and hVISA phenotypes were identified by the more accurate modified population analysis method (population analysis profile-area under the curve) (14). n nOf 106 glycopeptide-susceptible S. aureus strains, 10.4% had vancomycin MICs of 0.5 mg/liter, 85.8% had vancomycin MICs of 1 mg/liter, and 3.8% had vancomycin MICs of 2 mg/liter. The percentage of isolates classified as susceptible by the CSLI and EUCAST breakpoint (4 mg/liter) and a reduced breakpoint (2 mg/liter) was 100%, suggesting that no false positives would emerge from breakpoint reduction. Of 20 VISA strains, 55% exhibited vancomycin MICs of 4 mg/liter and 45% had vancomycin MICs of 8 mg/liter. Using the current breakpoint, 55% are classified as susceptible and 45% as intermediate. However, when using the reduced breakpoint, 100% are classified as intermediate (CSLI) or resistant (EUCAST). Of 157 hVISA strains, 2% exhibited vancomycin MICs of 1 mg/liter, 80.2% had vancomycin MICs of 2 mg/liter, and 17.8% had vancomycin MICs of 4 mg/liter. Using the current breakpoint, 100% are classified as susceptible; however, when using the reduced breakpoint, 82.2% would still be classified as susceptible but 17.8% would be classified as intermediate (CSLI) or resistant (EUCAST). By use of the reduced breakpoint, the correct classifications of hVISA and VISA would increase from 45% and 0% to 100% and 17.2%, respectively. Distribution of vancomycin MICs in the different phenotypes shows that 2 mg/liter is the most predominant in hVISA (Fig. u200b(Fig.1).1). The effect of a reduced breakpoint on the classification of coagulase-negative staphylococci would be minimal, with only 1.56% currently exhibiting a vancomycin MIC of 4 mg/liter (www.EUCAST.org). This evidence supports a modification of the contemporary interpretative guidelines for vancomycin susceptibility testing from ≤4 mg/liter for susceptible isolates to ≤2 mg/liter. n n n nFIG. 1. n nFrequency distribution of vancomycin MICs in VSSA, hVISA, and VISA strains.

Heterogeneity at the β-lactamase structural gene ampC amongst Citrobacter spp. assessed by polymerase chain reaction analysis: potential for typing at a molecular level

Considerable biochemical diversity and polynucleotide sequence variation have been reported amongst strains of Citrobacter spp. However, sequence heterogeneity has not been investigated at gene loci of clinical relevance. In this study, sequence heterogeneity in the beta-lactamase structural gene, ampC, amongst 91 clinical isolates of Citrobacter spp. that showed resistance to various third-generation cephalosporins was investigated. Variation was examined by high-stringency polymerase chain reactions (PCR) with primers homologous to the known ampC sequences of C. freundii strains OS60 and I113, and C. diversus NF85. If an isolate contained an ampC gene homologous to one of these three characterised ampC genes, a single PCR band of a predictable size was generated with the appropriate primer set; 50 (60%) of isolates gave a PCR product of the expected size with the OS60 primer set and nine (10%) gave a product with the I113 primer set. All these 59 isolates were identified as C. freundii by API-20E strips. Six isolates (7%) gave a product with the C. diversus NF85 primer set but only four of these were identified as C. diversus in API-20E tests; the other two isolates were identified as C. freundii. Of the 91 isolates, 28 (31%), were identified as either C. freundii or C. diversus, but gave no PCR product with any primer set tested. Five of these showed no homology to any of the reference strain ampC PCR products in hybridisation tests. Nevertheless, all showed beta-lactamase activity. Overall, this method allowed the identification of novel ampC gene loci, which may serve as a basis for the identification of Citrobacter spp. rapidly at a molecular level.

Frontline antibiotic therapy

The need to use front-line antibiotics wisely has never been greater. Antibiotic resistance and multi-drug resistant infection, driven by antibiotic use, remain major public health and professional concerns. To overcome these infection problems, use of older antibiotics active against multi drug-resistant pathogens is increasing - for example, colistin, fosfomycin, pivmecillinam, pristinamycin, temocillin and oral tetracyclines. The number of new antibacterials reaching clinical practice has reduced significantly in the last 20 years, most being focused on therapy of Gram-positive infection - eg linezolid, daptomycin, telavancin and ceftaroline. Recent guidance on antibiotic stewardship in NHS trusts in England is likely to provide a backdrop to antibiotic use in hospitals in the next 5 years.

Strain-Specific Expression Levels of pbp4 Exist in Isolates of Glycopeptide-Intermediate Staphylococcus aureus (GISA) and Heterogeneous GISA

A common characteristic of glycopeptide-intermediate-resistant Staphylococcus aureus (GISA) is a thickened cell wall, containing fewer cross-links and hence an abundance of d-Ala-d-Ala termini of the pentapeptide. A further phenomenon observed in GISA has been lowered or nonexistent expression of

Prediction of Fluoroquinolone Susceptibility Directly from Whole-Genome Sequence Data by Using Liquid Chromatography-Tandem Mass Spectrometry To Identify Mutant Genotypes

ABSTRACT Fluoroquinolone resistance in Gram-negative bacteria is multifactorial, involving target site mutations, reductions in fluoroquinolone entry due to reduced porin production, increased fluoroquinolone efflux, enzymes that modify fluoroquinolones, and Qnr, a DNA mimic that protects the drug target from fluoroquinolone binding. Here we report a comprehensive analysis, using transformation and in vitro mutant selection, of the relative importance of each of these mechanisms for fluoroquinolone nonsusceptibility using Klebsiella pneumoniae as a model system. Our improved biological understanding was then used to generate 47 rules that can predict fluoroquinolone susceptibility in K. pneumoniae clinical isolates. Key to the success of this predictive process was the use of liquid chromatography-tandem mass spectrometry to measure the abundance of proteins in extracts of cultured bacteria, identifying which sequence variants seen in the whole-genome sequence data were functionally important in the context of fluoroquinolone susceptibility.

‘Nappy pad’ urine samples for guiding investigation and treatment of urinary tract infection (UTI) in young children: Findings from the ‘DUTY’ prospective diagnostic cohort study

BACKGROUNDnThe added diagnostic utility of nappy pad urine samples and the proportion that are contaminated is unknown.nnnAIMnTo develop a clinical prediction rule for the diagnosis of urinary tract infection (UTI) based on sampling using the nappy pad method.nnnDESIGN AND SETTINGnAcutely unwell children <5 years presenting to 233 UK primary care sites.nnnMETHODnLogistic regression to identify independent associations of symptoms, signs, and urine dipstick test results with UTI; diagnostic utility quantified as area under the receiver operator curves (AUROC). Nappy pad rule characteristics, AUROC, and contamination, compared with findings from clean-catch samples.nnnRESULTSnNappy pad samples were obtained from 3205 children (82% aged <2 years; 48% female), culture results were available for 2277 (71.0%) and 30 (1.3%) had a UTI on culture. Female sex, smelly urine, darker urine, and the absence of nappy rash were independently associated with a UTI, with an internally-validated, coefficient model AUROC of 0.81 (0.87 for clean-catch), which increased to 0.87 (0.90 for clean-catch) with the addition of dipstick results. GPs working diagnosis had an AUROC 0.63 (95% confidence intervals [CI] = 0.53 to 0.72). A total of 12.2% of nappy pad and 1.8% of clean-catch samples were frankly contaminated (risk ratio 6.66; 95% CI = 4.95 to 8.96; P<0.001).nnnCONCLUSIONnNappy pad urine culture results, with features that can be reported by parents and dipstick tests, can be clinically useful, but are less accurate and more often contaminated compared with clean-catch urine culture.

‘Nappy pad’ urine samples for guiding investigation and treatment of urinary tract infection (UTI) in young children

BACKGROUNDnThe added diagnostic utility of nappy pad urine samples and the proportion that are contaminated is unknown.nnnAIMnTo develop a clinical prediction rule for the diagnosis of urinary tract infection (UTI) based on sampling using the nappy pad method.nnnDESIGN AND SETTINGnAcutely unwell children <5 years presenting to 233 UK primary care sites.nnnMETHODnLogistic regression to identify independent associations of symptoms, signs, and urine dipstick test results with UTI; diagnostic utility quantified as area under the receiver operator curves (AUROC). Nappy pad rule characteristics, AUROC, and contamination, compared with findings from clean-catch samples.nnnRESULTSnNappy pad samples were obtained from 3205 children (82% aged <2 years; 48% female), culture results were available for 2277 (71.0%) and 30 (1.3%) had a UTI on culture. Female sex, smelly urine, darker urine, and the absence of nappy rash were independently associated with a UTI, with an internally-validated, coefficient model AUROC of 0.81 (0.87 for clean-catch), which increased to 0.87 (0.90 for clean-catch) with the addition of dipstick results. GPs working diagnosis had an AUROC 0.63 (95% confidence intervals [CI] = 0.53 to 0.72). A total of 12.2% of nappy pad and 1.8% of clean-catch samples were frankly contaminated (risk ratio 6.66; 95% CI = 4.95 to 8.96; P<0.001).nnnCONCLUSIONnNappy pad urine culture results, with features that can be reported by parents and dipstick tests, can be clinically useful, but are less accurate and more often contaminated compared with clean-catch urine culture.