Determinants of response to antibiotic therapy

Biocide susceptibility testing of bacteria: Development of a broth microdilution method

Biocide susceptibility testing (BST) of bacteria lacks standardised methods. Based on a recently established broth macrodilution BST method, a broth microdilution method for BST was developed. To establish the respective protocol, four reference strains Staphylococcus aureus ATCC® 6538, Enterococcus hirae ATCC® 10541, Escherichia coli ATCC® 10536 and Pseudomonas aeruginosa ATCC® 15442 were investigated for their minimal inhibitory concentrations (MICs) towards quaternary ammonium compounds (benzalkonium chloride), cationic compounds (chlorhexidine), aldehydes (glutardialdehyde) and alcohols (isopropanol) using tryptic soy broth. All combinations of (i) inoculum preparation according to the German Veterinary Medical Society (DVG) or the Clinical and Laboratory Standards Institute (CLSI) with some modifications, (ii) use of 1^st subculture (SC) and 2^nd SC, (iii) direct colony suspension (DCS) with/without glass beads, and (iv) incubation at 37 °C for 24 h, 48 h, and 72 h were compared using seven independent replications. Overall, the reproducibility was high for all abovementioned strain/biocide/parameter combinations. In total, 86.9 % - 100 % of the results were within ± one dilution step of the mode value. The proposed method for a standardised BST protocol comprises (i) two different inoculum densities, (ii) the use of a fresh overnight culture (1^st SC or 2^nd SC), (iii) the preparation of the inoculum suspension by either of the two methods using DCS with or without glass beads, and (iv) the incubation at 37 °C for 24 h. This broth microdilution method will help to harmonize BST of bacterial pathogens in routine diagnostics.

Single-cell bacteria growth monitoring by automated DEP-facilitated image analysis

Growth monitoring is the method of choice in many assays measuring the presence or properties of pathogens, e.g. in diagnostics and food quality. Established methods, relying on culturing large numbers of bacteria, are rather time-consuming, while in healthcare time often is crucial. Several new approaches have been published, mostly aiming at assaying growth or other properties of a small number of bacteria. However, no method so far readily achieves single-cell resolution with a convenient and easy to handle setup that offers the possibility for automation and high throughput. We demonstrate these benefits in this study by employing dielectrophoretic capturing of bacteria in microfluidic electrode structures, optical detection and automated bacteria identification and counting with image analysis algorithms. For a proof-of-principle experiment we chose an antibiotic susceptibility test with Escherichia coli and polymyxin B. Growth monitoring is demonstrated on single cells and the impact of the antibiotic on the growth rate is shown. The minimum inhibitory concentration as a standard diagnostic parameter is derived from a dose-response plot. This report is the basis for further integration of image analysis code into device control. Ultimately, an automated and parallelized setup may be created, using an optical microscanner and many of the electrode structures simultaneously. Sufficient data for a sound statistical evaluation and a confirmation of the initial findings can then be generated in a single experiment.

Susceptible, intermediate, and resistant - the intensity of antibiotic action

INTRODUCTION

To date, the resistance of infectious agents has been assessed by widely varying criteria in different countries. Therefore, published data on resistance often cannot be meaningfully compared. In Germany, different laboratories can potentially report different results for identical microorganisms, since there is no uniform system for categorization. This situation is unsatisfactory.

METHODS

Selective literature review and evaluation of committee reports.

RESULTS

The new ISO standard 20776 for determination of the resistance of infectious agents and the harmonized evaluation system of the European Society for Clinical Microbiology and Infectious Diseases provide a new basis for susceptibility testing. The categorization of infectious agents as "susceptible," "intermediate," or "resistant" to particular antibiotics will become more reliable and will be consistent throughout Europe.

DISCUSSION

For a number of antibiotics, the criteria for evaluation of infectious agents as "susceptible," "intermediate", or "resistant" will change. Comparability with earlier resistance data will be compromised. However, the new evaluation criteria reflect current knowledge on the pharmacokinetics and pharmacodynamics of antimicrobial substances.

Evaluation and Prediction of Fluoroquinolone Pharmacodynamics in Bacterial Keratitis

Observational studies suggest that a pharmacodynamic index helps to predict the therapeutic outcome of respiratory and other infections. This study explored the prognostic importance of a ratio of the achievable corneal level for a fluoroquinolone to the fluoroquinolone's minimal inhibitory concentration (MIC) for corneal isolates of 391 patients with bacterial keratitis. The peak concentration and the area under the concentration curve (AUC) in the cornea were estimated from reported values achieved with topical ciprofloxacin. The inhibitory quotient (IQ) was calculated as the ratio of the estimated peak achievable corneal ciprofloxacin concentration to the ciprofloxacin MIC of keratitis isolates, and the area under the inhibitory curve (AUIC) was defined as the expected 24-hour AUC divided by the MIC. The probability of clinical improvement of ciprofloxacin-treated bacterial keratitis was 90% or more if ciprofloxacin's IQ was above 8 or the AUIC was greater than 151. A pharmacodynamic index relating corneal pharmacokinetic and susceptibility concentrations may correlate with the clinical response of bacterial keratitis to fluoroquinolone therapy.

Community-acquired pneumonia. Diagnostic and therapeutic approach

Optimal empiric therapy of CAP is with appropriate monotherapy (e.g., doxycycline, levofloxacin). Combination therapy is problematic because of potential side effects and high cost. Empiric coverage should have a high degree of activity against both typical and atypical pathogens. The antibiotic selected should have an excellent side-effect profile and be relatively inexpensive. Clinicians should be selective in their choice of antibiotic for CAP and choose an antimicrobial that has little or no resistance potential, is relatively inexpensive, and permits i.v.-to-PO switch monotherapy.

Antimicrobial survey of urinary tract isolates from a pediatric department

The epidemiology of urinary tract colonization/infection in children admitted during 1996 to the Pediatric Department of the University of Verona has been studied; 501/1959 urine cultures were positive (25.57%). 584 microorganisms (64.89% Gram-negative, 24.82% Gram-positive, 10.27% fungi) were isolated. The highest rate of Gram-negative isolation (80.0%) was observed in infants, while the highest rate of Gram-positive isolation (29.6%) was found in newborns admitted to the neonatal intensive care unit. Escherichia coli was the most frequently isolated microorganism in infants and children, but not in newborns. A 3-fold increase in resistant E. coli strains to cotrimoxazole/sulfamethoxazole and amoxicillin/clavulanic acid was documented during the last 3 years in this pediatric population. Our observations underline the importance of the survey of microbial maps in pediatric departments in order to optimize therapeutic and preventive choices.