Mathematical modelling of growth of Escherichia coli at subinhibitory levels of chloramphenicol or tetracyclines.

A mathematical model is suggested to describe the subinhibitory effects of chloramphenicol upon growth of Escherichia coli. It represents a generalization of Monod's equation. Our model was constructed entirely on the basis of biochemical factors, such as the reversibility of the interaction between the drug and its ribosomal target or intracellular uptake. The subinhibitory effects of chloramphenicol were satisfactorily described by this law within the range of 0.4-2.4 micrograms/ml. After simplifying the equation, this model was extrapolated to provide an accurate description of the mode of action of other bacteriostatic antibiotics which also inhibit the 50-S ribosomal subunit (tetracycline and doxycycline). The expanded model seems to accurately typify the effect of these antibiotics, whereas bactericidal aminoglycosides follow a completely different growth law.

[Use of an expert system as a tool to carry out urinary cyto-bacteriologic tests]. / Utilisation d'un système expert d'aide à la réalisation des examens cytobactériologiques urinaires.

An expert system for urine examinations was developed. Its validation was performed in a routine work, by comparison of expert decisions with software conclusions. Results of finished examinations or in progress were taken into account. The agreement was satisfactory. In addition to reliability, advantages constituted by availability, flexibility and adaptability were notified. Repeatability and reproducibility were also emphasized.

[An expert system as an aid to the validation of results of the antibiogram. Feasibility study based on the example of Staphylococcus aureus]. / Système expert d'aide à la validation des résultats de l'antibiogramme. Etude de faisabilité sur l'exemple de Staphylococcus aureus.

A model of expert system using Prolog language was developed, to verify the coherence of the results of the antibiotic sensitivity test. Biological knowledge was formalized in three different ways: a credibility coefficient based on epidemiological data is assigned to known observed resistance; co-existent resistances are described with lists of "implicit" resistances, reflecting phenotypes commonly observed within some antibiotic groups; every single or "implicit" resistance is connected to a "gregarius" status, expressing the plasmidic nature of resistance. Applied to Staphylococcus aureus, the expert system is able to detect the inconsistencies of the antibiotic sensitivity test and to identify required knowledges. It therefore allows phenotypic interpretation of results.

Mathematical modelling of bacterial growth at subinhibitory levels of aminoglycosides.

The subinhibitory effect of antibiotics has often been studied without any clear theoretical framework; we have chosen to use mathematical bacterial growth modelling as a useful tool to analyse these biological states in a more rigorous manner. Since their mode of action and molecular target are relatively well known, aminoglycosides were well suited for this more sophisticated study of subinhibitory action. We have shown that two models (the Monod and the logistic models) regularly used in bacteriology, were adequate to describe these effects in a glucose-limited medium. A change of model, according to antibiotic concentration, revealed the existence of two separate actions. At lower concentrations, inhibition affected mainly glucose use, the substrate remained limiting and growth mode did not change. As soon as the concentration exceeded a threshold, growth was totally disturbed, probably through a physiological "catastrophe". This threshold can be used to estimate bacterial susceptibility to these antibiotics.

[Bacteriostatic and bactericidal antibiotics: differences in subinhibitory concentration effects on the growth of Escherichia coli]. / Antibiotiques bactériostatiques et bactéricides: différences des effets des concentrations subinhibitrices sur la croissance de Escherichia coli.

The growth of Escherichia coli in synthetic medium was mathematically analysed at subinhibitory levels of bactericidal and bacteriostatic antibiotics. Over a threshold of concentration, bactericidal antibiotics induce an unexpected change of growth pattern. Bacteriostatic antibiotics induce a progressive effect linked to a continuously altered growth pattern that still remains close to the controls. This fundamental differential of antibiotic effects is reflecting well-known bacteriostatic or bactericidal phenomena. In vitro this analysis of the growth enables to measure the level of the antibiotic sensitivity. In vivo, these results must be taken into account when subinhibitory concentrations of antimicrobial agent are reached at the infection site.

VIDAS D-dimer: fast quantitative ELISA for measuring D-dimer in plasma.

VIDAS D-dimer (bioMérieux) is a new quantitative ELISA for D-dimer determination designed for the VIDAS automated system. The test contains single-dose, ready-to-use reagents and is completed within 35 min. Quantitative results are obtained from a calibration curve stored in the software of the system and expressed as fibrinogen equivalent units. The two-step capture/tag test relies on two complementary monoclonal anti-D-dimer antibodies, the second one being labeled with alkaline phosphatase. The upper limit of the measuring range is 1000 micrograms/L and the lower detection limit is <50 micrograms/L, which is below the lower limit of the reference interval (68-494 micrograms/L). Reproducibility (CV) within and between runs ranges from 5% to 7%. There is no interference from heparin, bilirubin, hemoglobin, fibrinogen degradation products, or plasma turbidity. Comparison with a conventional ELISA (y) gave good correlation (r= 0.91, n= 579) and comparable results (y= 1.35x - 148, S(y/x)= 750), especially for D-dimer concentrations ranging from 0 to 1000 micrograms/L (y= 1.09x - 10.6, r= 0.88, S(y/x)= 170).

Differential diagnosis of prostate cancer and benign prostate hyperplasia using two-dimensional electrophoresis.

Prostate specific antigen (PSA) is a protease which is characteristic of the prostate. It is widely used as a serum marker for the early diagnosis of prostate cancer (PCa). Nevertheless, for concentrations between 4 and 10 ng/mL, PSA does not enable PCa to be distinguished from benign diseases, such as benign prostate hyperplasia (BPH). In sera, the use of a ratio between free PSA (PSA uncomplexed with protease inhibitor) and total PSA (free PSA and PSA bound to alpha-1 anti-chymotrypsin) enables the "gray zone" to be reduced, but an important proportion of patients are still wrongly classed. Using two-dimensional electrophoresis, we demonstrated using 52 PCa and 40 BPH well-documented clinical cases that BPH sera show a significantly greater percentage of low-molecular-weight free PSA elements (IwPSA) than PCa sera. In our study, the use of a ratio between IwPSA and standard free PSA enables the correct diagnosis of 100% of PCa and 82.5% of BPH cases as against when 73.1% and 42.5% respectively were correctly diagnozed using the total PSA and the free/total PSA ratio. This important finding may be related to differences in the mechanism secreting PSA from the prostate into the bloodstream. We have shown how a tissue marker may be turned into a powerful tumor marker by events probably unrelated to its expression.