Progress and challenges in the use of fluorescence-based flow cytometric assays for anti-malarial drug susceptibility tests.

Drug-resistant Plasmodium is a frequent global threat in malaria eradication programmes, highlighting the need for new anti-malarial drugs and efficient detection of treatment failure. Plasmodium falciparum culture is essential in drug discovery and resistance surveillance. Microscopy of Giemsa-stained erythrocytes is common for determining anti-malarial effects on the intraerythrocytic development of cultured Plasmodium parasites. Giemsa-based microscopy use is conventional but laborious, and its accuracy depends largely on examiner skill. Given the availability of nucleic acid-binding fluorescent dyes and advances in flow cytometry, the use of various fluorochromes has been frequently attempted for the enumeration of parasitaemia and discrimination of P. falciparum growth in drug susceptibility assays. However, fluorochromes do not meet the requirements of being fast, simple, reliable and sensitive. Thus, this review revisits the utility of fluorochromes, notes previously reported hindrances, and highlights the challenges and opportunities for using fluorochromes in flow cytometer-based drug susceptibility tests. It aims to improve drug discovery and support a resistance surveillance system, an essential feature in combatting malaria.

Evaluating drug resistance in visceral leishmaniasis: the challenges.

For decades antimonials were the drugs of choice for the treatment of visceral leishmaniasis (VL), but the recent emergence of resistance has made them redundant as first-line therapy in the endemic VL region in the Indian subcontinent. The application of other drugs has been limited due to adverse effects, perceived high cost, need for parenteral administration and increasing rate of treatment failures. Liposomal amphotericin B (AmB) and miltefosine (MIL) have been positioned as the effective first-line treatments; however, the number of monotherapy MIL-failures has increased after a decade of use. Since no validated molecular resistance markers are yet available, monitoring and surveillance of changes in drug sensitivity and resistance still depends on standard phenotypic in vitro promastigote or amastigote susceptibility assays. Clinical isolates displaying defined MIL- or AmB-resistance are still fairly scarce and fundamental and applied research on resistance mechanisms and dynamics remains largely dependent on laboratory-generated drug resistant strains. This review addresses the various challenges associated with drug susceptibility and -resistance monitoring in VL, with particular emphasis on the choice of strains, susceptibility model selection and standardization of procedures with specific read-out parameters and well-defined threshold criteria. The latter are essential to support surveillance systems and safeguard the limited number of currently available antileishmanial drugs.

Inactivation of exogenous endoparasite stages by chemical disinfectants: current state and perspectives.

Chemical disinfection is common practice and inevitable to achieve sufficient control over parasites particularly in intensive animal housing systems. To identify suitable chemicals, reliable data on antiparasitic efficacy of disinfectants are required. This review summarizes recently published experience with procedures applied to evaluate the viability of a variety of endoparasites following physical or chemical stress. It is concluded that laboratory models used to assess antiparasitic efficacy of e.g. commercial disinfectants should consider the most resistant stages of both helminths and protozoa, i.e. ascarid eggs and coccidia oocysts. To ensure reproducibility and transparency, standardized protocols are pivotal. Such protocols are established on a national level (e.g. DVG guidelines in Germany); however, internationally accepted certification procedures are currently lacking.

Baseline in vitro activities of the antimalarials pyronaridine and methylene blue against Plasmodium falciparum isolates from Kenya.

We have analyzed the in vitro activities of pyronaridine and methylene blue against 59 Plasmodium falciparum isolates from Kenya in association with polymorphisms in Pfcrt (codon 76), Pfmdr1 (codon 86), and Pfnhe (full sequence). The median inhibitory concentrations that kill 50% of parasites were 13.5 and 3.3 nM for pyronaridine and methylene blue, respectively. Their activities were not associated with polymorphisms in these genes. The drugs' high in vitro activities indicate that they would be efficacious against Kenyan isolates in vivo.

Optimizing the HRP-2 in vitro malaria drug susceptibility assay using a reference clone to improve comparisons of Plasmodium falciparum field isolates.

BACKGROUND: Apparent emerging artemisinin-resistant Plasmodium falciparum malaria in Southeast Asia requires development of practical tools to monitor for resistant parasites. Although in vitro anti-malarial susceptibility tests are widely used, uncertainties remain regarding interpretation of P. falciparum field isolate values. METHODS: Performance parameters of the W2 P. falciparum clone (considered artemisinin "sensitive") were evaluated as a reference for the HRP-2 immediate ex vivo assay. Variability in W2 IC50s was assessed, including intra- and inter-assay variability among and between technicians in multiple experiments, over five freeze-thaw cycles, over five months of continuous culture, and before and after transport of drug-coated plates to remote field sites. Nominal drug plate concentrations of artesunate (AS) and dihydroartemisinin (DHA) were verified by LC-MS analysis. Plasmodium falciparum field isolate IC50s for DHA from subjects in an artemisinin-resistant area in Cambodia were compared with W2 susceptibility. RESULTS: Plate drug concentrations and day-to-day technical assay performance among technicians were important sources of variability for W2 IC50s within and between assays. Freeze-thaw cycles, long-term continuous culture, and transport to and from remote sites had less influence. Despite variability in W2 susceptibility, the median IC50s for DHA for Cambodian field isolates were higher (p <0.0001) than the W2 clone (3.9 nM), both for subjects with expected (less than 72 hours; 6.3 nM) and prolonged (greater or equal to 72 hours; 9.6 nM) parasite clearance times during treatment with artesunate monotherapy. CONCLUSION: The W2 reference clone improved the interpretability of field isolate susceptibility from the immediate ex vivo HRP-2 assay from areas of artemisinin resistance. Methods to increase the reproducibility of plate coating may improve overall assay interpretability and utility.

In vitro assessment of anticryptosporidial efficacy and cytotoxicity of adenosine analogues using a SYBR Green real-time PCR method.

OBJECTIVES: The aims of this study were to provide a cost-effective and valuable method for evaluating drug efficacy against Cryptosporidium parvum using a quantitative SYBR Green real-time PCR (qPCR) and to assess the efficacy of adenosine analogues as drug templates. METHODS: C. parvum HNJ-1 strain growing in human ileocaecal adenocarcinoma cells was employed as an in vitro culture system. To normalize the DNA extraction efficiency, a specific plasmid was added to each sample before DNA purification; the genomic DNA of infected cells was quantified by qPCR using specific primers to confirm drug efficacy and cytotoxicity. To determine the mechanism of action, enzymatic inhibition analyses were conducted using C. parvum S-adenosyl-l-homocysteine hydrolase (CpSAHH) recombinant protein. RESULTS: The dose-dependent growth inhibition of C. parvum was confirmed; 50% effective concentrations of neplanocin A (NPA) and 2-fluoroadenosine (2FA) were 139 µM and 0.842 µM, respectively. Cytotoxicity evaluation showed that the 50% growth inhibition concentration of 2FA was 1.18 µM; NPA did not exhibit any cytotoxicity up to 200 µM. The screening system revealed the specific but marginal efficacy of NPA and showed 2FA to be cytotoxic. Recombinant CpSAHH inhibition analyses showed that NPA competitively inhibited CpSAHH activity (K(i )= 0.395 µM), whereas 2FA did not. CONCLUSIONS: This novel qPCR system confirmed not only drug efficacy against C. parvum but also cytotoxicity to host cells. Moreover, since the SYBR Green method is cost effective, it could therefore be used in a wide variety of clinical and research-oriented applications of Cryptosporidium analysis.

Implementation of a reference standard and proficiency testing programme by the World Wide Antimalarial Resistance Network (WWARN).

BACKGROUND: The Worldwide Antimalarial Resistance Network (WWARN) is a global collaboration to support the objective that anyone affected by malaria receives effective and safe drug treatment. The Pharmacology module aims to inform optimal anti-malarial drug selection. There is an urgent need to define the drug exposure - effect relationship for most anti-malarial drugs. Few anti-malarials have had their therapeutic blood concentration levels defined. One of the main challenges in assessing safety and efficacy data in relation to drug concentrations is the comparability of data generated from different laboratories. To explain differences in anti-malarial pharmacokinetics in studies with different measurement laboratories it is necessary to confirm the accuracy of the assay methods. This requires the establishment of an external quality assurance process to assure results that can be compared. This paper describes this process. METHODS: The pharmacology module of WWARN has established a quality assurance/quality control (QA/QC) programme consisting of two separate components:1. A proficiency testing programme where blank human plasma spiked with certified reference material (CRM) in different concentrations is sent out to participating bioanalytical laboratories.2. A certified reference standard programme where accurately weighed amounts of certified anti-malarial reference standards, metabolites, and internal standards are sent to participating bioanalytical and in vitro laboratories. CONCLUSION: The proficiency testing programme is designed as a cooperative effort to help participating laboratories assess their ability to carry out drug analysis, resolve any potential problem areas and to improve their results - and, in so doing, to improve the quality of anti-malarial pharmacokinetic data published and shared with WWARN.By utilizing the same source of standards for all laboratories, it is possible to minimize bias arising from poor quality reference standards. By providing anti-malarial drug standards from a central point, it is possible to lower the cost of these standards.

Motility based assays using cultured fourth stage larvae fail to provide consistent discrimination between known avermectin-resistant and -susceptible isolates of Cooperia spp.

The fecal egg count reduction test (FECRT) is the only method commonly used for diagnosing anthelmintic resistance in gastrointestinal nematodes of cattle, but this method has several drawbacks that have limited its widescale implementation. Consequently, there exists a need to develop better methods for diagnosing resistance. Assays based on larval motility are used commonly for screening potential drug candidates, and for detecting drug resistance, but previous work in our lab demonstrated that the L3 stage failed to discriminate between avermectin-resistant and susceptible isolates of Cooperia spp. We hypothesized that the L4 may be a better stage for this purpose because it is a parasitic and actively feeding life stage without a double cuticle. L3 larvae of Cooperia spp. were exsheathed and cultured to L4 by maintaining them in media at 37 °C and 20 % CO2, with media changes and observation every 48 h for nine days. Three avermectin-resistant and two avermectin-susceptible GIN isolates (diagnosed by FECRT) containing >88 % Cooperia spp., were used. Three biological replicates were performed for each parasite isolate using both eprinomectin and ivermectin. Eleven drug concentrations from 0.01um to 40um and negative controls were evaluated. Motility readings were taken using the Worminator system before addition of the drug and at 24- and 48 -hs post drug exposure. Resistance ratios for ivermectin and eprinomectin ranged from 0.35 to 2.75 and 0.54-1.03, respectively. Though significant differences (p < 0.05) in percent inhibition were found at some drug concentrations in some assays, there were no consistent significant differences in the dose-response between susceptible and resistant isolates. Inhibition was greater in about half of the assays for the susceptible isolates, and in half the assays for the resistant isolates. The lack of consistency in these data indicate that motility of L4 is not a reliable diagnostic phenotype for measuring resistance to avermectin drugs in Cooperia spp.

In Vitro Growth Inhibition Assays of Leishmania spp.

In vitro growth (inhibition) assays have a dual application, either supporting the discovery of novel drugs or as a monitoring tool of drug resistance in patient isolates. From an experimental design point of view, both are quite different with regard to the infecting Leishmania species and strain, the wide variety of permissive host cells (primary cells versus cell lines), drug exposure times, detection methods and endpoint criteria. Recognizing the need for enhanced assay standardization to decrease interlaboratory variation and improve proper interpretation of results, a detailed description is given of the basic fundamental procedures and requirements for routine in vitro growth of Leishmania spp. with specific focus on the intracellular amastigote susceptibility assay. Although the described experimental procedures focus on visceral Leishmania species, the same assay principles may apply for the cutaneous species as well.

Standardization of the egg hatch test for the detection of benzimidazole resistance in parasitic nematodes.

The ability to reliably detect anthelmintic resistance is a crucial part of resistance management. If data between countries are to be compared, the same test should give the same results in each laboratory. As the egg hatch test for benzimidazole resistance is used for both research and surveys, the ability of different laboratories to obtain similar results was studied through testing of known isolates of cyathostomins, Haemonchus contortus, Ostertagia ostertagi, and Cooperia oncophora in programs supported by the EU (Cost B16 and FP6-PARASOL). Initial results showed difficulties in obtaining reproducible and similar data within and between laboratories. A series of ring tests, i.e., simultaneous and coordinated rounds of testing of nematode isolates in different laboratories was subsequently performed. By adopting identical protocols, especially the use of deionized water and making dilutions of thiabendazole in dimethyl sulfoxide in the final ring test, laboratories correctly identified both susceptible and resistant isolates. The protocols for the test and preparation of solutions of thiabendazole are described.

In-depth comparison of cell-based methodological approaches to determine drug susceptibility of visceral Leishmania isolates.

Monitoring the drug susceptibility of Leishmania isolates still largely relies on standard in vitro cell-based susceptibility assays using (patient-isolated) promastigotes for infection. Although this assay is widely used, no fully standardized/harmonized protocol is yet available hence resulting in the application of a wide variety of host cells (primary cells and cell lines), different drug exposure times, detection methods and endpoint criteria. Advocacy for standardization to decrease inter-laboratory variation and improve interpretation of results has already repeatedly been made, unfortunately still with unsatisfactory progress. As a logical next step, it would be useful to reach at least some agreement on the type of host cell and basic experimental design for routine amastigote susceptibility determination. The present laboratory study using different L. infantum strains as a model for visceral leishmaniasis species compared primary cells (mouse peritoneal exudate (PEC), mouse bone marrow derived macrophages and human peripheral blood monocyte derived macrophages) and commercially available cell lines (THP-1, J774, RAW) for either their susceptibility to infection, their role in supporting intracellular amastigote multiplication and overall feasibility/accessibility of experimental assay protocol. The major findings were that primary cells are better than cell lines in supporting infection and intracellular parasite multiplication, with PECs to be preferred for technical reasons. Cell lines require drug exposure of >96h with THP-1 to be preferred but subject to a variable response to PMA stimulation. The fast dividing J774 and RAW cells out-compete parasite-infected cells precluding proper assay read-out. Some findings could possibly also be applicable to cutaneous Leishmania strains, but this still needs cross-checking. Besides inherent limitations in a clinical setting, susceptibility testing of clinical isolates may remain problematic because of the reliance on patient-derived promastigotes which may exhibit variable degrees of metacyclogenesis and infectivity.

Multi-center screening of the Pathogen Box collection for schistosomiasis drug discovery.

BACKGROUND: Over the past five years, as a public service to encourage and accelerate drug discovery for diseases of poverty, the Medicines for Malaria Venture (MMV) has released box sets of 400 compounds named the Malaria, Pathogen and Stasis Boxes. Here, we screened the Pathogen Box against the post-infective larvae (schistosomula) of Schistosoma mansoni using assays particular to the three contributing institutions, namely, the University of California San Diego (UCSD) in the USA, the Swiss Tropical and Public Health Institute (Swiss TPH) in Switzerland, and the Fundação Oswaldo Cruz (FIOCRUZ) in Brazil. With the same set of compounds, the goal was to determine the degree of inter-assay variability and identify a core set of active compounds common to all three assays. New drugs for schistosomiasis would be welcome given that current treatment and control strategies rely on chemotherapy with just one drug, praziquantel. METHODS: Both the UCSD and Swiss TPH assays utilize daily observational scoring methodologies over 72 h, whereas the FIOCRUZ assay employs XTT (2,3-bis(2-methoxy-4-nitro-5-sulfophenyl)-5-[(phenylamino)carbonyl]-2H-tetrazolium hydroxide) at 72 h to measure viability as a function of NAD+/NADH redox state. Raw and transformed data arising from each assay were assembled for comparative analysis. RESULTS: For the UCSD and Swiss TPH assays, there was strong concordance of at least 87% in identifying active and inactive compounds on one or more of the three days. When all three assays were compared at 72 h, concordance remained a robust 74%. Further, robust Pearson's correlations (0.48-0.68) were measured between the assays. Of those actives at 72 h, the UCSD, Swiss TPH and FIOCRUZ assays identified 86, 103 and 66 compounds, respectively, of which 35 were common. Assay idiosyncrasies included the identification of unique compounds, the differential ability to identify known antischistosomal compounds and the concept that compounds of interest might include those that increase metabolic activity above baseline. CONCLUSIONS: The inter-assay data generated were in good agreement, including with previously reported data. A common set of antischistosomal molecules for further exploration has been identified .

Molecular epidemiology of malaria in Cameroon. XXIII. Experimental studies on serum substitutes and alternative culture media for in vitro drug sensitivity assays using clinical isolates of Plasmodium falciparum.

Correlation studies on the in vitro drug response of field isolates of Plasmodium falciparum and molecular markers for drug resistance are becoming important as many malaria control programs abandon monotherapies and resort to combination therapies. The standardization and optimization of the in vitro drug sensitivity assay are one of the prerequisites for validating molecular markers in the field. The present study was designed to assess and compare the growth of freshly obtained isolates for at least the first erythrocytic cycle in various culture media and determine the in vitro response to chloroquine in alternative media. Parasite growth was consistently higher in Dulbecco's modified Eagle's medium (DME)-human serum, Iscove's modified Dulbecco's medium (IMDM)-human serum, RPMI 1640 medium-goat serum, and a serum-free medium containing 1:1 (v/v) mixture of IMDM and F-12 supplemented with an ammonium sulfate fraction of adult bovine serum than in RPMI 1640 medium-human serum mixture. The level of chloroquine response determined in human serum-supplemented DME, IMDM, and RPMI 1640 media did not differ significantly (P > 0.05) from the control (RPMI 1640-human serum). This study suggests that alternative media may be used to optimize parasite growth during the critical initial phase of transition from in vivo to in vitro conditions. The capacity of these media to support long-term cultivation of P. falciparum requires further investigation.

Anti-infective potential of natural products: how to develop a stronger in vitro 'proof-of-concept'.

Natural products, either as pure compounds or as standardized plant extracts, provide unlimited opportunities for new drug leads because of the unmatched availability of chemical diversity. To secure this, a number of pivotal quality standards need to be set at the level of extract processing and primary evaluation in pharmacological screening models. This review provides a number of recommendations that will help to define a more sound 'proof-of-concept' for antibacterial, antifungal, antiviral and antiparasitic potential in natural products. An integrated panel of pathogens is proposed for antimicrobial profiling, using accessible standard in vitro experimental procedures, endpoint parameters and efficacy criteria. Primary requirements include: (1) use of reference strains or fully characterized clinical isolates, (2) in vitro models on the whole organism and if possible cell-based, (3) evaluation of selectivity by parallel cytotoxicity testing and/or integrated profiling against unrelated micro-organisms, (4) adequately broad dose range, enabling dose-response curves, (5) stringent endpoint criteria with IC(50)-values generally below 100microug/ml for extracts and below 25microM for pure compounds, (6) proper preparation, storage and in-test processing of extracts, (7) inclusion of appropriate controls in each in vitro test replicate (blanks, infected and reference controls) and (8) follow-up of in vitro activity ('hit'-status) in matching animal models ('lead'-status).

World association for the advancement of veterinary parasitology (WAAVP) guideline for testing the efficacy of ectoparasiticides for fish.

This guideline is intended to assist in the planning and execution of studies designed to assess the efficacy of ectoparasiticides for fish. It is the first ectoparasite-specific guideline to deal with studies set in the aquatic environment and therefore provides details for the maintenance of environmental standards for finfish. Information is included on a range of pre-clinical study designs as well as clinical studies in commercial/production sites, set within a regulatory framework. It provides information on the study animals, their welfare, husbandry and environmental requirements during the study. The most commonly pathogenic ectoparasites are presented with relevant points regarding life history, host challenge and numeric evaluation. Preparation and presentation of both topical and oral test treatments is provided, together with guidance on data collection and analysis. The guideline provides a quality standard or efficacy studies on finfish, which will assist researchers and regulatory authorities worldwide and contribute to the wider objective of harmonisation of procedures.

Molecular epidemiology of malaria in Cameroon. XV. Experimental studies on serum substitutes and supplements and alternative culture media for in vitro drug sensitivity assays using fresh isolates of Plasmodium falciparum.

In vitro drug sensitivity assay is an important tool for various on-going studies aiming to establish the correlation between candidate molecular markers for drug resistance and drug response in laboratory-adapted strains and field isolates of Plasmodium falciparum. A widespread use of this technique in the field would require a suitable substitute that can replace human serum. In this study, several alternative sources of serum substitutes and supplements were evaluated for their capacity to sustain parasite growth for a single life cycle and their compatibility with in vitro assays for clinical isolates that have not been adapted to in vitro culture. Albumax, a commercial preparation of lipid-enriched bovine albumin, did not support parasite growth as much as human serum and fetal calf serum in several isolates. Other serum supplements (AmnioMax and Ultroser) supported parasite growth relatively well. The 50% inhibitory concentrations (IC50s) of chloroquine and antifolates determined with 0.05% Albumax were generally two or three times higher than with human serum. With 10% fetal calf serum, IC50s for chloroquine and antifolates were approximately two times higher and three times lower than with human serum, respectively. The use of AmnioMax and OptiMAb resulted in a greater than two-fold increase in IC50s and several uninterpretable assays. Despite possible batch-to-batch differences, fetal calf serum may be a suitable substitute for in vitro drug assays while awaiting the results of further studies on other serum substitutes and supplements.

Evaluation under field conditions of the colourimetric DELI-microtest for the assessment of Plasmodium falciparum drug resistance.

It has been frequently stressed that improved methods are needed to monitor the fast spread of drug-resistant Plasmodium falciparum parasites in endemic areas. We recently developed a colourimetric microtest, double-site enzyme-linked lactate dehydrogenase enzyme immunodetection assay (DELI), to assess drug resistance in vitro. This method, which proved highly effective under laboratory conditions, was evaluated under field conditions in 2 African areas (in Senegal and Burkina Faso) in 1997 and 1998, respectively. The sensitivities of isolates from symptomatic (n = 50) and asymptomatic individuals (n = 26) infected with P. falciparum were assessed in parallel by the new DELI-microtest and the isotopic-microtest. IC50 values of the isolates determined for chloroquine, quinine, amodiaquine and mefloquine were well correlated (r = 0.79, P < 0.001). The proportions of sensitive and resistant isolates determined using the 2 methods were similar. The DELI-microtest proved to be faster to implement than the isotopic-microtest, easier to perform, and did not require sophisticated equipment. Moreover, a larger number of isolates can be tested since parasitaemias as low as 0.005% could be reliably measured with the DELI-microtest. These initial field studies thus support the value of the DELI-microtest for large-scale drug-sensitivity monitoring.

[Standardization of the method proposed by the World Health Organization for determining the susceptibility levels of leishmaniasis vectors to insecticides]. / Estandarización del método propuesto por la Organización Mundial de la Salud para determinar los niveles de susceptibilidad de los vectores de leishmaniasis a insecticidas.

The WHO method for determining insecticide resistance was standardized for several species of Lutzomyia sand flies under laboratory and field conditions. The biological assays were applied solely to optimize the conditions for the control, i.e., without insecticide, and to estimate mortality due to handling or other unfavorable conditions. Adult female flies from 3 laboratory colonies and one field strain were tested: two laboratory strains of Lutzomyia longipalpis, one laboratory strain of Lutzomyia serrana and one field-collected strain of Lutzomyia quasitownsendi. The WHO method was compared with one modified in which, during the post-exposure period, the recommended plain tube apparatus was replaced with a plastic container layered with damp plaster of Paris. Three paper substrate types were compared under each condition: olive oil additive, silicon oil additive and plain paper. The measured variable was percent mortality in 24 h. For the WHO protocol, the L. longipalpis strains indicated a 0-10% mortality, L. serrana 20-80% and L. quasitownsendi 10-50%. With the modified WHO apparatus, the average mortality was < 4% for all species. No significant differences were observed among the paper treatments. These results indicate a strong species-specific effect of post-exposure conditions on sand flies. To establish baseline levels of insecticide resistance in Lutzomyia sand flies, the WHO method is recommended only for L. longipalpis, and the modified method for L. serrana, L. quasitownsendi and closely related species.

Evaluation of a laboratory test to detect resistance to closantel in Haemonchus contortus.

OBJECTIVE: To evaluate a laboratory test for closantel resistance in Haemonchus contortus. PROCEDURE: Field isolates of H contortus, known to be resistant to closantel, were tested in the assay. In addition, mixtures of closantel-susceptible and closantel-resistant laboratory reference strains were tested to develop a method of predicting the proportion of resistant worms in a sample from the field. RESULTS: The assay correctly identified as resistant all of the closantel-resistant field isolates of H contortus. It also identified one isolate with an in vivo efficacy of 98% as having emerging resistance. Testing of the mixtures of laboratory reference strains revealed that an isolate would be classified as resistant when it consists of about 25% or more resistant worms. Test samples that are not fully susceptible yet contain less than 25% resistant worms may be classified as emerging resistance. CONCLUSION: The in vitro migration assay is a sensitive method of detecting closantel resistance in H contortus.

Commentary: A classification system for antimicrobial activity based on MIC-values: fake or reality?

In the literature circulates a classification system for antimicrobial activity, which has no basis whatsoever. In this commentary the non-existence of this classification system will be clearly indicated.