| Summary: | The harmful effects of microplastics are not yet fully revealed. This study tested harmful effects of polyethylene (PE), polypropylene (PP), polystyrene (PS), polyvinyl chloride (PVC), and polyethylene terephthalate (PET) microplastics were tested. Growth inhibition tests were conducted using three microorganisms with different characteristics: <i>Scenedesmus</i> sp., <i>Pseudomonas putida</i>, and <i>Saccharomyces cerevisiae</i>. The growth inhibition test with <i>Scenedesmus</i> sp. is relatively widely used, while the tests with <i>Pseudomonas putida</i> and <i>Saccharomyces cerevisiae</i> were, to our knowledge, applied to microplastics for the first time. The influence of concentration and size of microplastic particles, in the range of 50–1000 mg/L and 200–600 µm, was tested. Determined inhibitions on all three microorganisms confirmed the hazardous potential of the microplastics used. Modeling of the inhibition surface showed the increase in harmfulness with increasing concentration of the microplastics. Particle size showed no effect for <i>Scenedesmus</i> with PE, PP and PET, <i>Pseudomonas putida</i> with PS, and <i>Saccharomyces cerevisiae</i> with PP. In the remaining cases, higher inhibitions followed a decrease in particle size. The exception was <i>Scenedesmus</i> sp. with PS, where the lowest inhibitions were obtained at 400 µm. Finally, among the applied tests, the test with <i>Saccharomyces cerevisiae</i> proved to be the most sensitive to microplastics.
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