<p>The effect of yeast extract on the cytotoxicity and the colloidal behavior of positively charged polystyrene latex （PSL） nanoparticles （NPs） toward yeast cells were investigated. Confocal observation revealed that the cells were alive and the NPs were dispersed when adding the yeast extract into 5 mM NaCl, results that were quite different from the toxicological effects in 5 mM NaCl without the yeast extract. This is because the negatively charged organic molecules in the yeast extract adhered on the NPs, leading that the NP surface changed negative. Additionally, the PSL NPs showed no toxicity when the yeast cells were incubated in the perfect media （yeast extract and glucose）. These results suggest that the PSL NPs display little or no toxicity toward the environmental microorganisms. Nanocolloidal probe atomic force microscopy revealed that the adhesion forces between the PSL NP and the yeast cell drastically decrease when adding the yeast extract.</p><p></p>