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To determine the impact of stress response on enumeration, cell association status and the viability of Escherichia coli DH5, Staphylococcus aureus ATCC 13565 and Listeria monocytogenes CDC 7762 were evaluated using fluorescence microscopy and were compared with the outcomes of traditional plate count and optical density measurements. Fluorescence microscopy revealed that organic acid stress (acetic and lactic, pH 2.7-3.3) induced cell clumping with little loss of viability in Escherichia coli DH5. Significantly lower values for cell enumeration were found for plate counts and OD600 measurement, likely due to cell clumping in response to organic acid stress. Gram-negative bacteria Escherichia coli DH5 showed higher levels of clumping and subsequent resistance against organic acid stress. Increased cell surface hydrophobicity was found in cells that exhibited more evident clumping. However, inorganic acid stress (hydrochloric and sulfuric, pH 3.0-3.3) induced only very low level of clumping in stationary-phase Escherichia coli DH5 and almost no clumping in other cultures. Osmotic stress, heat and cold shock were not found to induce cell clumping. It has been determined that traditional enumeration methods have significantly underestimated the number of viable bacterial cells when organic acid stress is involved. Plate counts and OD600 measurement therefore need to be reassessed as tools for accurate evaluation of pathogens in food industry. / Food Science and Technology