Seasonal variation in chemical composition of essential oils obtained from Satureja parnassica subsp. parnassica Heldr. & Sart ex Boiss. and Satureja thymbra L. was determined by GC and GC-MS analysis. The in vitro antibacterial activities of these essential oils were evaluated at two different initial levels against two common foodborne bacteria. Their minimum inhibitory concentration (MIC) and noninhibitory concentration (NIC) values were determined via an innovative automated technique, which was applied on a Malthus apparatus combining the conductance measurements with the common dilution method. The essential oil concentration fluctuated greatly during the period examined and established that the Satureja oils obtained during the flowering period were the most potent, exhibiting the lowest MIC values and retaining remarkable bactericidal properties. Thus, they represent an inexpensive source of natural antibacterial substances that exhibit potential for use in food systems to prevent the growth of foodborne bacteria and to extend the shelf life of the processed food.