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The antibacterial activity of essential oils (EOs) isolated from ginger (Zingiber officinale), eucalyptus (Eucalyptus camaldulensis) and sweet orange (Citrus sinensis) was evaluated against fish spoilage and fish‐borne pathogenic bacteria by agar well diffusion and microbroth dilution method. Chemical profiles of EOs by gas chromatography–mass spectrometer revealed that zingiberene, α‐phellandrene and limonene were the major components of ginger, eucalyptus, and sweet orange EOs, respectively. Among the three EOs evaluated, ginger oil had the best antibacterial activity against all the bacteria tested. Bacillus subtilis was found as the most sensitive bacterium against ginger EO. Vibrio parahaemolyticus and Aeromonas hydrophila were the most resistant to all the EOs. Antibiotic resistant Yersinia enterocolitica was found as the most sensitive bacteria among gram‐negative bacteria tested. PRACTICAL APPLICATIONS The results of the present study reveal the scope of the plant essential oils (EOs) in the shelf life extension of fish. The fresh fish after catching need to be ice‐preserved before undergoing further processing. In addition, the preservation during the postharvest handling and transportation of fish is important, during which there are chances for microbial contamination from the food contact surfaces, containers and transport vehicles. The use of plant EOs will definitely help in reducing the level of microbial contamination, preserving the fish and extending their shelf life. They are cheaper in economical terms as well and easy to use by dip or spray treatment. The fishermen can use the plant EOs onboard to preserve the fish. The retailers and vendors can use it for the quality maintenance of fish.

A pure bacterial culture is composed of clonal cells in different physiological states. Separation of those subpopulations is critical for further characterization and for understanding various processes in the cultured cells. We used density-dependent cell sorting with Percoll to separate subpopulations from cultures of a marine bacterium, Vibrio parahaemolyticus. Cells from cultures in the exponential and stationary phases were fractionated according to their buoyant density, and their culturability and ability to maintain culturability under low-temperature and low-nutrient stress (stress resistance) were determined. The buoyant density of the major portion of the cells decreased with culture age. The culturability of stationary-phase cells increased with increasing buoyant density, but that of exponential-phase cells did not. Stress resistance decreased with increasing buoyant density regardless of the growth phase. The results indicate that density-dependent cell sorting is useful for separating subpopulations of different culturabilities and stress resistances. We expect that this method will be a powerful tool for analyzing cells in various physiological states, such as the viable but nonculturable state.A bacterial culture started from one colony is a clonal population. This, however, does not mean that all the cells behave identically (20,23). The presence of subpopulations with different physiological and morphological characteristics is known, especially among cells in the stationary phase of culture (16). Recent studies indicate that the cell's physiological states are controlled by a set of genes specifically expressed in various growth phases (16,17,25,35).The presence of subpopulations is also expected for cells in the viable but nonculturable state (10, 39). It is commonly observed that when bacterial cells are transferred from rich to poor medium, some cells start losing culturability, although they still show certain metabolic activities (19,26). Entry into the viable but nonculturable state is dependent on the cellular growth phase or physiological state. In general, cells in stationary phase or those experiencing stress conditions tend to retain culturability longer (18,27). In a culture entering the viable but nonculturable state, there may be at least three subpopulations, that is, cells retaining culturability, cells lacking culturability but retaining metabolic activity (i.e., viable but nonculturable state), and cells showing no detectable biological activity. Because they coexist in one batch culture, the separation of cells in the viable but nonculturable state from other subpopulations is critical for further detailed investigations. However, currently no simple, reliable method is available for this purpose.Density-gradient centrifugation is a useful technique for separating cells or organelles of different densities. In bacteriology, this technique has been applied to separation of cells in different stage of the cell cycle (6, 13, 15, 31), separation of competent cells for transfor...