Hiromitsu Kojima

Hydrogen Peroxide as an Antibacterial Factor in Zinc Oxide Powder Slurry

Abstract Using four kinds of antibiotics, an investigation was made to determine whether or not H2O2 generated from a ZnO powder slurry was related to its antibacterial activity. Changes in the sensitivity of Escherichia coli to the antibiotics suggested that H2O2 was one of the primary factors concerned in the antibacterial activity of the ZnO powder slurry.

ANTIBACTERIAL CHARACTERISTICS OF MAGNESIUM OXIDE POWDER

The antibacterial activity of magnesium oxide (MgO) was studied. Inhibitory zones appeared around the MgO powder slurry put directly on nutrient agar plates seeded with Escherichia coli or Staphylococcus aureus. However, no zone was observed using a penicillin cup to avoid contact between the bacteria and the MgO powder. Moreover, the supernatant solution of the MgO powder slurry and a MgCl2 solution containing Mg2+ at a concentration of the solubility of MgO did not affect the growth of E. coli and S. aureus. Moreover, elevated shaking speed increased the death of E. coli in the slurry, indicating that the contact frequency between bacterial cells and MgO powders affected the antibacterial activity. It was considered that the contact between MgO powder and bacteria was important for the occurrence of its antibacterial activity. Since the generation of active oxygen, such as O2−, from the MgO powder slurry was detected by chemiluminescence analysis, an investigation was carried out to determine whether active oxygen generated from MgO powder slurry was related to its antibacterial activity. The changes in the antibiotic sensitivity in E. coli treated by MgO powder agreed with those by active oxygen treatment. These results suggested that the active oxygen generated from the MgO powder slurry was one of the primary factors in its antibacterial activity.

Kinetic analysis of the bactericidal action of heated scallop-shell powder.

Shell powder of scallop (Patinopecten yessoensis) was exposed to heat treatment at between 200 and 1000 degrees C, and the bactericidal action of the powder slurry was investigated. Shell powder heated at 700 degrees C or higher exhibited bactericidal action against Escherichia coli, Salmonella typhimurium, Staphylococcus aureus and Bacillus subtilis (vegetative cells). The death of bacteria in the shell powder slurry followed first-order reaction kinetics, and the apparent death rate constant (k) was determined. An increase in exposure temperature enhanced the bactericidal action. The bactericidal action is due to calcium oxide that is converted from calcium carbonate, which is the main component of the shell powder, by heat treatment. The slurry temperature is found to significantly affect the bactericidal action of the shell powder. The slope of the Arrhenius plot of k for E. coli and S. aureus that were grown at 37 degrees C exhibited a discontinuous point at approximately 22 degrees C, at which the values of activation energy for the death of bacteria in the powder slurry changed. This temperature corresponds to that of the phase transition of cell membrane lipids. The bactericidal action of the shell powder is greater than that of a NaOH solution of identical pH. Although the pH of the shell powder slurry is high, the slurry was considered to possess other antibacterial mechanisms in addition to that of alkalinity.

Effect of pressure on volumetric mass transfer coefficient and gas holdup in bubble column

Abstract Volumetric mass transfer coefficient and gas holdup in bubble column was studied experimentally under pressurized condition with variation of liquid phase, diameter of single nozzle used as gas disperser. The pressure in the bubble column was in the range 0.1–1.1 MPa. Gaz holdup and volumetric mass transfer coefficient increased with pressure. The effect of pressure on gas holdup and volumetric mass transfer coefficient appeared significantly in the case where the single nozzle with smaller diameter were used as gas disperser. Experimental equations for both gas holdup and volumetric mass transfer coefficients under small volume of gas chamber were presented incorporating the effect of pressure, diameter of single nozzle used as gas disperser and gas velocity.

Heated scallop-shell powder slurry treatment of shredded cabbage.

The main component of scallop-shell powder is calcium carbonate (CaCO3). Through heat treatment, CaCO3 in the shell is converted to CaO, which exhibits antibacterial activity. The disinfecting effect of heated scallop-shell powder on shredded cabbage was investigated for various powder concentrations (0.1 to 1.0 g dm(-3)) and treatment temperatures (10 to 40 degrees C). Scallop-shell powder treatment was found to reduce the aerobic bacteria count in cabbage, with increasing effectiveness at higher powder concentrations and treatment temperatures. Coliforms were completely eliminated within 5 min with as little as 0.1 g dm(-3) powder treatment. During storage at 4 degrees C, aerobic bacterial counts did not increase after powder treatment, whereas counts increased with water-washing or sodium hypochlorite treatment at 200 microg dm(-3). The inactivation pattern of bacterial cells in shredded cabbage involved an accelerated decline followed by an extended tail at powder concentrations of 0.1 and 0.5 g dm(-3). We postulate that a fraction of bacterial cells in the initial population becomes tolerant to the shell powder. A proposed model accurately predicts the reducing bacterial counts on shredded cabbage by scallop-shell powder treatment. The decrease in the L-ascorbic acid content of shredded cabbage was approximately 20 to 30% for scallop-shell powder treatment at 0.1 and 0.5 g dm(-3) (20 degrees C), which is almost identical to that by sodium hypochlorite treatment at 200 micorg dm(-3).

Kinetic analysis of death of bacteria in CaO powder slurry

The process of Escherichia coli and Staphylococcus aureus death in calcium oxide (CaO) powder slurry was assumed to follow first-order reaction kinetics. The apparent death rate constant (k) was found to increase with increasing powder concentration and was higher than that for a NaOH solution having the same pH as the CaO powder slurry. The slurry temperature was found to significantly affect the bactericidal action of the CaO powder slurry against both E. coli and S. aureus. The slope of the Arrhenius plot of k for both bacteria changed at a slurry temperature of approximately 22°C, suggested to correspond to a change in the activation energy required to induce the death of bacteria in CaO powder slurry as a result of a phase transition of the cell membrane.

Sporicidal kinetics of Bacillus subtilis spores by heated scallop shell powder.

Scallop shell powder heated at 1,000 degrees C for 1 h exhibited sporicidal action against Bacillus subtilis spores. The sporicidal kinetics of this action were analyzed with the use of a nonlogarithmic model. Apparent death rate constants (k) were obtained under various conditions. The value of k increased with powder concentration but became constant beyond the concentration representing the solubility of Ca(OH)2. A linear inverse relationship between k and temperature was found, and from this relationship the activation energy required for the death of B. subtilis spores in the heated shell powder slurry could be determined.

Indirect conductimetric assay of antibacterial activities

The applicability of indirect conductimetric assays for evaluation of antibacterial activity was examined. The minimal inhibitory concentration (MIC) obtained by the indirect method was consistent with that by the direct conductimetric assay and the turbidity method. The indirect assay allows use of growth media, which cannot be used in the direct conductimetric assay, making it possible to evaluate the antibacterial activity of insoluble or slightly soluble materials with high turbidity, such as antibacterial ceramic powders.

Effect of mixing ratio on bactericidal action of MgO–CaO powders

MgO–CaO powders were prepared with different molar ratios (MgO/CaO) at 1400 °C for 2 h in air. By using the powder samples obtained, the change in bactericidal effect as a function of MgO–CaO composition was studied by colony count method. From the XRD measurements, it was found that CaO solid solution was formed by the replacement of Mg2+ ion with larger Ca2+ ion. However, no formation of MgO solid solution was observed. The average particle size and the specific surface area of the samples used in this study were about 0.2 μm and 10.5m2g−1, respectively. The pH values of physiological saline containing powder samples increased with the increase of CaO content, and the value reached 12.1 in sample with the molar ratio (MgO/CaO) of 0.25. From the results of bactericidal tests for Staphylococcus aureus, it was found that the bactericidal effect increased with the increase of CaO content in the samples.

Far-infrared irradiation-induced injuries to Escherichia coli at below the lethal temperature

Escherichia coli in phosphate-buffered saline irradiated with far-infrared (FIR) energy was injured and killed even under the condition where the bulk temperature of the suspension was maintained below the lethal temperature. Using four kinds of antibiotics (penicillin G, chloramphenicol, nalidixic acid and rifampicin), we investigated the FIR irradiation-induced damage to E. coli on the basis of the sensitivity changes to the antibiotics. FIR irradiation increased the organism’s sensitivity to rifampicin both below and above the lethal temperature. The increase in sensitivity to chloramphenicol was observed only when FIR irradiation occurred above the lethal temperature. These results suggest that the mechanism of FIR irradiation-induced death in E. coli differs according to whether the radiation exposure occurs above or below the lethal temperature. Journal of Industrial Microbiology & Biotechnology (2000) 24, 19–24.