Gamma-irradiated Aspergillus conidia show a growth curve with a reproductive death phase.

In this study, we evaluated the effects of gamma irradiation on the germination of Aspergillus conidia and mycelial growth using microscopy and predictive microbiological modeling methods. A dose of 0.4 kGy reduced the germination rate by 20% compared to the untreated control, indicating interphase death due to the high radiation dose. The number of colonies formed (5.5%) was lower than the germination rate (69%), suggesting that most colonies died after germination. Microscopic observations revealed that mycelial elongation ceased completely in the middle of the growth phase, indicating reproductive death. The growth curves of irradiated conidia exhibited a delayed change in the growth pattern, and a decrease in slope during the early stages of germination and growth at low densities. A modified logistic model, which is a general purpose growth model that allows for the evaluation of subpopulations, was used to fit the experimental growth curves. Dose-dependent waveform changes may reflect the dynamics of the subpopulations during germination and growth. These methods revealed the occurrence of two cell death populations resulting from gamma irradiation of fungal conidia and contribute to the understanding of irradiation-induced cell death in fungi.

Five-year trends in bacterial contamination of dialysis equipment.

Bedside dialysis monitoring equipment for hemodialysis are located in the bioburden section upstream of the endotoxin-retentive filter for dialysis fluid sterilization. We observed 26 equipment at our institution for bacterial contamination at least once every 4 weeks for 5 years with another ultrafiltration membrane upstream to prevent bacterial contamination. Bacterial contamination levels were highest and most diverse at the time of the first flush. During subsequent initial cleanng, the contamination level decreased, and bacterial species converged almost exclusively to one genus, namely Methylobacterium spp. During clinical use, the equipment were cleaned and disinfected daily after dialysis, and daily operations and maintenance were performed using aseptic techniques. Although the frequency of bacterial detection decreased annually, the same bacterial genotypes observed at the first flush were isolated even after long time periods and were thought to persist in the equipment possibly by forming biofilm. Pseudomonas aeruginosa was newly detected after the replacement of parts during breakdown maintenance, indicating the need to sterilize replacement parts. Thus, the bioburden should be assessed regularly as part of the management of in-house-produced dialysis fluid.

Theory and application of growth delay analysis of colony formation for evaluation of injured population of the stressed fungal conidia.

A new concept of injured population assessment is proposed, in which the size of the injured population in stressed mold spores is evaluated by analyzing the colony formation process on a solid agar medium. In this method, a small paper disc containing mold spores is placed on a subculture agar plate, and the linear increase in the radius of the colony formed by development from the spore is measured over time. Then, the principle of the previously reported growth delay analysis (GDA) method originally using a liquid medium is applied to obtain the integrated viable ratio (IV) of the stressed population from the delay time relative to the growth of the unstressed population. On the other hand, the viable ratio (V) to the initial value as the colony count obtained with the stressed culture is obtained; the difference between the logarithms of V and IV is determined as the log number of the injured population. Applying this analysis method to heated spores of Cladosporium sphaerospermum, we determined the size of the injured population that occurred. This method was considered to be effective as a new method for quantifying injured populations using a solid medium.

Different patterns of germination inhibition by carvacrol and thymol in Bacillus subtilis spores.

This study aimed to clarify how the phenolic monoterpene carvacrol and its structural isomer thymol both as essential oil components (EOCs) inhibit the germination of Bacillus subtilis spore. Germination was evaluated by the OD600 reduction rate in a growth medium and phosphate buffer containing either l-alanine (l-Ala) system or l-asparagine, d-glucose, d-fructose plus KCl (AGFK) system. The germination of the wild-type spores in the Trypticase Soy broth (TSB) was found to be greatly inhibited by thymol than by carvacrol. Such a difference in the germination inhibition was confirmed by the dipicolinic acid (DPA) release from germinating spores in the AGFK buffer system, but not in the l-Ala system. Similar to the wild-type spores, no difference in the inhibitory activity between the EOCs was also indicated with the gerB, gerK-deletion mutant spores in the l-Ala buffer system and the above substantial difference was also done with the gerA-deleted mutant spores in the AGFK. Fructose was found to release spores from the EOC inhibition and inversely even stimulated. Increased concentrations of glucose and fructose partially suppressed the germination inhibition by carvacrol. The results obtained should contribute to the elucidation of the control effects of these EOCs on bacterial spores in foods.

Synergy analysis with extended isobolography and its conceptual basis for the evaluation of antimicrobial effects of combined physical and chemical treatments.

To evaluate the synergistic effects of physical and chemical antimicrobial treatments, whether for cidal or static effects, I proposed the extended isobologram (EIBo) analysis method, which is a modification of the isobologram (IBo) analysis commonly used to evaluate synergy between drugs. As the method types for this analysis, the growth delay (GD) assay reported previously by the author was introduced, in addition to the conventional endpoint (EP) assay. The evaluation analysis consists of five stages, analytical procedure establishment, antimicrobial activity assay, dose-effect analysis, IBo analysis, and synergy analysis. In EIBo analysis, the fractional antimicrobial dose (FAD) is introduced for the normalization of the antimicrobial activity of each treatment. For the synergy evaluation, the synergy parameter (SP) is defined as a measure of the degree of the synergistic effect of a combined treatment. This method makes it possible to quantitatively evaluate, predict, and compare various combination treatments as a hurdle technology.

Injury modes and physiological characteristics of injured microorganisms with a special reference to heat injury.

Various sterilization and disinfection processes are used to control harmful microorganisms in food, medicine, and the environment. During killing, microorganisms often remain between life and death, being called injured microorganisms. The degree of injury of the injured microorganisms depends on the load of the disinfection treatment, and the treatment conditions and varies not only quantitatively but also qualitatively. Knowing how they are injured by sublethal and lethal stresses of disinfection, how they repair themselves, what makes the difference between life and death, and their physiological characteristics, will lead to appropriate microbial testing and optimization of disinfection conditions for practical viability and growth potential, and will deepen our understanding of the effectiveness of the treatment. Focusing on mainly heat injury and using Escherichia coli as a model microorganism, in this review, I will discuss the classification of injury modes in injured microorganisms caused by disinfection treatment, including "λ injury" (delayed resumption of growth）," µ injury" (reduced growth rate injury）," ß injury" (secondary injury）, and other derivatives.

Combined effects of microorganism control and the concept of their evaluation methods.

Various combination treatments are used for microorganism control in food, medicine, and the environment. Especially in food, combination treatments have been studied using antimicrobial compounds in pasteurization and sterilization but comprehensive quantitative evaluation methods, have not yet been established to evaluate their effectiveness. This review introduces the author's recently published methods for evaluating the effects of combination treatments on the control of harmful microorganisms in food. Particularly important items are 1) the type of action of the control treatment and the mode of the combined method, 2) the choice of endpoint method and growth delay method for analytical evaluation, 3) the construction of extended isobolography that allows the application of conventional isobologram (IBo) for chemicals to various control methods, 4) the extended use of combined index (CI）, and 5) the introduction of synergistic parameter (SP) for quantitative evaluation of synergistic effects. In addition, I describe the characteristics of the action of antimicrobial compounds and disinfectants in their combined effects with heating, and insist on the advantages of using combined treatments and their evaluation methods in the food industry.

Theoretical Base for the Application of the DiVSaL Method to Bacterial Spores to Evaluate Injured Populations Occurring After Exposure to Lethal Stress.

To assess injury in bacterial spore populations exposed to lethal stress, we proposed a theoretical basis for applying the DiVSaL method, which has already been reported for general microorganisms as a double subculture method. We constructed a mathematical model in which both injuries to the germination system and the spore body were taken into the theory. In this theory, we reasonably assumed that the viable and germinable spore count is constant before the subsequent vegetative growth and that the delay of germination and outgrowth can be included in the concept of λ injury previously reported as the growth-independent injury. By introducing these assumptions, the double subculture method can be considered to apply to spores as well. As examples of the application of this theory, the growth delays of Bacillus subtilis spores treated with heat and UV irradiation were analyzed and the numbers of injured spores were evaluated. Based on the results obtained, heat is indicated to have a higher injury generation ability than UV irradiation. The applicability of the DiVSaL method as a tool for food preservation and sanitation designs is presented.

Low Temperature Heating-Induced Death and Vacuole Injury in Cladosporium sphaerospermum Conidia.

The mechanism of thermal death of mold conidia has not been understood in detail. The purpose of this study is to analyze the death kinetics of heated conidia of Cladosporium sphaerospermum and to ascertain the expectant cell injury responsible for the death. The death of the dormant (resting) conidia of Cladosporium sphaerospermum was examined at temperatures of between 43 and 54℃ with the conventional colony count method. The death reaction apparently followed the first order kinetics, but the Arrhenius plot of the death rate constant demonstrated seemingly a break. The linearity at temperatures higher than that at the break was lost at lower temperatures, suggesting the involvement of an unusual mechanism in the latter temperatures. In the cell morphology, we observed with quinacrine staining the vacuole rupture at a lower temperature but not at a high temperature. Interestingly, the vacuole rupture by low-temperature heating was found to correlate with the viability loss. Furthermore, active protease originally locating in vacuoles was detected in the cytoplasm of the conidia after heated at a low temperature. The results obtained suggest the involvement of potent autophagic cell death induced by low temperature heating of C. sphaerospermum conidia.

Evaluation of distinct modes of oxidative secondary injury generated in heat-treated cells of Escherichia coli with solid/liquid and complex/semi-synthetic media sets.

AIMS: To characterize and evaluate oxidative secondary injury generated in heat-treated Escherichia coli cells during recovery cultivation either on agar or in a broth of a semi-synthetic enriched M9 (EM9) medium and a complex Luria broth (LB) medium with different types of antioxidants. METHODS AND RESULTS: E. coli cells grown in the EM9 and LB broth were heated at 50°C in a buffer (pH 7.0). Heated cells were recovered on the same kind of agar medium as that used for growth, with or without different antioxidants. Although these antioxidants mostly protected the cells from oxidative secondary injury on the recovery media, sodium thiosulphate and sodium pyruvate were most protective on EM9 and LB agars, respectively. Determination of viability using the most probable number and growth delay analysis methods showed significant reductions in the protective effects of antioxidants in the EM9 and LB media. CONCLUSION: Oxidative secondary injury generated in heated E. coli cells was found to be qualitatively and quantitatively diverse under cellular and environmental conditions. SIGNIFICANCE AND IMPACT OF THE STUDY: Our results suggest that different modes of oxidation should be considered in viability determination and injured cell enumeration of heat-treated cells.

Detection Time Distribution of Microcolonies Formed by Individual Heat-Injured Cells of Escherichia coli.

The microcolony formation at 30℃ on an enriched minimal salts agar plates by individual Escherichia coli cells heated at 50℃ was monitored with a time-lapse shadow image analysis system, MicroBio µ3DTM AutoScanner. While the time course of microcolony count detected every half an hour for the unheated cells seemingly demonstrated a normal distribution, that for the heated cell population demonstrated totally the growth delay probably resulting from cell injury and also interestingly distributed in its rather deformed pattern with a tailing. Those patterns of the cumulative counts of appearing microcolonies during the post-heating cultivation period were expressed in three different mathematical models. This approach may be proposed as a rapid cultivation method predictable for enumeration of viable and repairable injured cells in practical use.

A Modified Double Subculture Method for the Two-Mode Injuries Evaluation in a Stressed Fungal Spore Population.

In order to evaluate injury of a stressed fungal spore population, a modification of formerly presented double subculture method, which consists of both the conventional plate count method and the growth delay analysis method, was proposed. In this method, an apparent logarithmic growth kinetics was assumed and the previous kinetic model was improved to be able to estimate injured subpopulations in two different modes containing early occurring growth-independent and late occurring growth-dependent injuries, called the λ and µ injuries, respectively. Based on the kinetic theory developed here, this novel method was applied to heat-treated conidia of Cladosporium cladosporioides and these two mode injuries were evaluated.

Inhibitory Effect of Spice Powders on the Development of Heated and Irradiated Bacillus subtilis Spores as Evaluated by Calorimetry.

Inhibitory effects of the powders of paprika, red pepper, black pepper, sage, oregano and thyme in a solid medium after heat treatment and gamma-irradiation on the development from spore of Bacillus subtilis were examined using calorimetry. Based on the f(t) curve (Antoce et al., 1996) from the thermogram obtained, two parameters, the growth rate constant and the growth retardation time, were used to evaluate the inhibitory effect. The inhibitory effects of paprika and red pepper powders were enhanced by the spore pretreatment with heat, but not significantly with irradiation. The inhibitory enhancement by preheating depended upon the kind of spices used. Sage, oregano and thyme powders per se inhibited the development from spores completely even at a low concentration of 0.04 g/ml. Inhibitory effects of paprika and red pepper powders were obviously observed with heat treatment but not with irradiation. With black pepper powder, by contrast, substantial enhancement was neither observed with heat treatment nor gamma-irradiation. The results suggested that the addition of those spice powders might be useful in the thermal inactivation process of solid foods contaminated with Bacillus subtilis spores.

A Novel Double Subculture Method and Its Theory for the Enumeration of Injured Cells in Stressed Microbial Population.

　A novel double subculture method, termed DiVSaL (Differential Viabilities between Solid and Liquid media) method, for the enumeration of injured cell population of a microorganism, which occurs after some sublethal to lethal treatment, was proposed. In this method injured cells were enumerated as the differential value between viabilities determined with two different techniques, the conventional plate counting using a solid agar medium and the growth delay analysis using a liquid medium. In the former technique, the viable cell number is obtained as colony forming unit (CFU) formed on an agar medium where sublethally injured cells are as much rescued as possible. In the latter technique, on the other hand," the integrated viability" defined by Takano and Tsuchido (1982) is introduced and is calculated from the growth delay of a stressed population, referred to unstressed one. For the growth delay analysis, in this paper, not only the original theoretical model, where the specific growth rate (and therefore the defined G10 value) does not change after the exposure to a stress treatment, but also a novel modified theory, where the parameter changes, is proposed. On the theoretical background, this DiVSaL method as a double subculture method can be used to enumerate the injured cells without selection by addition of some inhibitor or by nutritional shortage.

Antifungal Activity of Diglycerin Ester of Fatty Acids against Yeasts and Its Comparison with Those of Sucrose Monopalmitate and Sodium Benzoate.

The antifungal activities of diglycerin monoester of fatty acids (DGCs), which have been employed as food emulsifiers, were examined against three yeasts, Saccharomyces cerevisiae, Candida albicans and Candida utilis and were compared with those of sucrose monoester of palmitic acid (SC16) as another type of emulsifier and sodium benzoate (SB) as a weak acid food preservative. When the minimum growth inhibitory concentrations (MICs) of diglycerin monolaurate (DGC12) against these yeasts were determined 2 d after incubation in YM broth at pH5.0, they were relatively low, being 0.01% (w/v), for both S. cerevisiae and C. utilis, whereas was high, being 4.0% (w/v), for C. albicans. On the contrary, the MICs of sucrose monopalmitate (SC16) were high, being 3.0 and 4.0% (w/v), for the former two yeasts, respectively, but 0.6% (w/v) for the last yeast. In contrast to these emulsifiers, the MICs of sodium benzoate (SB) were similar independently upon the yeast strain, being in order 0.4, 0.3 and 0.5% (w/v), for the above yeasts, respectively. The anti-yeast activities of DGC12 and SC16 were gradually increased with a decrease in pH, in a manner similar to that of SB, except for the action of SC16 on C. albicans, for which the activity was more effective at pHs 5.0 and 6.0 than at pHs 4.0 and 7.0. Among DGCs tested having different fatty acid moieties in the molecule, lauroyl ester (DGC12) was more effective than myristoyl and palmitoyl esters against S. cerevisiae and C. utilis. The inhibitory effect of DGC12 on the yeast growth depended upon both the cell density and the strength of aeration during the treatment. Further, DGC12 was found to kill S. cerevisiae and C. utilis cells at a rather low concentration of 0.005% (w/v) in 50mM acetate buffer at pH5.0, although, against C. albicans cells, only slight fungicidal activity was demonstrated at a high concentration of 0.5% (w/v). The results obtained support the effectiveness of practical application of DGC12 to acidic foods for the control of growth and survival of general yeasts such as S. cerevisiae and C. utilis.

Killing effect of peppermint vapor against pink-slime forming microorganisms.

The killing effect of peppermint vapor (PMV) against pink-slime forming microorganisms, Methylobacterium mesophilicum as a bacterium and Rhodotorula mucilaginosa as a yeast, was investigated by the agar vapor assay. In this method, microbial cells were spread over the agar surface exposed to PMV in a petri dish, and then transferred into a recovery liquid. When 60µl of the peppermint liquid was added to a paper disc, a marked killing effect of PMV was observed after 48h against M. mesophilicum and after 168h against R. mucilaginosa. M. mesophilicum and R. mucilaginosa were found to be more resistant to PMV than Escherichia coli and Candida albicans, used as reference microorganisms, respectively. With the addition of 0.03% sodium pyruvate as a hydrogen peroxide scavenger in agar, the killing effect of PMV against E. coli and C. albicans was decreased, whereas it was little changed against M. mesophilicum and R. mucilaginosa. In fact, the properties of the killing effect of hydrogen peroxide solution at 0.2-1.0mM was in accord with those of PMV. M. mesophilicum and R. mucilaginosa were more resistant to the oxidant than E. coli and C. albicans, respectively. Results obtained suggested that reactive oxygen species (ROS) may be involved in the killing action of PMV and therefore pink-slime formers are more resistant to PMV than non-pink-slime formers because of the presence of carotenoids as an antioxidant in cells. We also suggest that the use of PMV appeared to be a potential tool for the control of pink-slime forming microorganisms occurring in wet areas of houses such as the bathroom and washing room.

All genomic mutations in the antimicrobial surfactant-resistant mutant, Escherichia coli OW66, are involved in cell resistance to surfactant.

The spontaneous antimicrobial surfactant-resistant mutant, Escherichia coli OW66, has been isolated, and its physiological properties have been characterized in our previous paper (Ishikawa et al., J Appl Microbiol 92:261-268, 2002b). This report revealed that strain OW66 had seven mutations in their chromosomal DNA by comparative genomic hybridization microarray, and that their alternative functions were involved in cell resistance to antimicrobial surfactants. These mutations were located in oppB, ydcR, IVR(vacJ-yfdC), rpoN, rpoB, rpoC, and soxR. Furthermore, seven of the single-mutated isogenic strains and seven of the six-mutated isogenic strains were constructed from strains OW6 (NBRC106482) and OW66, respectively, through homologous recombination, and their resistances to an antimicrobial surfactant were measured using the minimum inhibitory concentration method. These results revealed that all six-mutated strains were more sensitive than strain OW66, and that the soxR66 mutation was independently involved in antimicrobial surfactant resistance of E. coli cells. Expression of soxR66 and soxS was increased in both strains OW66 and OW6-soxR66 without the surfactant treatment by the quantitative real time-polymerase chain reaction analysis, compared with strain OW6. Two-dimensional polyacrylamide gel electrophoresis analysis also revealed that some proteins in the soxRS regulon, including Mn-SOD, were overexpressed in both strains OW66 and OW6-soxR66. These results indicate that the soxR66 mutation leads to the constitutive expression of the soxRS regulon, resulting in the acquired resistance of E. coli cells to an antimicrobial surfactant.

Isolation and characterization of novel bisphenol-A--degrading bacteria from soils.

One hundred and seven soil samples were collected from various places in Japan, and their bisphenol-A (BPA, 2,2-bis(4-Hydroxyphenyl) propane) degradative capacities were evaluated. Eighty-five soil samples possessed BPA degradative capacities, and 26 bacterial strains could be isolated as BPA-degrading bacterium. Sequence analysis of their 16S rRNA genes indicated that 22 isolates belonged to proteobacteria groups, and three of four Gram-positive bacterial strains, YA27, NO13, and NO15, were classified as Bacilli. All isolates except strain YA27 completely degraded 115 microg/mL BPA in L medium but strain YA27 degraded only 50 microg/mL BPA. Strain YA27 and three Sphingomonas sp. strains could also grow in basal salt media containing BPA as a sole carbon source (BSMB medium). In HPLC analyses, some isolates, including the three Sphingomonas strains, produced some BPA metabolites in their cultures although the others, including strain YA27, produced no detectable metabolite. Furthermore, the Pseudomonas strains SU1 and SU4 produced some BPA metabolites that were different from the metabolites detected in the degradation of BPA by the S. bisphenolicum strain AO1. These results suggested that all isolates could be applicable to the bioremediation of BPA-polluted soil and water. Furthermore, we suggest that Bacillus sp. YA27 and Pseudomonas SU1 and SU4 may exhibit novel BPA metabolism pathways that are distinct from that of S. bisphenolicum AO1.

Properties of bacterial corrosion of stainless steel and its inhibition by protamine coating.

We investigated characteristics of the corrosion of stainless steel specimens by bacteria and the effects of using antimicrobial coating on the surface for inhibiting corrosion. Bacillus sp. 2-A and Staphylococcus sp. 2-1 cells adhered tightly to a stainless steel SUS304 specimen, formed a microcolony or biofilm, and had highly corrosive activities. Microbially influenced corrosion (MC) was observed under or around adhering cells. However, dead cells were markedly less active than viable cells not only in corroding the specimen but also in adhering to its surface. The culture supernatant was not able to induce the corrosion of SUS304 effectively. A protamine coating on the specimen killed bacterial cells only on its surface, interfered with cell adhesion, and inhibited MC. From these results, adhesion of viable cells to the surface of a SUS304 specimen led to the outbreak of MC. Protamine was also found to be an effective substance tested for protecting the specimen from both cell adhesion and surface MC. We suggest that a protamine coating can be applied as a convenient and inexpensive corrosion prevention method.