• Journal of Microbiology
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• v.38 no.3
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• pp.117-121
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• 2000

Recent advances in studies of bacterial gene expression and light microscopy show that cell-to cell communication and communication and community behavior are the rule rather than the exception. One type of cell-cell communication, quorum sensing in Gram-negative bacteria involves acyl-homoserine lactone signals. This type of quorum sension represents a dedicated communication system that enables a given species to sense when it has reached a critical population density. and to respond by activating expression of specific genes. The LuxR and LuxI proteins of Vibrio fisheri are the founding members of the acyl-homoserine lactone quorum sensing signal receptor and signal generator families of proteins. Acyl-homeserine lactone signaling in Pseudomonas aeruginosa is one model for the relationship between quorum sensing community behavior, and virulence. In the P. aeruginosa model. quorum sensing is required for normal biofilm maturation and virulence. There are multiple quorum-sensing circuits that control the expression of dozens of specific genes in P. aeruginosa.

• Journal of Microbiology
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• v.37 no.1
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• pp.1-9
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• 1999

The Biolog redox technology was carried out for evaluation of acidification effect on microbial communities at each stage of pH gradient microcosm. While the number of heterotrophic bacterial population and activities of extracellular enzyme decreased as the pH decreased, the number of total bacteria in the microcosm was not affected. The average color development of sample at each pH-gradient showed a sigmoidal curve, and at higher pH, more overall color development appeared in Biolog plates. Average color development value in Biolog plates was stabilized at 50 hours as an optimum incubation time. The color production in the Biolog plates was caused by cell density at above pH 5.0, but by cell activity below pH 4.0. Principal component analysis of color responses revealed distinctive patterns among the pH-gradient microcosm samples.

• Journal of Ecology and Environment
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• v.29 no.4
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• pp.399-404
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• 2006

This study was carried out to investigate the population densities, R/S ratios, and identification of heterotrophic bacteria on the rhizosphere soil of halophyte Suaeda japonica found on the western and southern mudflats of Korea. The population densities of aerobic and anaerobic heterotrophic bacteria on the rhizosphere soil of Suaeda japonica were in the range of $1.3\;{\pm}\;0.3\;{\times}\;10^6\;{\sim}\;6.3\;{\pm}\;3.3\;{\times}\;10^7\;and\;2.8\;{\pm}\;1.3\;{\times}\;10^4\;{\sim}\;1.8\;{\pm}\;0.7\;{\times}\;10^7\;cfu\;g^{-1}\;d.\;wt.$, respectively. In case of physiologically specific bacteria, population densities of amylolytic bacteria on the rhizosphere soil of Suaeda japonica were in the range of $4.4\;{\pm}\;0.6\;{\times}\;10^6\;{\sim}\;2.5\;{\pm}\;1.2\;{\times}\;10^7\;cfu\;g^{-1}\;d.\;wt.$, those of cellulolytic bacteria were from $8.5\;{\pm}\;6.0\;{\times}\;10^4\;{\sim}\;2.3\;{\pm}\;1.6\;{\times}\;10^6\;cfu\;g^{-1}\;d.\;wt.$, and those of proteolytic bacteria were from $3.8\;{\pm}\;1.8\;{\times}\;10^5\;{\sim}\;4.2\;{\pm}\;2.9\;{\times}\;10^6\;cfu\;g^{-1}\;d.\;wt.$, respectively. The R/S ratios were ranged from 2.33 to 2.39. Among eleven isolates from the roots of halophyte Suaeda japonica of Goheung bay by using 16S rDNA analysis, five clones were closely related to ${\gamma}-Proteobacteria$ group and six clones were closely related to ${\alpha}-Proteobacteria$ group. Among four isolates from Suncheon bay, two strains were related to ${\gamma}-Proteobacteria$ group and another two were related to Actinobacteria and Bacilli group, respectively.

• Microbiology and Biotechnology Letters
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• v.40 no.2
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• pp.83-91
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• 2012

Quorum sensing (QS) is a cell-to-cell communication system, which is used by many bacteria to regulate diverse gene expression in response to changes in population density. Bacteria recognize the differences in cell density by sensing the concentration of signal molecules such as N-acyl-homoserine lactones (AHL) and autoinducer-2 (AI-2). In particular, QS plays a key role in biofilm formation, which is a specific bacterial group behavior. Biofilms are dense aggregates of packed microbial communities that grow on surfaces, and are embedded in a self-produced matrix of extracellular polymeric substances (EPS). QS regulates biofilm dispersal as well as the production of EPS. In some bacteria, biofilm formations are regulated by c-di-GMP-mediated signaling as well as QS, thus the two signaling systems are mutually connected. Biofilms are one of the major virulence factors in pathogenic bacteria. In addition, they cause numerous problems in industrial fields, such as the biofouling of pipes, tanks and membrane bioreactors (MBR). Therefore, the interference of QS, referred to as quorum quenching (QQ) has received a great deal of attention. To inhibit biofilm formation, several strategies to disrupt bacterial QS have been reported, and many enzymes which can degrade or modify the signal molecule AHL have been studied. QQ enzymes, such as AHL-lactonase, AHL-acylase, and oxidoreductases may offer great potential for the effective control of biofilm formation and membrane biofouling in the future. This review describes the process of bacterial QS, biofilm formation, and the close relationship between them. Finally, QQ enzymes and their applications for the reduction of biofouling are also discussed.

• The Plant Pathology Journal
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• v.15 no.3
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• pp.162-167
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• 1999

Integration of microbial antagonists with fungicides was tried to control the gray mold caused by Botrytis cinerea on pepper in greenhouse conditions and to reduce fungicide uses. All of the selected bacterial antagonists, Bacillus amyloliquefaciens BL3, Paenibacillus polymyxa BL4, and Pseudomonas putida Cha94, completely inhibited the conidial germination of B. cinerea until 30 days after treatment. However, bacterial colonization of pepper phylloplane was poor in BL4, while the other bacterial isolates and the fungal antagonist Trichoderma harzianum TM colonized well on the phylloplane, maintaining the population density of 104-105 cfu/g until 15 days after microbial treatments. Out of 13 kinds of selected fungicides used for gray mold diseases, polyoxin B and BKF 1995 showed the most discriminatory activity on the fungal growth between B. cinerea and TM. TM grew readily on the media containing those fungicides, while B. cinerea showed poor or no mycelial growth on them. The selected fungicides and antagonists alone reduced incidence of gray mold on pepper, showing disease indices of about 2.4 to 3.0, while its was increased up to 4.2 in the untreated control. Alternate treatments with the antagonists and 2-fold diluted fungicides inhibited the disease incidence as much as the antagonists or fungicides alone, and reduced the secondary inoculum more than the single treatments. This suggests that integration of antagonists and fungicides may be an efficient way to reduce fungicide sprays with reliable control efficacy of the disease. However, there was not much difference in the early and mid-term disease progress among the treatments and the untreated control, probably due to extremely favorable environmental conditions for the disease development in this experiment.

• The Korean Journal of Mycology
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• v.26 no.1 s.84
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• pp.60-68
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• 1998

The effect of P. agarici and P. tolaasii causing the bacterial disease of mushrooms on the mycelial growth and fruitbody formation of F. velutipes was evaluated in laboratory. When the pathogenic bacteria was inoculated simultaneously with F. velutipes or 5 days after inoculation of F. velutipes, they significantly deterred both mycelial growth and fruitbody formation of F. velutipes in sawdust culture and showed strong inhibition under high population density. They appeared to be tender or milky in exhibiting symptom on F. velutipes by inoculating the concentration of $10^2{\sim}10^6$ of unit/g media, and their growth seemed to be stopped under $10^8\;cfu/g$ media. On $10^2\;cfu/g$ media of P. agarici and $10^4\;cfu/g$ media of P. tolaasii, there was no effect on the fruitbody yield of F. velutipes. P. tolaasii was more suppressive in the mycelial growth of F. velutipes than P. agarici, while on fruitbodies formation of F. velutipes, P. agarici showed slightly higher inhibition than that of P. tolaasii. When the bacteria was inoculated 10 days after inoculation of F. velutipes, both mycelial growth and fruit body formation were not affected nearly.

• The Korean Journal of Mycology
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• v.25 no.1 s.80
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• pp.10-19
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• 1997

Fusarium wilt of radish (Raphanus sativus L.) is caused by the Fusarium oxysporum f. sp. raphani (FOR) which mainly attacks Raphanus spp. The pathogen is a soil-borne and forms chlamydospores in infected plant residues in soil. Infected pathogen colonizes the vascular tissue, leading to necrosis of the vascular tissue. Growth promoting beneficial organisms such as Pseudomonas fluorescens WCS374 (strain WCS374), P. putida RE10 (strain RE10) and Pseudomonas sp. EN415 (strain EN415) were used for microorganisms-mediated induction of systemic resistance in radish against Fusarium wilt. In this bioassy, the pathogens and bacteria were treated into soil separately or concurrently, and mixed the bacteria with the different level of combination. Significant suppression of the disease by bacterial treatments was generally observed in pot bioassy. The disease incidence of the control recorded 46.5% in the internal observation and 21.1% in the external observation, respectively. The disease incidence of P. putida RE10 recorded 12.2% in the internal observation and 7.8% in the external observation, respectively. However, the disease incidence of P. fluorescens WCS374 which was proved to be highly suppressive to Fusarium wilt indicated 45.6% in the internal observation and 27.8% in the external observation, respectively. The disease incidence of P. putida RE10 mixed with P. fluorescens WCS374 or Pseudomonas sp. EN415 was in the range of 10.0-22.1%. On the other hand, the disease incidence of P. putida RE10 mixed with Pseudomonas sp. EN415 was in the range of 7.8-20.2%. The colonization by FOR was observed in the range of $2.4-5.1{\times}10^3/g$ on the root surface and $0.7-1.3{\times}10^3/g$ in the soil, but the numbers were not statistically different. As compared with $3.8{\times}10^3/g$ root of the control, the colonization of infested ROR indicated $2.9{\times}10^3/g$ root in separate treatments of P. putida RE10, and less than $3.8{\times}10^3/g$ root of the control. Also, the colonization of FOR recorded $5.1{\times}10^3/g$ root in mixed treatments of 3 bacterial strains such as P. putida RE10, P. fluorescens WCS374 and Pseudomonas sp. EN415. The colonization of FOR in soil was less than that of FOR in root part. Based on soil or root part, the colonization of ROR didn't indicate a significant difference. The colonization of introduced 3 fluorescent pseudomonads was observed in the range of $2.3-4.0{\times}10^7/g$ in the root surface and $0.9-1.8{\times}10^7/g$ in soil, but the bacterial densities were significantly different. When growth promoting organisms were introduced into the soil, the population of Pseudomonas sp. in the root part treated with P. putida RE10 was similar in number to the control and recorded the low numerical value as compared with any other treatments. The population density of Pseudomonas sp. in the treatment of P. putida RE10 indicated significant differences in the root part, but didn't show significant differences in soil. The population densities of infested FOR and introduced bacteria on the root were high in contrast to those of soil. P. putida RE10 and Pseudomonas sp. EN415 used in this experiment appeared to induce the resistance of the host against Fusarium wilt.

• Journal of Korean Society of Forest Science
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• v.101 no.3
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• pp.501-508
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• 2012

This study was carried out to compare species diversity of soil bacteria from Baekdudaegan mountain forests (Bonghwa-gun, Mungyeong-si and Sangju-si) in Gyeongsangbuk-do and to analyze the effects of soil environments on diversity and population of soil bacteria. Soil bacteria were isolated from soil samples by streak plate method, and identified by DNA extaction and 16S rDNA sequence analyses. The population of soil bacteria from the soil samples of Bonghwa-gun was the highest with $5.1{\times}10^5cfu/g$, and followed by those from Mungyeong-si and Sangju-si with $1.9{\times}10^5cfu/g$ and $1.1{\times}10^5cfu/g$, respectively. The population of soil bacteria from surface layer soil was the highest, and then gradually decreased according to soil depth. The increase in population of soil bacteria from soil samples of different sites was correlated with the increase of the altitude of soil sampling site, depth of A horizon, liquid phase among three phases of soil, water content and bulk density of soil. Two hundreds and sixty eight bacterial colonies from Bonghwa-gun were classified into 10 species, 8 genera. One hundred and thirty four bacterial colonies from Mungyeong-si were classified into 15 species, 9 genera. Forty four bacterial colonies from Sangju-si were classified into 5 species, 2 genera. The dominant species (occupancy rate) from Bonghwa-gun and Mungyeong-si were Bacillus weihenstephanensis (36% and 40%, respectively), and Sangju-si was Bacillus cereus (39%). The relationships between soil environment and community structure of soil bacteria were analyzed statistically by using ecological indices. The diversity, evenness and dominance indices of soil bacteria were 6.30, 2.04 and 0.59 in Bonghwa-gun, 9.09, 2.94 and 0.51 in Mungyeong-si, and 4.55, 2.34 and 0.71 in Sangju-si, respectively. The diversity and evenness indices were increased by the increase of water content, drainage condition and gravel content of soil, while the dominance index was decreased.

• Korean Journal of Environmental Biology
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• v.24 no.3
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• pp.258-267
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• 2006

Distribution of population densities of heterotrophic bacteria, E. coli, and Salmonella and Shigella in seawater and sediments at 40 stations near Samcheonpo Bay were measured for 3 times from July to December, 2003. Population densities of heterotrophic bacteria in seawater during survey periods were in the range of $1.7{\pm}0.9{\times}10^3{\sim}2.4{\pm}0.9{\times}10^5$ CFU $mL^{-1}$ and the highest density was shown at St. 34 during the sampling period of September, 2003. Population densities of heterotrophic bacteria were shown higher values on September than those of July and December at all sampling stations. Population densities of anaerobic heterotrophic bacteria in sediments during survey periods were in the range of $2.2{\pm}0.2{\times}10^3{\sim}2.0{\pm}0.2{\times}10^5$ CFU $mL^{-1}$ and their population densities at sampling stations far from Samcheonpo Bay measured lower values than those near Samcheonpo Bay. Population densities of anaerobic heterotrophic bacteria in the sediments were not affected by physico-chemical factors of upper water environment. E. coli were detected only at 8 stations in seawater and 4 stations in the sediments among 40 sampling stations on July and were not detected during September and December. Salmonella and Shigella were detected only a few stations on July and September during sampling periods.

• The Plant Pathology Journal
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• v.22 no.4
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• pp.364-368
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• 2006

Survival of biocontrol agents and their effective colonization of rhizhosphere are the essential components for successful disease suppression. The effects of hydrogel supplement on bacterial survival and disease control were evaluated in pot and in the field. Addition of 2% hydrogel material to potting soil resulted in significant enhancement of colonization of biocontrol agent Serratia plymuthica A21-4 both in soil and rhizosphere of pepper plants. Rhizosphere colonization of S. plymuthica A21-4 retrieved from 40 days old pepper seedlings indicated 100 times higher bacterial population in hydrogel treated soil than in ordinary pot soil. The pepper plants sown in hydrogelated potting soil showed higher seed germination rate and the better growth of pepper plant than those in ordinary commercial pot soil. Although the suppression of Phytophthora capsid density in the potting soil by treatment of biocontrol agent A21-4 was not significantly different between in hydrogelated soil and ordinary potting soil, the suppression of Phytophthora blight between two treatments was significantly different. A21-4 treatment in hydrogelated potting soil was completely disease-free while same treatment in ordinary potting soil revealed 36% disease incidence. Our field study under natural disease occurrence also showed significantly less disease incidence(12.3%) in the A21-4 treatment in the hydrogelated soil compared to other treatments. Yield promotion of pepper by the A21-4 treatment in the hydrogelated potting soil was also recognized. Our results indicated that hydrogel amendment with biocontrol agent in pot soil would be a good alternative to protect pepper seedlings and increase plant yield.