Antagonism of Bacillus spp. against Xanthomonas campestris pv. campestris

AGRICULTURE, AGRIBUSINESS AND BIOTECHNOLOGY

Antagonism of Bacillus spp. against Xanthomonas campestris pv. campestris

Leila Monteiro^I; Rosa de Lima Ramos Mariano^II; Ana Maria Souto-MaiorI, ^*

^IDepartamento de Antibióticos; Universidade Federal de Pernambuco; Cidade Universitária; 50.670-90; Recife - PE - Brazil

^IIDepartamento de Agronomia/Fitossanidade; Universidade Federal Rural de Pernambuco; Dois Irmãos; 52.071-900; Recife - PE - Brazil

ABSTRACT

The antagonism of eight Bacillus isolates was investigated against nine strains of Xanthomonas campestris pv. campestris (causal agent of crucifers black rot) to assess the role of lipopeptides in this process. Antimicrobial and hemolytic (surfactant) activity tests were performed in vitro using agar diffusion methods. Antibiosis and hemolysis were positive for four Bacillus isolates against all X. campestris pv. campestris strains. The correlation observed between antimicrobial and hemolytic activities indicated that lipopeptides were involved in the antibiosis mechanism of the studied antagonists. Fermentation studies were carried out with the isolates that showed highest antimicrobial and hemolytic activities, to follow up growth and production of bioactive and surfactant compounds. Production of bioactive and surfactant compounds was observed during the late growth phase of the Bacillus isolates.

Key words: Bacillus, biological control, Xanthomonas campestris pv. campestris, crucifers black rot

RESUMO

Investigação sobre o antagonismo de oito isolados de Bacillus: B. subtilis R14, B. megaterium pv. cerealis RAB7, B. megaterium pv. cerealis C211, B. megaterium C116, Bacillus sp. RAB9, B. cereus C240, Bacillus sp. C11 e B. cereus C210, contra nove linhagens de X. campestris pv. campestris (bactéria responsável pela podridão negra das crucíferas) foi realizada para se verificar a participação de lipopeptídeos neste mecanismo. Testes de atividades antimicrobiana e hemolítica (surfactante) foram realizados, utilizando-se o método de difusão em ágar. Antibiose e hemólise foram positivas para quatro isolados de Bacillus: R14, RAB7, C116 e C210. A correlação observada entre as atividades antimicrobiana e a hemolítica indica que lipopeptídeos estão envolvidos no mecanismo de antibiose dos isolados investigados. As fermentações foram realizadas com os isolados que demonstraram melhores resultados nos testes de atividades antimicrobiana e hemolítica: R14, RAB7 e C116, para acompanhar o crescimento e a produção de compostos bioativos e surfactantes. As fermentações foram realizadas em mesa agitadora, usando frascos Fernbach. A produção de compostos bioativos e tensoativos foi observada durante a fase final de crescimento dos isolados Bacillus estudados.

INTRODUCTION

Xanthomonas campestris pv. campestris is the causal agent of crucifers black rot, a disease responsible for severe economic losses. Plants belonging to this family are susceptible to this disease in all developmental stages. The black rot occurs more frequently in humid soils and temperatures ranging from 20 to 30ºC, which are common in tropical and subtropical regions (Mariano et al., 2001). The symptoms are characterized by yellow V-shaped lesions that begin on the leaf margins and progress to the center through the vascular tissue, resulting, in general, in the leaf necrosis (Assis et al., 1997).

The use of chemical compounds has failed to control plant diseases due to resistance, environment pollution, and damage to human health. Because of these disadvantages, the use of microorganisms for pathogen control and for plant growth promotion is becoming more common. However, the success of biocontrol and yield increase depends on the nature of the antagonistic properties and on the mechanisms of action of the organism. The modes of action are widely varied and can be, for instance, nutrient competition, direct parasitism, and production of secondary metabolites (Melo, 1998).

The genus Bacillus is one of the most utilized in the biocontrol of phytopathogens. This genus comprehends a heterogeneous group of Gram-positive, aerobic or facultative anaerobic, endospore-forming bacteria. The endospores are termotolerant structures, resistant to dryness, to ultraviolet radiation and to organic solvents. These properties, associated to the ability of producing peptide antibiotics, contribute to the utilization of this genus on the biocontrol of several root and foliar diseases (Backman et al., 1997; Kloepper, 1997; Melo, 1998). Bacillus spp. have been formulated and registered for commercial use in the United States, and one of the products, Serenade® , is recommended for foliar diseases of several crops (Gardener and Fravel, 2002).

Assis et al. (1996) investigated the antagonism of 32 epiphytic Bacillus spp., isolated from cabbage, kale and radish. Among these isolates, 13 reduced 100% the incidence of black rot in kale under greenhouse conditions. In another study (Assis et al., 1997), 13 isolates reduced incidence in cabbage, ranging from 48% to 78%, in field experiments. Among them, B. cereus C210, B. megaterium C116, B. subtilis R14 and B. cereus C240 reduced 78%, 74%, 73% and 71%, respectively.

Bacillus spp. are involved in the control of plant diseases through a variety of mechanisms of action, such as competition, systemic resistance induction and antibiotic production. The mechanism of antibiosis has been shown to be one of the most important (Tomashow and Weller, 1996). Among several peptide antibiotics, Bacillus spp. produce lipopeptides, which are amphiphilic compounds with surfactant activity (Zuber et al., 1993). In the present study, the production of lipopeptides and the role of these compounds in Bacillus antagonism against X. campestris pv. campestris were investigated.

MATERIALS AND METHODS

Microorganisms

Eight epiphytic Bacillus studied, B. subtilis R14, B. megaterium pv. cerealis RAB7, Bacillus sp. RAB9, B. megaterium pv. cerealis C211, B. megaterium C116, B. cereus C240, Bacillus sp. C11 and B. cereus C210, were isolated from cabbage, radish and kale. Nine strains of X. campestris pv. campestris: C3, C4, C8, C10, C11, C12, C18, S2 and S6, were isolated from cabbage.

Antimicrobial Activity

Antimicrobial activity was determined by agar diffusion technique. One hundred microliters of X. campestris pv. campestris suspensions (around 10⁹ cells/mL) were mixed to yeast-malt agar in pour plates. After solidification, 1 µL of Bacillus suspensions were placed on the agar surface and incubated at 37ºC for 12 h, followed by incubation at 30ºC for 24 h. After the 36 h of incubation, inhibition halos were measured and antimicrobial activity (mm) was expressed as the difference between diameter of inhibition zone and diameter of Bacillus colony (Monteiro, 2002).

Hemolytic Activity

Hemolytic activity was also determined by agar diffusion technique. One microliter of Bacillus suspensions were placed on the surface of plates containing blood agar medium and incubated at 30ºC and 37ºC. At 72 h of incubation, the diameters of hemolysis zones were measured, and the results expressed as hemolytic activity (mm) (Monteiro, 2002).

Statistical Analysis

All experiments were performed in a completely randomized design. The results were subjected to analysis of variance (ANOVA) and means were compared by Tukey Test (P < 0,05) using the software SANEST^® ("Sistema de Análises Estatísticas, Instituto Agronômico de Campinas" – IAC, 1989).

Fermentation

Cell growth, substrate consumption and production of bioactive compounds were followed during cultivations in a medium proposed by Kim et al. (1997) for biosurfactant production. The fermentations were carried out in Fernbach flasks, containing 500 mL of medium in a rotatory shaker (New Brunswick Scientific) at 150 rpm and 30º C. Biomass was measured as dry-weight, after filtering 10 mL samples through 0.2 µm Millipore membranes and drying to constant weight at 80ºC for 24 h. Glucose concentration was determined by an enzymatic method (BioMérieux, kit 61269). Production of bioactive compounds, expressed as antimicrobial activity (mm), was determined by agar diffusion method using paper disks and X. campestris pv. campestris C10 as microorganism-test. Biosurfactant production was estimated by superficial tension measurements (CSC–Dunoüy interfacial tensiometer). Methodology proposed by Cooper et al. (1981) and Kowall et al. (1998) was used to isolate the surfactant compounds (Monteiro, 2002).

RESULTS AND DISCUSSION

Bioactivity of Bacillus Isolates

Growth inhibition of all X. campestris pv. campestris strains by four of the Bacillus isolates tested: R14, RAB7, C116, and C210, are presented in Table 1. The other four isolates: RAB9, C211, C240, and C11, did not show any inhibition halo or showed a negligible one. Analysis of the data showed that there was specificity among the antagonists and the X. campestris pv. campestris strains. Not all antagonists inhibited the phytopathogenic strains with the same efficiency and not all the strains had the same sensitivity to the antagonists. This fact could be explained by the genetic variability of both the phytopathogens and the antagonists. B. subtilis R14 and B. megaterium pv. cerealis RAB7 were the most efficient of all antagonists, without showing difference between themselves.

Hemolytic Activity of Bacillus Isolates

The hemolytic activity presented by lipopeptides can be used for selecting lipopeptide-producing microorganisms. Therefore, hemolytic activity tests were performed to investigate the possible role of these compounds in the antimicrobial activity of the Bacillus isolates. The results are shown in Table 2. The same isolates that showed antimicrobial activity also produced hemolysis zone on blood agar plates. Only three isolates showed hemolytic activity at 30º C: RAB7, R14 and C116, without significant differences among them. At 37º C, four of the antagonists tested showed activity: C116, RAB7, R14 and C210. In addition, the isolates that presented hemolytic activity showed wider zones at 37º C. The isolates C116 and RAB7 differed significantly from R14 and C210, and they all differed from the others isolates, which did not show any hemolysis zone.

B. megaterium pv. cerealis RAB7 showed both high antimicrobial activity and high hemolytic activity. B. subtilis R14 was the best antagonist among all, but showed less hemolytic activity then B. megaterium C116, which showed the highest activity on the hemolytic activity tests. B. cereus C210 activity was less significant in both experiments when compared to the other antagonists.

The correlation found between hemolytic activity and growth inhibition of the phytopathogenic bacterium indicated that lipopeptides were involved in the antagonism. However, although lipopeptides showed surfactant as well as antimicrobial activities, a potent lipopeptide surfactant could show low antimicrobial activity and vice versa. Moreover, more than one lipopeptide, with different antimicrobial and surfactant activities, were normally produced by Bacillus (Ohno, et al., 1995), and synergistic effects have been observed, as discussed below.

Hiraoka et al. (1992) studied the activity of the lipopeptides surfactin (a potent surfactant) and iturin, either together or separately. Surfactin alone did not show antimicrobial activity against the phytopathogenic fungus Fusarium oxysporum f. sp. lycopersici, causal agent of crown and root rot of tomato. Nevertheless, the combination of both lipopeptides enhanced pathogen inhibition, in relation to iturin alone. B. subtilis RB14, a surfactin and iturin producer, inhibited not only Fusarium oxysporum f. sp. lycopersici, but also the growth of other phytopathogens, more significantly than B. subtilis NB22, which produced only iturin A.

Production of Bioactive and Surfactant Compounds by Bacillus Isolates

The three isolates that showed the highest antimicrobial and hemolytic activities: RAB7, R14 and C116, were selected for fermentation studies. Figure 1 shows biomass production, glucose consumption and antimicrobial activity production during the growth of the three isolates. B. megaterium pv. cerealis RAB7 and B. subtilis R14 had a longer growth phase then B. megaterium C116, but in all cases the antimicrobial activity production was observed at the late growth phase. The antimicrobial activities in liquid medium were similar to that observed on solid medium, where the isolates R14 and RAB7 produced wider inhibition zones then C116.

Fermentations were carried out at the same growth conditions to follow the variation of the medium superficial tension, which indicated the production of biosurfactants. The results are shown in Table 3. B. megaterium C116 showed the fastest growth and the superficial tension of the medium was reduced more quickly in comparison to the other two isolates. Nevertheless, the reduction of superficial tension occurred similarly for the three Bacillus isolates, at the end of the growth phase.

Biosurfactants produced during microorganism growth in liquid medium tended to concentrate on the foam, so less activity was observed in the liquid (Cooper et al., 1981). This could be the reason for the superficial tension be higher in this study than that found in the published literature, where it was lower than 30 mN/m (Kim et al. 1997). In order to isolate tensoactive compounds, HCl was used to precipitate peptides in the final sample of each culture, which was centrifuged, and the sediments dissolved in deionized water at pH 12. The supernatant, the water, and the water with dissolved precipitates had their superficial tension measured.

As shown in Table 4, after the removal of precipitates, the medium superficial tension raised, indicating that tensoactive compounds were removed through the precipitation. The water with dissolved precipitates showed superficial tension lower than pure water (65.9 mN/m), indicating the presence of biosurfactants in these precipitates.

The production of both antimicrobial and surfactant activities during growth of B. megaterium pv. cerealis RAB7, B. subtilis R14 and B. megaterium C116 on solid and liquid media indicated that lipopeptides could have a major role on the reduction of the incidence of black rot observed previously in greenhouse and field trials using these microorganisms as biocontrol agents. More than one lipopeptide, with different antimicrobial and surfactant activities, might have been produced by each Bacillus isolate.

ACKNOWLEDGEMENT

The authors are grateful to "Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)" for a scholarship to L. M.

Received: November 28, 2003;

Revised: April 23, 2004;

Accepted: July 14, 2004.

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*

Author for correspondence

Publication Dates

• Publication in this collection
  15 Apr 2005
• Date of issue
  Jan 2005

History

• Accepted
  14 July 2004
• Reviewed
  23 Apr 2004
• Received
  28 Nov 2003