Advances in Biological Sciences Research, volume 14 Proceedings of the 3rd KOBI Congress, International and National Conferences (KOBICINC 2020) Antimicrobial Activity of Pseudomonas fluorescens and Bacillus subtilis on Different Dilution Concentrations Against Various Citrus Post-Harvest Pathogens Unun Triasih1,* Yogi Adhi Nugroho1 Sri Widyaningsih1 1Indonesian Citrus and Subtropical Fruits Research Institute *Corresponding author. Email: [email protected] ABSTRACT Pseudomonas flourescens and Bacillus subtilis are common antagonistic bacteria to various pathogens. These bacteria can produce various antimicrobial substances such as chitinase, siderophore, and antibiotic. This research aims to obtain the optimum dilution concentrations of Pseudomonas flourescens and Bacillus subtilis that can effectively inhibit the growth of Penicillium digitatum, P. paradoxum, and Aspergillus sp. This research was carried out in a complete randomized design with five different dilution concentrations of Pseudomonas flourescens and Bacillus subtilis ranging from 10-1 to 10-5. The antimicrobial activity tests against fungal pathogens were carried out in triplicate using a dual culture method. P. flourescens and B. subtilis displayed strong antimicrobial activities against all target pathogens. The highest inhibition activity of P. flourescens and B. subtilis was recorded against Aspergillus sp. with 45.7 % and 35.92 % and cell density of 168 x 106 CFU/mL and 14 x 106 CFU/mL, respectively. The highest antimicrobial activity was recorded at the 10-1 and 10-4 for P. flourescens and B. subtilis, respectively. The antimicrobial activity of P.flourescens and B.subtilis against P. digitatum and P. paradoxum are ranging from 17.59 % to 27.73 %. Keywords: Antimicrobial, Bacillus subtilis, dilution, post-harvest, Pseudomonas flourescens delicate way to prolong its commercial period and 1. INTRODUCTION reduce the risk of postharvest loss due to Citrus is one of the most economically important contaminations by various microorganisms such as crops in the world. Citrus is known for its high fungi and bacteria [1]. In the developing country like nutritional values that are found beneficial to improve India and Indonesia citrus loss due to postharvest immunity and source of vitamins and minerals. Citrus mishandling and contamination are very high, can also be made into various food products, namely reaching the range of 25-30% annually [2]. These are marmalade and jam. Due to its benefits and mostly caused by the contaminations of various versatilities, the worldwide demand for high-quality foodborne postharvest fungal pathogens such as citrus fruit is very high. To meet this target, every Penicilium italicum, P. digitatum and Colletothricum phase in citrus productions needs to be maintained in gloeosporiodes. These isolates are known to be cause stringent condition to ensure its quality and quantity, of blue mold, green mold and anthracnose diseases starting from its land managements to its post-harvest that are commonly found during citrus postharvest handling. period. One of the most essential phases in determining Controlling fungal contaminations during the quality of a citrus fruits is its post-harvest phase. postharvest period is difficult and have caused During this phase, the harvested citrus fruits are concerns among public, farmers and scientists [3]. handled, processed, stored and transported in a Often times, fungal contaminations in postharvest Copyright © 2021 The Authors. Published by Atlantis Press B.V. This is an open access article distributed under the CC BY-NC 4.0 license -http://creativecommons.org/licenses/by-nc/4.0/. 535 Advances in Biological Sciences Research, volume 14 period is controlled through fungicide treatments measured using hemocytometer. Target fungal [4,5]. However, this method is no longer considered pathogens were grown in Potato Dextrose Agar effective because not only it can be dangerous if (PDA) medium for further use. consumed, it will also contaminate the environment if used in long run [6-8]. To replace fungicide, several 2.2 Antagonistic Tests of P. flourescens and feasible and environmentally friendly alternatives B. subtilis against Various Citrus Postharvest have been proposed to control or avoid the Fungal Pathogens contaminations of these pathogens [9]. In vitro antagonistic tests of P. flourescens and B. The use of microorganisms as biopreservative subtilis against various citrus postharvest pathogens agent for food products have been gaining its were carried out using dual culture method proposed popularity lately. Biopreservative is considered safe by [14] with modifications. Sterile filter papers (Ø: because it doesn’t leave chemical residue to the 0.5 mm) were dipped into each bacterial culture for environment and normally will not cause harm to 15 minutes. Filter papers then placed on PDA plate human if consumed. Biopreservative microorganisms alongside the target fungal pathogens with 3 cm work by directly inhibiting the growth target distances. The plate was incubated at room foodborne pathogens through the means of toxic temperature for 7 days. Fungal colony diameter was secondary metabolites production and competition measured and used to determined the inhibition for space and nutrient uptake [10]. Pseudomonas activity (%) for each bacterial isolate. The inhibition flourescens and Bacillus subtilis are one of the most activity (%) was determined according to: common microorganisms used to control foodborne postharvest pathogens. P. flourescens reported to R1 – R2 have antifungal activity against various fungal I = x 100 % foodborne pathogen such as Aspergillus niger, A. (1) flavus, Penicillium italicum, P. digitatum and R1 Fusarium sp. [11,12]. On the other hand, B. subtilis also exhibited antifungal activity against various I: Inhibition activity (%) foodborne pathogen such as P. digitatum. [13] R1: colony diameter of untreated pathogen (control) reported that B. subtilis can inhibit the growth P. R2: colony diameter of treated pathogen digitatum with 82.4 % inhibition zone when tested in in vivo dual culture tests by inhibiting the mycelial 2.3 Data Analysis growth P. digitatum. However, despite showing promising antifungal activities against various fungal Statistical significance in this experiment was foodborne pathogens, the potency of P. flourescens determined using Analysis of Variance (ANOVA) at and B. subtilis as antifungal for citrus postharvest 5 % significance level followed by Tukey HSD test pathogen have not yet reported. Therefore, this in SPSS. research aim to study and evaluate the antifungal activity of P. digitatum and B. subtilis against citrus 3. RESULT AND DISCUSSION postharvest pathogens namely P. digitatum, P. Table 1. Cell density of both isolates in different paradoxum and Aspergillus sp. dilution level 2. MATERIALS AND METHODS Bacterial isolates Dilution level P. flourescens B. subtilis 2.1 Research Design and Culture 10-1 169,8 x 106 140,05 x 106 Preparation 10-2 53,6 x 106 77,8 x 106 10-3 51 x 106 24 x 106 -4 6 6 This research is carried out in complete 10 46,2 x 10 14 x 10 10-5 17,25 x 106 7,5 x 106 randomized design with three replications. Bacterial culture P. flourescens and B. subtilis are challenged The increase in serial dilution level can against three citrus postharvest pathogens namely P. significantly reduce the cell density within the digitatum, P. paradoxum and Aspergillus sp., P. bacterial culture (Table 1), and these cultures were flourescens and B. subtilis were cultivated in King’s later evaluated for its antifungal activity against B medium at room temperature for 48 hours. Both target pathogens. All bacterial culture in this bacterial cultures were diluted to 10-5 serial dilutions. experiment displayed good antagonistic activity The cell density of each bacterial culture was against all target pathogens. These antagonistic can 536 Advances in Biological Sciences Research, volume 14 Figure 1. Inhibition activity (%) of P. flourescens in different dilution level against three citrus postharvest pathogens Figure 2. Inhibition activity (%) of B. subtilis in different dilution level against three citrus postharvest pathogens be seen by the impaired growth and significant their research P. digitatum treated with P. flourescens colony diameter reduction of the target pathogens experienced significant loss in its mycelial growth, when treated with P. flourescens and B. subtilis. spore germinations and germ tube elongations. The ability of P. flourescens to inhibit various Similar to its counterpart, B. subtillis also kinds of fungal pathogens also have been reported demonstrated strong inhibition activity against all before. [15] reported that P. flourescens isolated from target pathogens. The highest inhibition activity of B. wheat rhizosphere could inhibit the growth of its host subtilis was recorded against Aspergillus sp. at 10-4 pathogens notably Fusarium sp. and Aspergillus sp. dilution level with 35.92 % of inhibition. B. subtilis with high efficacy and inhibition zone up to 20 mm in displayed slightly weaker inhibition activities against diameter. In other study, [16] reported that P. P. paradoxum and P. digitatum with 21.60 % and flourescens cell culture suspended in growth medium 28.02 % inhibition respectively (Figure 2). or sterile distilled water could significantly inhibit the Antifungal activity of B. subtilis have been growth of the citrus postharvest pathogen P. reported in many studies. In a study conducted by digitatum based