Study of the Soil Isolates for Antimicrobial Activity

During the process of screening for a potent antimicrobial compound, a new strain was isolated from the soil sample of Thalaikunda village in Ooty, Tamil Nadu. That organism was name as NK2. It was found to be antagonistic to both bacterial and fungal test organisms. Production of antibiotic was more in a newly formulated broth. Antibiotic production reached maximum at the end of the 70 h of fermentation by stirred flask culture. The antimicrobial compound was extracted in n-butanol, ethyl acetate and methanol. Antimicrobial compound, which was produced by the soil isolate NK2 did not showed cytotoxic activity on Vero cell lines.

Over five thousand antibiotics have been identified from the culture of gram-positive, gram-negative and Þ lamentous fungi but only hundred antibiotics have been commercially used to treat human, animal and plant disease 1 . A major feature of industrial antibiotic production is directed to screening programmes for new potent antibiotic producing organism either from natural sources or from established cultures. Screening for antibiotics producing microorganism, can be detected and isolated by the use of highly selective procedure which allows detection and isolation of only those microorganism of interest from a large population is possible. Soil is the largest source of microorganisms 2 . Majority of antibiotics so far isolated were produced from Streptomycetes, which are common inhabitants of the soil 3 . There are 23,000 known secondary metabolite, 42% of which are produced by Actinobacteria, 42% by fungi (Penicillium spp) and 16% by other bacteria 3 Streptomycetes spp. as the microorganisms become resistant after some time to a particular antibiotic, it is becoming necessary to find newer antibiotics to which the microorganism is sensitive 4 . In the present study, some bacteria and fungal strains were tested for antibiotic sensitivity. Soil isolate named as NK 2 was found to be active on the selected microorganisms. Taxonomical studies were performed. The secondary metabolite of the soil isolate NK 2 showed antimicrobial activity. In this paper isolation, characterization, bioprocessing and evaluation of product, obtained from the soil isolate, NK 2 are described. Fermentation media for NK 2 was formulated based upon carbon utilization pattern 8. A new fermentation medium consisting of soluble starch-20 g, sucrose-15 g, glucose-5, soya bean meal-20 g, yeast extract powder-5 g, CaCO 3 -3.2 g, MgSO 4 .7H 2 O-2.5 g, K 2 HPO 4 -5 g, MnCl 2 -0.2 g, NaCl-0.01 g, FeSO 4 .4 H 2 O-0.002 g and silicone oil as an antifoaming agent-0.3 ml/ l. The seeded medium consisted of glucose-10 g, soluble starch-10 g, yeast extract powder-5 g, beef extract-3 g, CaCO 3 -2 g/l. A 250 ml conical flask containing 10 ml of seed medium was inoculated with a loopful growth of the selected strain grown on slants. The flask was incubated at 28 0 for 48 h. Ten millilitres of NK 2 seed culture was transferred to a 1 l conical flask containing 100 ml of the same medium and then it was incubated at 28 0 , the second stage seed culture was used as the inoculum to initiate the fermentation in a 5 l containing 3 l of fermentation medium. The fermentation was carried out at 28 0 with sufficient aeration and agitation at 200 rpm until the pH reached neutrality. Culture growth was evaluated by centrifuging the fermented broth for 10 min at 5000 revolution per minute 9 . The percentage of packed cell volume, change in pH, antibiotic production was noted.
Soil isolate, NK 2 , was studied for its morphology and its characteristics by agar block method. The morphology was observed under scanning electron microscopy ( fig. 1) Carbon utilization studies were performed and based on the new fermentation medium was formulated 9 . Development of melanin pigment in Wakesman No. 42 medium was evaluated by streaming the soil isolate on the slants followed by incubation at 28 0 and the results were observed at the interval of 12 h for 4 days and it was recorded [10][11][12][13] .
Nitrate reducing property was evaluated by inoculating the isolate in organic nitrate broth and incubated at 28 0 for 5 days. From the 5 th day, the cultures were observed for the nitrate reduction by using the reagents such as α-naphthal solution and sulphonilic acid. Development of the pink colour indicated the nitrate reducing property of the isolate [10][11][12][13][14] . Proteolytic activity of the isolate was evaluated by inoculating them in pasteurized milk and observing for the reduction of litmus paper, formation of white band, change in pH, formation of whey like brownish translucent band and gas formation up to 48 h [10][11][12][13] . Presence or proteolytic enzymes was determined by growing the soil isolate over the starch agar medium and incubated at 28 0 for 5-7 days. The development of clear zone indicated the hydrolysis of starch and it was ß ooded with Lugol's iodine solution for conÞ rmation [10][11][12][13] .
The soil isolate was streaked and incubated for 4 days at 28 0 in a medium containing ferric ammonium citrate, dibasic potassium phosphate, Na 2 S 2 O 4 , yeast extract and agar [10][11][12][13] . The Nutrient gelatin medium was employed to grow the soil isolate. The protein gelatin is expected to be hydrolyzed by exoenzyme, if secreted by the isolate. The solid character of the medium depends on gelatin remained in the gel state [10][11][12][13] . The isolate was inoculated in glucose nutrient broth along with bromothymol blue as indicator and incubated at 28 0 for 15 days. At every 12 h of interval, change in colour was noted [10][11][12][13] .
Inoculum (24 h old) was used to seed the ß ask at 10 % level. Fermentation was carried out for 5 d with 200 rpm at 28 0 . The active constituents were extracted from both filtrate and mycelia after separation by centrifugation from the fermented cultured broth. One part of the Þ ltrate was extracted three times with equal volume of n-butanol and another part with ethyl acetate. The mycelia were extracted with methanol. All the three extracts were concentrated at 40 0 to obtain crude extracts. All the crude extracts of NK 2 obtained from the fermented media were subjected to chromatographic analysis. Based on R f values, crude antibiotic fractions were classiÞ ed 14,15 .
Test bacteria were grown on nutrient agar and fungi were grown on Sabouraud dextrose agar medium. The extracts were dissolved in corresponding solvents and 100 µl of the samples were placed in to the corresponding cups. Zone of inhibition was measured after 24 h incubation at 37 0 for bacteria and after 48 h incubation at 28 0 for fungi. The antimicrobial activity was estimated by measuring the diameter of the inhibitory zone 16 . All extracts obtained from broth culture of NK 2 were tested for its cytotoxic activity on Vero cell lines using the Trypan Blue exclusion techniques 9 . The samples were tested at various concentrations between 125-500 µl/ml.
In the process of screening of soil actinomycetes, the isolate NK 2 was found to be capable of producing antibiotic against bacteria and fungi. The soil isolate NK 2 gave slightly positive results for the nitrate reduction and starch hydrolysis and it gave intense results for the acid production test. It does not have the capacity to produce H 2 S and Melanin pigments. NK 2 showed white band and solid formation after 24 h for milk coagulation test and at 48 h it showed white band, more whey like brownish medium, solid formation and gas formation. NK 2 -isolate showed growth in medium containing lactose, good growth in medium containing sucrose, fructose and d(+) sorbitol. it showed good growth with fermentation in the medium containing glucose and maltose and good growth with no fermentation in the medium containing d(+)mannitol.
Scanning electron microscope revealed a rectangular shape with irregular grouping of the NK 2 isolate Þ g. 1.
The production of antibiotic was carried out in stirred ß ask culture. The production began after inoculations, gradually reached the maximum at 70 h and slowly decreased. At 70 h the pH was 6.8 and there was slight increase to 7. Biomass reached the maximum at 94 h and then remained at the same level till 118 th h. Fermentation parameters are listed in Table 1. After extraction with n-butanol, the fermented broth gave a cream colour powder with the percentage of yield 0.689%. Extraction with ethyl acetate yielded a brownish  yellow powder with a yield of 0.0560% and methanol extract yielded a yellowish brown powder with a yield of 0.0548%. By trial and error method the optimal solvent system for TLC studies of NK 2 was found to be butanol, acetic acid and water in the ratio of 9:0.5:0.5. The R f (Retardation factor) values for the n-butanol, ethyl acetate and methanol was found to be 0.36, 0.08, 0.30, respectively.  (Table 3), it concluded that compound obtained from the isolate was an antibiotic belongs to the Streptomycetes spp. Further work can be continued for the structural elucidation of the compounds obtained by the fermentation process and it can be compared with the standard antibiotics to prove its potency.