Streptomyces noursei var saccharicus: An Antibiotic Producer from Soils

Abstract

During our continuous search for antibiotic producing actinomycetes, a variant of Streptomyces species was isolated from soils of Andhra Pradesh in India. The morphological, cultural, physiological and biochemical characters were studied, compared to known species and identified as a new variant of Streptomyces noursei and designated as Streptomyces noursei var saccharicus. The antibiotic activity of the strain was tested against both Gram-positive and Gram-negative bacteria as well as fungi and yeasts. INTRODUCTION Since the isolation of actinomycin in 1940 and streptomycin in 1944 by Waksman, the actinomycetes have received tremendous attention of the scientists. Soils, composts and fodders are common sources of actinomycetes. Waksman [1] recognized a few natural substrates as ideal sources for the isolation of actinomycetes and other streptomycetes. The nature of a Streptomyces colony is an important property in characterizing a culture. Krainsky [2] used the structure, size, shape and texture of the colony as one of the major diagnostic criteria. According to Pridham and Lyons [3] and International Subcommittee [4], the best way to handle streptomycete classification nomenclature and identification is through application of the genus-species-subspecies concept. The majority of antibiotic producing actinomycetes found in these species led to growing economic importance of these organisms which resulted in the isolation and description of numerous new species. It is reported that the only genus Streptomyces, the member of Actinomycetales accounts for approximately 93% producing secondary metabolites [5]. The present communication deals with the isolation and characterization of an antibiotic producer from soils.

Keywords

Species, Streptomycete, Streptomyces noursei var saccharicus, Gram- positive, Gram- negative, Sporophores, spiral spore chains, Aerial mycelium, Vegetative mycelium, Diaminopimelic acid.

Introduction

Since the isolation of actinomycin in 1940 and streptomycin in 1944 by Waksman, the actinomycetes have received tremendous attention of the scientists. Soils, composts and fodders are common sources of actinomycetes. Waksman [1] recognized a few natural substrates as ideal sources for the isolation of actinomycetes and other streptomycetes. The nature of a Streptomyces colony is an important property in characterizing a culture. Krainsky [2] used the structure, size, shape and texture of the colony as one of the major diagnostic criteria. According to Pridham and Lyons [3] and International Subcommittee [4], the best way to handle streptomycete classification nomenclature and identification is through application of the genus-species-subspecies concept.

The majority of antibiotic producing actinomycetes found in these species led to growing economic importance of these organisms which resulted in the isolation and description of numerous new species. It is reported that the only genus Streptomyces, the member of Actinomycetales accounts for approximately 93% producing secondary metabolites [5].

The present communication deals with the isolation and characterization of an antibiotic producer from soils of Andhra Pradesh.

Materials and Methods

Isolation

Soil samples were collected from different locations of Andhra Pradesh, India. Actinomycetes were isolated by plating on Half-strength nutrient agar medium ,Starch –Casein agar medium[6]and AV agar medium[7] and incubating at 280 C for 14 days. The media were supplemented with Benzyl penicillin (0.8mg), Nystatin(50μg/ml) to minimize the bacterial and fungal contamination. A total of 359 actinomycetes were isolated from 8 samples. Among 359 actinomycetes, isolate D50 with moderate antibacterial activity particularly against Gram-positive bacteria and sporophores are arranged in groups and formed spiral spore chains was found to be interesting and it was selected for detailed taxonomic study.

Antimicrobial Activity

The isolate D50 was inoculated into a production medium [8] with pH 7.2 and incubated at 28°C for 6 days on a rotary shaker. The antimicrobial activity was determined by standard cup-plate method [9]. The potency of the isolate was measured by the degree of inhibition zone (Table.1). All the test organisms employed in the present studies were supplied by the National Chemical Laboratory, Pune.

Table 1: Antimicrobial spectrum of D₅₀ culture filtrate.

Characterization

Characterization of the isolate D50 was done according to ISP procedures [10]. The studies include morphological, cultural, physiological tests and carbon source utilization pattern. The data of cultural characteristics, physiological & biochemical properties, carbon source utilization pattern, growth in the presence of various nitrogen sources and resistance to various antibiotics, growth in the presence of various inhibitory compounds and tolerance to sodium chloride of isolate D50 are presented in Tables 2 to 7.

Table 2: Cultural characteristics of D₅₀

Table 3: Physiological and biochemical properties of D₅₀.

Table 4: Carbon source utilization pattern of D₅₀

Table 5: Growth of D₅₀ in the presence of various nitrogen sources.

Table 6: Resistance to various antibiotics.

Table 7: Effect of inhibitory chemical compounds on D₅₀.

Characterization of the selected isolate has been made by following the standard procedure [10] For identification, the International Streptomyces Project (ISP) reports [11-13]. Bergey’s Manual of Determinative Bacteriology [14] and Bergey’s Manual of Systematic Bacteriology [15] have been followed.

Results and Discussion

As shown in Table.1, the isolate showed moderate antibacterial activity predominantly active against Gram-positive and no or negligible activity was observed against Gram- negative bacteria, fungi and yeasts. Therefore the isolate D50 was selected for further study.

The most significant characteristics of the strain D50 are summarized as follows

The strain grew well on most of the media. The micro-morphological studies revealed that the strain D50 has shown sporophores which occurred as spiral spore chains. Hence, it belongs to section ‘spira (s)’. The aerial mycelium developed moderately to good on most of the media and it was brown to pale gray to gray in colour. The vegetative mycelium was pale gray to gray on most of the media. The strain was non-chromogenic without any characteristic diffusible pigment and it did not produce any other soluble pigment.

The strain D50 was H2S and tyrosinase negative . It showed good diastatic activity and it could hydrolyze the casein and gelatin. It could coagulate and peptonise milk. It exhibited weak nitrate reduction. It grew well at 28° C. It did not grow at 100 C and 20° C and it showed poor growth at 37° C (Table.2&3). It showed good growth on meso- inositol. It exhibited poor to moderate growth on arabinose, sucrose, glucose, D-xylose, D-mannitol, D-fructose and L(+)-rhamnose and no growth on raffinose and cellulose. (Table 4).

The strain D50 exhibited good growth on L-arginine and potassium nitrate. It showed moderate growth on L-histidine and L-asparagine but it did not show any growth on L-cysteineHCl and L-valine. (Table.5). It exhibited resistance against penicillin G and cephalexin and sensitivity to streptomycin, tetracycline & gentamicin. It showed resistance to rifampicin after 7 days. (Table 6).

The analysis of cell wall hydrolyzates demonstrated the presence of LL—DAP (Diaminopimelic acid) & glycine. No characteristic sugars were present. The above data suggested that the strain D50 belongs to cell Type I and Type C sugar pattern. It could tolerate upto 7% NaCl but failed to grow at 10% and 13% NaCl. It did not grow in the presence of phenol but it could grow in the presence of crystal violet and potassium tellurite (Table.7).

A detailed survey of the literature indicates that our strain D50 is related to Streptomyces noursei [14-17] in respect of spiral sporophore , non-chromogenicity and some biochemical reactions.

However, some qualitative and quantitative differences could be noticed, the strain D50 differed from the reference culture in the following respects: brown to pale gray to gray colour aerial mycelium, production of antibacterial antibiotic, utilization of arabinose, xylose and rhamnose.

Conclusions

In view of large number of similarities and a few differences, it is felt that the strain D50 can be considered as a new variant of Streptomyces noursei . Hence it is named as Streptomyces noursei var saccharicus. Saccharicus is referred to sugar cane field soil from which the organism was isolated.

Acknowledgements

We thank the University Grants Commission, New Delhi for providing the Financial Assistance.

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