The current study focused on the isolation of actinomycetes from a few sites in Algeria in order to select strains with high antagonistic power. Eighty-eight isolates of actinomycetes were isolated from different samples. Actinomycetes are widely distributed in natural ecosystems including soil, aquatic ecosystems, deserts and even in Antarctica [32, 33]. In order to reduce the microbial load in the samples and promote the proliferation of actinomycetes, we performed a preheating. Baskaran et al.  reported that the predominance of other bacteria and fungi inhibits the colonization of actinomycetes. Heat treatment is often used to reduce the number of Gram-negative bacteria often invading isolation dishes . Using three isolation media allowed us to cover more species. SCA medium covers more isolates than CMC-agar and N-Free agar media. However, the latter two media allowed us to isolate high antagonistic actinomycetes that SCA medium does not cover. For this, it is interesting to use a range of media for the isolation of antagonistic actinomycetes. Actinomycetes present a potential source of bioactive compounds. They are remarkable antibiotic producers, making quarters out of all known pharmaceuticals . In order to screen our actinomycetes isolates for their antagonism against pathogenic bacteria, we opted for the cross-streak method. The results of this screening showed that the isolates CSM21, NSA21 and SSA112 possessed excellent antibacterial activity against all pathogens tested except E. coli. Baskaran et al.  screened 42 actinomycetes isolated from mangrove sediments against pathogenic bacteria and found that 22 species exhibited antagonism, of which isolate A107, identified as Streptomyces spp., had the maximum activity against all pathogens targeted. Ganesan et al.  screened 106 strains of actinomycetes against several pathogens using the cross-streak method. Only 41.50% showed good antagonism against the pathogens tested. The cross streak method has been reported to be used for rapid screening of antagonistic microorganisms .
By analyzing the results of the antibacterial activity of the crude supernatants and of the organic extracts (ethyl acetate) of the fermentations in liquid medium we found that the crude supernatants of all the isolates have a greater antibacterial activity than that of the organic extracts which is sometimes absent. The significant antibacterial activity recorded in actinomycete isolates cultured in TSB medium can be explained by the composition of this medium that promotes the production of antibacterial substances. Thus, it has been reported that there are both quantitative and qualitative variations in the antibiotics produced. Substrates and habitats greatly influenced the production of antibiotics by actinomycetes . Contrary to our results, several authors have reported the effectiveness of ethyl acetate in extracting large amounts of antibiotics from crude supernatants of actinomycete cultures [21, 19]. This result can be explained by the difference in polarity of the antibacterial metabolites produced by our isolates with that of ethyl acetate. The study of the production of antibacterial metabolites as a function of incubation time showed early and important production in isolate CSM21 against the three pathogenic bacteria. From the economic aspect, it is interesting to select actinomycetes which produce large quantities of antibacterial metabolites in a short time, which reduces production costs. Likewise, many actinomycetes have been reported to begin production of antibiotics after 4–6 days of incubation .
With the population explosion, large amounts of waste contaminated with pathogens are generated. It is therefore obvious that this waste must be properly contained and managed because it can cause infections in humans , hence the need to develop new effective biological methods to combat these agents. Screening, isolation and characterization of promising strains of actinomycetes producing bioactive compounds is an important area of research worldwide. Actinomycetes, especially Streptomyces species, are widely recognized as important microorganisms with potential applications . It has been reported that members of the genus Streptomyces isolated from terrestrial or aquatic environments are responsible for the largest share of production of antimicrobial substances . View their metabolisms, actinomycetes can be proposed as biological control agents against the presence and persistence of pathogenic bacteria in contaminated waste. To produce antibacterial metabolites and take advantage of organic waste, SSF was adopted in this study where we used SS as a substrate. We have recorded a considerable production which results in the high diameter of the inhibition zones of the pathogenic bacteria tested, especially when the substrates are moistened with distilled water which makes the production costs less expensive. In addition, SS is a highly available and inexpensive waste. Several authors have reported the production of antibacterial metabolites by actinomycetes in SSF, particularly, by Streptomyces species. Gebreyohannes et al.  showed the antibacterial activity of crude extracts from an SSF carried out by strains of actinomycetes using rice grains as a substrate. Khaliq et al.  studied the production of tylosin in SSF by Streptomyces fradiae NRRL-2702 and its mutant γ-1 using different agro-industrial wastes. Wheat bran, as a solid substrate, gave the best production by the mutant γ-1. Deen et al.  showed the efficiency of erythromycin production in SSF by Saccharopolyspora erythraea NCIMB 12462 using several agro-industrial wastes where sugarcane bagasse was found to be the best. Other antibiotics have also been produced by SSF, namely tetracycline by Streptomyces viridifaciens ATCC 11989, neomycin by Streptomyces marinensis, meroparamycin by Streptomyces sp. TUE01 . SSF offers a number of advantages over liquid fermentation. The production medium is often simple, using agro-industrial by-products such as wheat bran, rice bran and wheat straw . In addition, it can promote the production of new secondary metabolites compared to liquid fermentation . The application of antagonistic actinomycetes with significant and diverse enzymatic activity can increase the rate of elimination of pathogenic bacteria in contaminated waste. Parmar et al.  have proven the efficacy of proteases in the elimination of pathogenic bacteria in SS.
The selected isolate CSM21 was identified using macro and microscopic characters. Macroscopic examination on different media allows us to follow the growth of colonies, the arrangement of SM and AM as well as the morphology of the spores which can be taken into account when characterizing actinomycetes strains . In addition, the straight to flexible, hook, loop or spiral spore chains are characteristics of the genus Streptomyces which is the most abundant genus in nature. AM consists of three to more mature spores. Colonies can produce a wide variety of pigments responsible for staining SM and AM [36, 37]. Actinomycetes are ubiquitous microorganisms that may have existed in normal to extreme ecosystems with acidic or alkaline pH, low or high temperature, high salinity, etc. . A large number of Streptomyces, Nocardia and Micromonospora are grown at temperatures between 25 and 30°C. Streptomyces sp. JUBM-35-NS-1 and Nocardia sp. JUBM-35-NS-2 exhibit growth at temperatures between 25 and 45°C and tolerance of up to 8 and 2% NaCl, respectively . Loqman et al.  described a new species of Streptomyces named S. thinghirensis which tolerates 7% NaCl with growth occurring between 28 and 42°C. Marine ecosystems are distinguished from other ecosystems by several characteristics, thus promoting the development of particular microorganisms with particular physiological and cultural characteristics allowing them to produce bioactive secondary metabolites . The results of the cultural and morphological properties obtained allow us to suggest that the CSM21 isolate is affiliated with the genus Streptomyces. However, confirmation of this identification requires genetic analysis of 16S rDNA.
Screening a collection of actinomycetes for its antagonism to pathogenic bacteria allowed us to select a potential strain affiliated with the genus Streptomyces named CSM21 isolated from sea sand. This strain showed these potentials to produce antibacterial metabolites in liquid and solid fermentation. The recovery of organic waste offers an economic and environmental opportunity, which can reduce the problems of its conventional disposal. For the first time, SS are used as a solid substrate to produce antibacterial metabolites in SSF by actinomycetes. This well-known and available waste can promote the production of new antibacterial molecules. Thus, the use of SSF product rich in antagonist isolate (CSM21), antibacterial metabolites and hydrolytic enzymes in contaminated waste storage areas and in the various waste treatment processes is proposed as one of the biological methods for use in the control of pathogenic bacteria.