The emergence of Multi Drug Resistant (MDR) pathogens is a serious threat to the public health, especially in the developing countries like Bangladesh (Faiz & Ariful, 2011; MacGowan & Macnaughton, 2017; Rahman & Huda, 2014). Although the records lack precision and verification, the people usually suffer heavily from several infectious diseases connected to MDR (Barai et al., 2017). The emergence of MDR strains could be attributed to several factors including frequent and improper use of antibiotics, prescribing antibiotics without antimicrobial susceptibility testing, presence of MDR strains in food, feed and environment, emergence of multidrug resistance in common pathogens, horizontal and vertical transfer of antibiotic resistance genes, and improper hospital waste management etc. (“Progress on antibiotic resistance,” 2018; Rahman & Huda, 2014; van Belkum et al., 2018). The current situation in Bangladesh is very alarming since the resistance by major Gram negative bacteria such as Escherichia coli, Klebsiella sp., Salmonella sp., Pseudomonas sp., Acinetobacter sp. and major Gram positive bacteria such as Staphylococcus aureus, Enterococcus sp was reported against imipenem (3- 84 %), third generation cephalosporin (61.6- 94.9 %), aminoglycosides (10.8- 88.6%), ciprofloxacin (56- 90.1%), cotrimoxazole (58- 80.3%), nitrofurantoin (14.3- 91.7%), tazobactum+piperacillin (20.8- 81.4% ) and colistin (2.2- 16.4%) (Barai et al., 2017).
Although research and development for new generation of antibiotics initially helped in solving the resistance problems, the situation deteriorated with time. To combat the multidrug resistance, a sustainable holistic approach including production of new potent antibiotics from novel sources is urgently needed (A & M, 2012; Ahmad et al., 2017). Since, numerous essential bioactive compounds e.g. antibiotics, enzymes, hormones, vitamins, anticancer and antiviral drugs, herbicides, fungicides, insecticides, immunomodifiers, therapeutic agents, were obtained from different microorganisms, systematic screening for microbes with potential antimicrobial agents is always a focus of interest to the researchers (Ferdous, Shishir, Khan, & Hoq, 2018; Khatun, Haque, & Islam, 2018; Lee, Chan, Stach, Wellington, & Goh, 2018; Mustafa, A., & Cem, 2004). Discovery and development of new antimicrobial agents is therefore beside other approaches to combat antibiotic resistance, an important area (Ahmad et al., 2017; Bérdy, 2005; V & S, 2018).
Among 22,500 bioactive compounds, almost half were reported to be produced by actinomycetes (mainly Streptomyces) followed by fungi and other bacteria (Bérdy, 2005; Lee et al., 2018; Ser et al., 2017). A large number of antibiotics (nearly 80% of all) currently in use including Erythromycin, Streptomycin, Rifamycin and Gentamycin are obtained from soil actinomycetes which necessitates the novel biotopes, niche, ecosystems and extreme environments to be explored on regular basis for more potential and novel actinomycetes (Ahmad et al., 2017; Crits-Christoph, Diamond, Butterfield, Thomas, & Banfield, 2018; Lee et al., 2018; S. Ningthoujam & Sanasam, 2011). Among the soil actinomycetes, two major groups i.e. Streptomyces and Micromonospora are predominant in obtaining different antibiotics.
Streptomyces is renowned for producing secondary metabolites with different biological activities, such as antibacterial, antifungal, antiparasitic, herbicidal, antitumor, anticancer and anti-immunosuppressant activities (Anderson & Wellington, 2001; Jiang et al., 2018; Nishat & Alam, 2017). Like other members of actinobacteria, Streptomyces are filamentous Gram-positive bacteria with high GC content in the genome, mostly spore-forming and noted for their distinct "earthy" odor (Ahmad et al., 2017; Martinko & Madigan, 2005). In fact, thousands of antibiotics obtained to date represent a small portion of the repertoire of bioactive compounds and hence, it is very likely to discover novel Streptomyces with potent bioactive compounds (Bérdy, 2005; Crits-Christoph et al., 2018; Ser et al., 2017).
Bangladesh is a tropical country with vast ecological diversity which increases the chance of obtaining diverse Streptomyces with novel antimicrobials. But this area remained mostly untouched in this country except few recent studies (Abony, Alam, Banik, Jannat, & Datta, 2017; Khatun et al., 2018; Nishat & Alam, 2017; Sharmin, Rahman, Sayeed, Anisuzzaman, & Islam, 2017). Streptomyces bangladeshensis, a new species of Streptomyces, from the soil of Natore, Bangladesh was reported to produce bis-(2-ethylhexyl)-phthalate, an antibacterial and antifungal agent (Al-Bari, 2005) and Streptomyces banglaensis strain ANTS-1T obtained from the soil of Rajshahi, Bangladesh was reported to produce Actinomycin D, an antitumor protein (Sharmin et al., 2017). The present study was therefore undertaken with the objectives of characterizing 8 potential indigenous Streptomyces isolates based on certain physico-chemical parameters, biotyping based on their carbohydrate utilization ability and to determine the influence of pH levels on their antimicrobial production.