Antibacterial activity of an ethyl acetate extract from Streptomyces albus culture supernatant against Staphylococcus aureusAbstract
The increasing resistance of Staphylococcus aureus highlights the urgent need for new antibacterial alternatives. In this context, Streptomyces, a genus of actinobacteria widely recognized for its biosynthetic potential, represents a promising source of bioactive compounds. This study evaluated the antibacterial activity of a crude ethyl acetate extract obtained from the culture supernatant of Streptomyces albus against S. aureus strains. The extract was produced by submerged fermentation followed by liquid–liquid extraction with ethyl acetate. Antibacterial activity was assessed using the agar diffusion method, as well as by determining the Minimum Inhibitory Concentration (MIC) and Minimum Bactericidal Concentration (MBC). Two clinical strains (IC 3792 and IC 2141) and one reference strain (ATCC 29213) were evaluated. The extract produced inhibition zones ranging from 18.00 ± 1.0 mm to 22.33 ± 0.58 mm. MIC and MBC analyses demonstrated both inhibitory and bactericidal activity. Notably, strain IC 2141 exhibited an MIC of 625 µg/mL and a corresponding MBC, resulting in an MBC/MIC ratio ≤ 2, indicative of a bactericidal effect. These findings demonstrate that the crude extract obtained from S. albus exhibits antibacterial activity, including against strains with reduced susceptibility to conventional antibiotics. Overall, soil-derived actinobacteria represent a promising source of bioactive compounds, reinforcing the relevance of microbial bioprospecting in the development of new therapeutic strategies to address increasing bacterial resistance.
Keywords:
actinobacteria; antibacterial activity; Staphylococcus; Streptomyces
Resumo
A crescente resistência de Staphylococcus aureus reforça a necessidade urgente de novas alternativas antibacterianas. Nesse contexto, Streptomyces, um gênero de actinobactérias amplamente reconhecido por seu potencial biossintético, destaca-se como uma fonte promissora de compostos bioativos. Este estudo avaliou a atividade antibacteriana de um extrato bruto em acetato de etila obtido do sobrenadante de cultura de Streptomyces albus contra cepas de S. aureus. O extrato foi produzido por fermentação submersa, seguida de extração líquido-líquido com acetato de etila. A atividade antibacteriana foi avaliada por meio do método de difusão em ágar, bem como pela determinação da Concentração Inibitória Mínima (CIM) e da Concentração Bactericida Mínima (CBM). Foram testadas duas cepas clínicas (IC 3792 e IC 2141) e uma cepa de referência (ATCC 29213). Os halos de inibição variaram de 18.00 ± 1.0 mm a 22.33 ± 0.58 mm. As análises de CIM e CBM demonstraram atividade inibitória e bactericida. Destaca-se que a cepa IC 2141 apresentou CIM de 625 µg/mL e CBM correspondente, resultando em uma razão CBM/CIM ≤ 2, indicativa de efeito bactericida. Os resultados demonstram que o extrato bruto obtido de S. albus apresenta atividade antibacteriana, inclusive contra cepas com susceptibilidade reduzida a antibióticos convencionais. De forma geral, actinobactérias derivadas do solo constituem uma fonte promissora de compostos bioativos, reforçando a importância da bioprospecção microbiana no desenvolvimento de novas estratégias terapêuticas frente ao aumento da resistência bacteriana.
Palavras-chave:
actinobacteria; atividade antibacteriana; Staphylococcus; Streptomyces
1. Introduction
Actinobacteria, especially those from the genus Streptomyces, are widely recognized for producing secondary metabolites with potent antimicrobial activity (Selim et al., 2021; Kadirova et al., 2025). Several antibiotics used in clinical practice such as streptomycin, erythromycin, chloramphenicol, and tetracycline were originally isolated from strains of this genus, which accounts for a significant portion of the antimicrobial agents currently available (Alam et al., 2022; Vasconcelos et al., 2025). In addition, these microorganisms are capable of synthesizing enzymes, enzyme inhibitors, antitumor agents, and other molecules of biotechnological interest (Boubekri et al., 2022).
Beyond the production of bioactive compounds, these bacteria belong to a broad group of Gram-positive microorganisms with unique morphological characteristics, including a filamentous structure composed of chains or hyphae, resembling that of filamentous fungi (Dahal, 2022). This structural complexity favors the synthesis of biologically active molecules, reinforcing their potential as a promising source of new antimicrobial agents (França et al., 2024).
The rise of bacterial resistance and the scarcity of new antibiotic classes have spurred the search for effective alternatives. In this context, actinobacteria have gained prominence due to their biological potential and extensive metabolic diversity, making them promising candidates for overcoming microbial resistance mechanisms (Jakubiec-Krzesniak et al., 2018; Oliveira et al., 2024).
Bacterial resistance may arise from several factors, which can be classified as either intrinsic or acquired. Intrinsic factors refer to the natural ability of bacteria to resist the action of antibiotics through their own structural or functional characteristics (Iskandar et al., 2022). Acquired factors, on the other hand, are related to external elements and result from the acquisition of new resistance mechanismsoften driven by the indiscriminate use of antimicrobial agents (Galgano et al., 2025). This process can lead to the emergence of resistance in microorganisms that were previously susceptible, resulting in strains resistant to one or more antibiotics (Rosini et al., 2020).
A relevant example of bacterial resistance is Staphylococcus aureus (S. aureus), particularly methicillin-resistant strains (MRSA), which pose a major clinical challenge due to their resistance to multiple classes of antibiotics (Cheung et al., 2021). Although this microorganism is part of the normal microbiota of human skin and mucous membranes, it can become pathogenic in situations of immune imbalance or when natural body barriers are compromised (Plumet et al., 2022).
Under such conditions, S. aureus can cause a wide range of infections, from mild skin infections to severe conditions such as pneumonia, septicemia, and endocarditis, affecting both humans and animals (Costa et al., 2022). Furthermore, MRSA is associated not only with hospital-acquired infections but also with community-acquired infections, which increases its public health impact and further complicates effective treatment (Heaton et al., 2020).
Thus, this study aims to investigate the antibacterial activity of an actinobacterium from the genus Streptomyces against S. aureus, using both clinical strains and a reference strain (ATCC), in order to evaluate the potential of the compounds present in the extract produced by these bacteria as an alternative for the development of new antimicrobial agents.
2. Methodology
2.1. Microorganisms
The actinobacterial strain used in this study was identified as Streptomyces albus, previously isolated from soil samples collected in an area impacted by solid waste disposal (open dump site) in the municipality of Imperatriz, Maranhão, Brazil (5°53'18.359'' S; 47°48'22.451'' W). After collection, samples were transported to the Laboratory of Molecular Biology of Pathogenic Microorganisms at CEUMA University, where isolation, identification, and maintenance of the strain were performed.
Three Staphylococcus aureus strains were included in this study: two clinical isolates (IC 2141 and IC 3792) and one reference strain (S. aureus ATCC 29213). The clinical isolates were obtained from a private clinical analysis laboratory located in São Luís, Maranhão, Brazil, where they had been previously identified and characterized regarding their antimicrobial susceptibility profiles using standard clinical laboratory procedures.
According to the resistance profile presented in Table 1, strain IC 2141 was classified as methicillin-resistant Staphylococcus aureus (MRSA), while strain IC 3792 was classified as methicillin-susceptible Staphylococcus aureus (MSSA). The reference strain ATCC 29213 was used as a quality control strain and was not included in the resistance profile analysis.
Susceptibility profile of clinical and reference strains of Staphylococcus aureus to the tested antibiotics.
Prior to the experiments, all S. aureus strains were cultured in Mueller–Hinton broth and incubated at 37 °C for 18–24 hours to ensure active growth and standardization of the inoculum.
2.2. Preparation of the crude extract
The crude extract was obtained from the culture supernatant of S. albus following a protocol adapted from Trisuwan et al. (2008). Briefly, 50 mL of Potato Dextrose Broth (PDB) was inoculated with three agar plugs (5 mm in diameter) containing actively growing S. albus. The cultures were incubated at 28 °C for 15 to 20 days under constant agitation (150 rpm).
After incubation, the biomass was removed by centrifugation (6000 rpm for 10 minutes), and the resulting supernatant was subjected to liquid–liquid extraction with ethyl acetate (1:1, v/v). The organic phase was collected, and the solvent was removed under reduced pressure to obtain the crude extract, which was subsequently used in the antibacterial assays.
2.3. Agar diffusion assay
The crude extract was standardized at a concentration of 10,000 µg/mL and diluted in 1% dimethyl sulfoxide (DMSO). Antibacterial activity was evaluated using the agar well diffusion method, according to Bauer et al. (1966) and CLSI (2015) guidelines.
Petri dishes containing 20–25 mL of Mueller–Hinton agar were inoculated with standardized bacterial suspensions. Wells of 6 mm in diameter were prepared, and 100 µL of the extract was added to each well. The plates were incubated at 37 °C for up to 48 hours, after which the inhibition zones were measured.
Chloramphenicol (500 µg/mL) was used as a positive control due to its well-established antibacterial activity, while 1% DMSO was used as a negative control to confirm that the observed effects were not related to the solvent.
2.4. Determination of Minimum Inhibitory Concentration (MIC) and Minimum Bactericidal Concentration (MBC)
The Minimum Inhibitory Concentration (MIC) was determined using the broth microdilution method, following CLSI (2015) guidelines. The crude extract was initially prepared at a concentration of 10,000 µg/mL and subjected to two-fold serial dilutions in 96-well microplates containing Mueller–Hinton broth.
Bacterial inocula were prepared in sterile saline solution and adjusted according to the McFarland standard (approximately 1 × 108 CFU/mL). Each well received 100 µL of Mueller–Hinton broth and 100 µL of the extract, followed by the addition of 10 µL of the bacterial suspension.
Control wells were included as follows: growth control (bacteria without extract), sterility control (broth only), and extract control (extract without inoculum). All assays were performed in triplicate and incubated at 37 °C for 24–48 hours.
Bacterial growth was assessed by the addition of 30 µL of resazurin solution (0.1%), followed by further incubation. A color change from blue to pink indicated metabolic activity and bacterial growth, whereas the absence of color change indicated inhibition.
The Minimum Bactericidal Concentration (MBC) was determined by subculturing 10 µL aliquots from wells showing no visible growth onto Mueller–Hinton agar plates. After incubation at 37 °C, the MBC was defined as the lowest concentration at which no bacterial growth was observed. The MBC/MIC ratio was calculated to classify the antibacterial effect as bactericidal or bacteriostatic
3. Results
3.1. Antibacterial activity of Streptomyces albus-derived extract against antibiotic-resistant S. aureus
The S. aureus strains evaluated in this study exhibited heterogeneous resistance profiles to clinically relevant antibiotics, including reduced susceptibility to last-resort agents such as vancomycin. This resistance landscape underscores both the clinical relevance of the selected strains and the pressing need for alternative antimicrobial strategies capable of overcoming established resistance mechanisms.
The antibacterial activity of the ethyl acetate extract obtained from the culture supernatant of S. albus was assessed using the agar diffusion assay (Figure 1). Notably, despite the resistant phenotypes observed among the tested strains, the extract, at 10.000 µg/mL, consistently induced the formation of well-defined inhibition zones across all isolates, including the reference strain ATCC 29213. This observation indicates that the extract retains inhibitory activity even in strains exhibiting reduced susceptibility to conventional antibiotics.
Formation of inhibition zones produced by the actinobacteria extract and the controls: chloramphenicol (CLO) and DMSOagainst Staphylococcus aureus strains. (A): Formation of inhibition halos produced by the Streptomyces albus extract; (B): Formation of inhibition halos produced by the positive control (chloramphenicol - CLO) and the negative control (1% DMSO). Source: Elaborated by the author.
As shown in Table 2, the diameters of the inhibition zones varied among the strains, reflecting differences in susceptibility while maintaining a consistent inhibitory profile. The positive control (chloramphenicol) exhibited the expected antibacterial effect, whereas no inhibition was observed for the negative control (1% DMSO), confirming that the observed activity was attributable to the extract.
Mean diameter of inhibition zones (mm) with standard deviation, obtained by the actinobacterial extract and controls (chloramphenicol and 1% DMSO) in the agar diffusion assay against S. aureus strains.
The use of chloramphenicol as a control is particularly relevant in this context, as it represents a clinically established antibiotic originally isolated from Streptomyces venezuelae, a soil-derived actinomycete (Saikia and Chetia, 2024). This highlights the well-recognized role of Streptomyces species as a prolific source of bioactive secondary bioactive compounds and reinforces the biological plausibility of the antibacterial activity observed for the extract derived from S. albus.
Collectively, these findings demonstrate that the extract exerts reproducible antibacterial activity against S. aureus, including strains with clinically relevant resistance profiles. In the context of the global rise in antimicrobial resistance and the diminishing discovery pipeline of novel antibiotics, these results support the continued exploration of Streptomyces-derived compounds as a relevant source of antimicrobial agents, while underscoring the need for further studies to elucidate their active components and mechanisms of action.
3.2. Strain-dependent inhibitory and bactericidal activity of a Streptomyces albus-derived extract against S. aureus
Building upon the inhibitory activity observed in the agar diffusion assay, the antimicrobial potency of the extract was further quantified through the determination of the Minimum Inhibitory Concentration (MIC). The extract exhibited inhibitory activity against all tested S. aureus strains, with MIC values varying among isolates, as detailed in Table 3. This variability indicates differential susceptibility within the same species, despite the consistent antibacterial effect observed across the entire strain panel.
Determination of Minimum Inhibitory Concentration (MIC) and Minimum Bactericidal Concentration (MBC) of Streptomyces albus extract against S. aureus strains.
To further characterize the nature of this activity, the Minimum Bactericidal Concentration (MBC) was determined (Table 4). The MBC values, together with the calculated MBC/MIC ratios, enabled the classification of the extract’s effect as either bactericidal or bacteriostatic depending on the strain (Makade et al., 2024). These findings demonstrate that, beyond inhibiting bacterial growth, the extract is capable of exerting bactericidal activity under specific conditions, reinforcing its functional versatility against S. aureus.
Determination of the MBC/MIC ratio and classification of the antimicrobial effect of the Streptomyces albus extract against S. aureus strains.
These results contribute to addressing a persistent gap in the current literature, in which many studies report antimicrobial activity of Streptomyces-derived products without systematically integrating inhibitory and bactericidal parameters across multiple strains (Ait Assou and El Hassouni, 2025). The strain-dependent variability observed here aligns with growing evidence that intra-species heterogeneity plays a critical role in antimicrobial response, particularly in clinically relevant pathogens such as S. aureus.
Importantly, the demonstration of both inhibitory and bactericidal effects using a non-fractionated extract highlights the biological relevance of the metabolite mixture produced by S. albus, even prior to compound isolation. Given that Streptomyces species are historically recognized as one of the most prolific sources of clinically relevant antibiotics, these findings reinforce the continued relevance of this genus in the search for new antimicrobial agents in the context of escalating antimicrobial resistance (Souza et al., 2026).
Nevertheless, it is important to emphasize that the use of a crude extract represents an initial step in the characterization of bioactive potential. Further studies are required to isolate and identify the active compounds, as well as to elucidate their mechanisms of action and potential synergistic interactions. Such investigations will be essential to determine the therapeutic applicability of these findings.
Taken together, the results presented here provide consistent evidence of antibacterial activity across multiple S. aureus strains, with both inhibitory and bactericidal effects, supporting the potential of S. albus-derived products as a promising source of antimicrobial agents and warranting further investigation.
4. Discussion
The ability of Streptomyces spp. to produce a wide array of specialized compounds has long attracted considerable attention, largely due to their ecological roles and biotechnological potential. These compounds function as adaptive elements that enhance microbial fitness, enabling survival and competitive dominance in complex and resource-limited environments (Shepherdson et al., 2023; Santamaria et al., 2022). This remarkable biosynthetic capacity underpins the historical and ongoing importance of this genus as a major source of clinically relevant antimicrobial agents.
S. aureus is a leading cause of both community- and healthcare-associated infections worldwide, contributing substantially to global morbidity, mortality, and healthcare burden (Touaitia et al., 2025). Recent estimates indicate that antibiotic-resistant S. aureus, particularly methicillin-resistant S. aureus (MRSA), was responsible for more than 100,000 deaths globally in 2019, placing it among the most critical bacterial pathogens associated with antimicrobial resistance (Murray et al., 2022). In clinical settings, S. aureus is implicated in a wide spectrum of conditions, ranging from skin and soft tissue infections to severe invasive diseases such as bacteremia, endocarditis, osteomyelitis, and pneumonia. In addition to its clinical impact, MRSA infections are associated with significantly increased healthcare costs, prolonged hospital stays, and higher mortality rates compared to infections caused by susceptible strains (Hirabayashi et al., 2024). Despite advances in antimicrobial therapy, resistance rates remain high in both hospital and community settings, with emerging reports of reduced susceptibility even to last-line agents such as vancomycin, further underscoring the urgent need for novel therapeutic strategies (Zha et al., 2021).
The antimicrobial activity observed in the present study aligns with previous reports demonstrating the broad-spectrum potential of Streptomyces-derived compounds (Souza et al., 2026). The production of such bioactive compounds confers a selective advantage by suppressing competing microorganisms, thereby shaping microbial community structure and stability (Chevrette et al., 2019; Silva and Nogueira, 2021). The inhibitory effects detected across all tested S. aureus strains, including those with clinically relevant resistance profiles, are consistent with this ecological and functional framework.
Notably, although several studies have investigated antimicrobial compounds derived from Streptomyces spp., reports specifically evaluating Streptomyces albus against S. aureus, particularly in the context of multidrug-resistant clinical isolates, remain limited. This positions the present findings as a relevant contribution to the field, particularly by providing integrated data on inhibitory and bactericidal activity across multiple strains. Previous studies have largely focused on isolated compounds or on different Streptomyces species, often under distinct experimental conditions, which limits direct comparison but reinforces the importance of expanding investigations involving S. albus.
The results obtained for MIC and MBC further support the antimicrobial potential of the S. albus extract. The extract demonstrated notable activity against clinical isolates IC 2141 and IC 3792, both of which exhibit resistance profiles that include vancomycin, a key antibiotic in the treatment of multidrug-resistant S. aureus. These isolates presented MIC values of 625 µg/mL and exhibited a bactericidal profile (MBC/MIC ratio ≤ 2), indicating a high level of susceptibility under the tested conditions.
The observed activity against strains with reduced susceptibility to conventional antibiotics suggests that the bioactive compounds present in the extract may act through mechanisms that differ from those targeted by commonly used antimicrobials. This hypothesis is supported by the extensive chemical diversity described within the genus Streptomyces, which includes multiple classes of antibiotics with distinct mechanisms of action, such as chloramphenicol (protein synthesis inhibition), tetracyclines (ribosomal inhibition), and glycopeptides (cell wall synthesis inhibition). Many of these compounds, originally derived from Streptomyces spp., exhibit potent activity against S. aureus, including resistant strains, reinforcing the biological plausibility of the activity observed in the present study (Bansal et al., 2021).
Importantly, the demonstration of antibacterial activity using a non-fractionated extract highlights the intrinsic bioactivity of the metabolite mixture produced by S. albus, even in the absence of compound purification. This observation is particularly relevant given that synergistic interactions among microbial compounds present in the extract have been increasingly recognized as contributors to antimicrobial efficacy. At the same time, it is important to acknowledge that crude extracts represent an initial stage in the drug discovery pipeline. Further studies are required to isolate and structurally characterize the active compounds, as well as to elucidate their mechanisms of action and potential synergistic effects.
These findings reinforce the growing body of evidence supporting the genus Streptomyces as a prolific and still underexplored source of bioactive compounds with activity against multidrug-resistant pathogens. By demonstrating consistent inhibitory and bactericidal effects against clinically relevant S. aureus strains, including those with resistance to key antibiotics, the present study contributes to addressing a critical gap in the search for new antimicrobial agents. These results provide a robust foundation for future investigations aimed at translating this bioactivity into therapeutically viable compounds.
5. Conclusion
This study demonstrates that the ethyl acetate extract from the culture supernatant of S. albus exhibits consistent antibacterial activity against S. aureus, including strains with clinically relevant resistance profiles. In addition to growth inhibition, the extract displayed bactericidal activity under specific conditions, highlighting its functional versatility.
The observed activity against strains with reduced susceptibility to key antibiotics, such as vancomycin, underscores the relevance of these findings in the context of antimicrobial resistance. Although the use of a non-fractionated extract limits mechanistic insights, the results provide clear evidence of intrinsic bioactivity.
Together, these findings support the potential of Streptomyces-derived products as sources of antibacterial agents and warrant further investigation to identify active compounds and elucidate their mechanisms of action.
Acknowledgements
We would like to thank CAPES for the financial support provided by the program, which granted me a master's scholarship. Furthermore, we would like to thank CEUMA University and the master’s Program in Biosciences Applied to Health for the infrastructure and institutional support that facilitated the execution of the activities and the dissemination of the results obtained in this research. These resources were essential both for the completion of this work and for the comprehensive dissemination of its findings. To all who contributed in some way, our sincere thanks.
Data Availability Statement
The dataset analyzed or produced in this study can be requested from the corresponding author.
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Editor:
Takako Matsumura Tundisi
