The emergence of multidrug-resistant pathogens creates public health challenges, prompting a continuous search for effective novel antimicrobials. This study aimed to isolate marine actinomycetes from South Africa, evaluate their in vitro antimicrobial activity against Listeria monocytogenes and Shiga toxin-producing Escherichia coli, and characterize their mechanisms of action. Marine actinomycetes were isolated and identified by 16S rRNA sequencing. Gas chromatography–mass spectrometry (GC–MS) was used to identify the chemical constituents of bioactive actinomycetes’ secondary metabolites. Antibacterial activity of the secondary metabolites was assessed by the broth microdilution method, and their mode of actions were predicted using computational docking. While five strains showed antibacterial activity during primary screening, only Nocardiopsis dassonvillei strain SOD(B)ST2SA2 exhibited activity during secondary screening for antibacterial activity. GC–MS identified five major bioactive compounds: 1-octadecene, diethyl phthalate, pentadecanoic acid, 6-octadecenoic acid, and trifluoroacetoxy hexadecane. SOD(B)ST2SA2′s extract demonstrated minimum inhibitory concentration and minimum bactericidal concentration, ranging from 0.78–25 mg/mL and 3.13 to > 25 mg/mL, respectively. Diethyl phthalate displayed the lowest bacterial protein-binding energies (kcal/mol): −7.2, dihydrofolate reductase; −6.0, DNA gyrase B; and −5.8, D-alanine:D-alanine ligase. Thus, marine N. dassonvillei SOD(B)ST2SA2 is a potentially good source of antibacterial compounds that can be used to control STEC and Listeria monocytogenes.

Marine actinomycetes are promising sources of antibacterial compounds that may alleviate antimicrobial resistance. This study aimed to evaluate the antibacterial potential of marine actinomycetes secondary metabolites against staphylococci and to characterize their mechanism of action. The actinomycetes were obtained from Salt Rock Beach, South Africa using actinomycetes isolation agar medium and identified by 16S rRNA sequencing. Antibacterial efficacy was evaluated using broth microdilution method. The effect of the secondary metabolites on the respiratory chain dehydrogenase activity and membrane integrity of staphylococci was analyzed by iodonitrotetrazolium chloride and 260 absorbing materials assays, respectively. Liquid chromatography-mass spectrometry analysis was done to identify the chemical constituents of the bacterial extract. Streptomyces SRST1SA2 was the only isolate whose secondary metabolites revealed anti-Staphylococcus activity. The extract from Streptomyces SRST1SA2 showed minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) values ranging from 0.25 to 1 mg/ml and 0.5–1 mg/ml, respectively. The extract inhibited bacterial growth by interfering with the activity of respiratory chain dehydrogenase and membrane integrity. The LC–MS spectrum revealed antibacterial compounds such as myristic acid (C₁₄H₂₈O₂), palmitic acid (C₁₆H₃₂O₂), and oleic acid (C₁₈H₃₄O₂). Streptomyces sp. SRST1SA2 has the potential to serve as a source of antibacterial compounds against staphylococci.

The emergence of multidrug-resistant pathogens such as Listeria monocytogenes and Shiga toxin-producing Escherichia coli (STEC) poses public health challenges, and this has led to a continuous search for effective novel antimicrobial agents. This study aimed to isolate marine actinomycetes from South Africa, evaluate their in vitro antimicrobial activity against Listeria and STEC, and characterize their mechanisms of action. Marine actinoymcetes were isolated and identified by 16S rRNA sequencing. Gas chromatography-mass spectrometry (GC-MS) was used to identify the chemical constituents of bioactive actinomycetes secondary metabolites. Antibacterial activity of the secondary metabolites was assessed by the broth microdilution method and their mode of actions were predicted using computational docking. While five strains showed antibacterial activity during primary screening, only Nocardiopsis dassonvillei strain SOD(B)ST2SA2 exhibited activity during secondary screening for antibacterial activity. GC-MS identified five major bioactive compounds: 1-octadecene, diethyl phthalate, pentadecanoic acid, 6-octadecenoic acid, and tri-fluoroacetoxy hexadecane. SOD(B)ST2SA2`s extract demonstrated the minimum inhibitory con-centration and minimum bactericidal concentration ranging from 0.78–25 mg/ml and 3.13 to > 25 mg/ml, respectively. Diethyl phthalate displayed the lowest bacterial protein binding energies (kcal/mol): −7.2, dihydrofolate reductase; −6.0, DNA gyrase B, and −5.8, D-alanine:D-alanine lig-ase. Thus, marine N. dassonvillei SOD(B)ST2SA2 is a potentially good source of antibacterial compounds that can be used to control STEC and Listeria monocytogenes.