Screening the cultivable cave microbial mats for the production of antimicrobial compounds and antibiotic resistance

The current work extends the phenotypic characterization of a bacterial culture collection obtained from white, yellow, grey or pink microbial cave wall colonies that are common in the caves of Slovenian Karst. We have determined antibiotic resistance to 22 natural and synthetic antibiotics in 69 isolates from the microbial mats. Thirty-eight isolates (52%) were resistant to 1-5 antibiotics; another 27 isolates (37%) were resistant to 6-10 antibiotics; and 7 isolates (0.1%) were resistant to 11-17 antibiotics. We screened for production of antimicrobial compounds by growing cave isolates on five different media and overlaying individual cultures with ten Gram-positive and Gram negative multidrug resistant isolates. Out of 78 isolates tested, 15.3% showed antimicrobial activity against Escherichia coli DH5, 15.3% against extended-spectrum β-lactamase producing Escherichia coli, 3.8% against Salmonella enterica serovar Typhimurium TL747, 9% against Klebsiella pneumoniae ATCC BAA-1706, 9% against carbapenem-resistant Klebsiella pneumoniae ATCC BAA-1705, 7.7% against Bacillus cereus, 20.5% against Bacillus subtilis, 9% against Listeria monocytogenes, 19.2% against methicillin-resistant Staphylococcus aureus, and 20.5% against methicillin-resistant Staphylococcus pseudointermedius. Results showed the potential of cave microbes to suppress the growth of multi-resistant pathogens, and, a relatively high resistance to antibiotics