| Summary: | Bacteria that thrive in extreme conditions and the bacteriophages that infect them are sources of valuable enzymes resistant to denaturation at high temperatures. Many of these heat-stable proteins are useful for biotechnological applications; nevertheless, none have been utilized as antibacterial agents. Here, we demonstrate the bactericidal potential of Ts2631 endolysin from the extremophilic bacteriophage vB_Tsc2631, which infects <i>Thermus scotoductus</i>, against the alarming multidrug-resistant clinical strains of <i>Acinetobacter baumannii</i>, <i>Pseudomonas aeruginosa</i> and pathogens from the Enterobacteriaceae family. A 2−3.7 log reduction in the bacterial load was observed in antibacterial tests against <i>A. baumannii</i> and <i>P. aeruginosa</i> after 1.5 h. The Ts2631 activity was further enhanced by ethylenediaminetetraacetic acid (EDTA), a metal ion chelator (4.2 log reduction in carbapenem-resistant <i>A. baumannii</i>) and, to a lesser extent, by malic acid and citric acid (2.9 and 3.3 log reductions, respectively). The EDTA/Ts2631 combination reduced all pathogens of the Enterobacteriaceae family, particularly multidrug-resistant <i>Citrobacter braakii</i>, to levels below the detection limit (>6 log); these results indicate that Ts2631 endolysin could be useful to combat Gram-negative pathogens. The investigation of <i>A. baumannii</i> cells treated with Ts2631 endolysin variants under transmission electron and fluorescence microscopy demonstrates that the intrinsic antibacterial activity of Ts2631 endolysin is dependent on the presence of its N-terminal tail.
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