Lysin LysMK34 of Acinetobacter baumannii bacteriophage PMK34 has a turgor pressure-dependent intrinsic antibacterial activity and reverts colistin resistance.

The prevalence of extensively and pandrug-resistant Acinetobacter baumannii leaves little or no therapeutic options for treatment for this bacterial pathogen. Bacteriophages and their lysins represent attractive alternative antibacterial strategies in this regard. We used the extensively drug-resistant A. baumannii strain MK34 to isolate the bacteriophage PMK34 (vB_AbaP_PMK34). This phage shows fast adsorption and lacks virulence genes, nonetheless its narrow host spectrum based on capsule recognition limits broad application. PMK34 is a Fri1virus member of the Autographiviridae and has a 41.8 kb genome (50 ORFs), encoding an endolysin (LysMK34) with potent muralytic activity (1499.9 ± 131 U/μM), a typical mesophilic thermal stability up to 55 °C and a broad pH activity range (4-10). LysMK34 has an intrinsic antibacterial activity up to 4.8 and 2.4 log units for A. baumannii and Pseudomonas aeruginosa strains, respectively, but only when a high turgor pressure is present. Addition of 0.5 mM EDTA or application of an osmotic shock after treatment can compensate for the lack of a high turgor pressure. The combination of LysMK34 and colistin results in up to 32-fold reduction of the MIC of colistin, and colistin-resistant strains are re-sensitized in both Mueller-Hinton broth and 50 % human serum. As such, LysMK34 may be used to safeguard the applicability of colistin as a last-resort antibiotic. ImportanceA. baumannii is one of the most challenging pathogens for which development of new and effective antimicrobials is urgently needed. Colistin is a last resort antibiotic and even colistin-resistant A. baumannii strains exist. Here, we present a lysin that sensitizes A. baumannii for colistin and can revert colistin resistance to colistin susceptibility. The lysin also shows a strong, turgor pressure dependent intrinsic antibacterial activity, providing new insights in the mode-of-action of lysins with intrinsic activity against Gram-negative bacteria.