The subject of the present invention are mutant strains of Escherichia coli MG6165 (ATCC nr 47076) lacking all potassium transporters: Kdp, Kup and Trk (?Ktrans), encompassing strains lacking at least one of the genes encoding mechanically gated channels mscS and mscL (?Ktrans ?mscS i/lub ?mscL). In particular, it concerns Escherichia coli MG61655 (ATCC No. 47076) lacking all potassium transporters: Kdp, Kup and Trk (?Ktrans), which additionally are devoid of the mechanically gated channel gene mscS - E. coli (?Ktrans ?mscS), the mechanically gated channel gene mscL - E. coli (?Ktrans ?mscL) and/or both the mechanically gated channel genes mscS and mscL - E. coli (?Ktrans ?mscS ?mscL). The subject of the present invention is also a method of testing chemical compounds for potential antibacterial properties, making use of the abovementioned strains. Another subject of the present invention is a kit for testing chemical compounds as potential antibacterial substances, containing the abovementioned strains.