An investigation into the effect of modified β-lysines on the growth rates of eubacterial cells is reported. It is shown that the effects observed are due to the post translational modification of Elongation Factor P (EFP) with these compounds catalysed by PoxA. PoxA was found to be remarkably promiscuous, which allowed the activity of a wide range of exogenous β-lysines to be examined. Two chain-elongated β-lysine derivatives which differ in aminoalkyl chain length by only 2 carbon units exhibited opposing biological activities - one promoting growth and the other retarding it. Both compounds were shown to operate through modification of EFP.