The invention relates to targeted post translational modification of metallo-beta-lactamase by truncation and insertion of a dipeptide at the amino terminal end to reduce amino terminal heterogeneity in a recombinant DNA production system. A protein K-T-E-ΔBL is expressed, and modified by host proteases to E-ΔBL. Appropriate nucleotide molecules, vectors and hosts are also described. E-ΔBL is useful in a pharmaceutical composition for treating antibiotic induced adverse effects in the intestine of patients treated with beta-lactam antibiotics.