Molecular characterization of plasmid-encoded ACC-1a4 β-lactamase produced by Salmonella strains isolated from water in a Tunisian hospital

A collection of non-typhoidal Salmonella strains, designated MMB1 to MMB18, resistant to third generation cephalosporins were isolated from water in a Tunisian hospital. Isolates harbored a new blaACC gene located on a small transmissible plasmid of 5 kb. The ACC-1a4 enzyme was produced in Escherichia coli, purified and characterized. Compared to the chromosomal blaACC-1a gene, the blaACC-1a4 exhibit three mutations. We determined the kinetic parameters of our enzyme for a representative series of β-lactam substrates. The ACC-1a4 enzyme was active on penicillins and cephalosporins, nitrocefin and cephalothin being its best tested substrates. This is a real cause for alarm since these mutations showed that the DNA sequence of blaACC is an intermediate between those of blaACC-1a and blaACC-4 and might increase the probability of the appearance and spreading of ACC-4-mediated resistance in the hospital environment.