Cell wall recycling-linked coregulation of AmpC and PenB β-lactamases through ampD mutations in Burkholderia cenocepacia

In many Gram-negative pathogens, mutations in the key cell wall-recycling enzyme AmpD (N-acetyl-anhydromuramyl-L-alanine amidase) affect the activity of the regulator AmpR, which leads to the expression of AmpC β-lactamase, conferring resistance to expanded-spectrum cephalosporin antibiotics. Burkholderia cepacia complex (Bcc) species also have these Amp homologs; however, the regulatory circuitry and the nature of causal ampD mutations remain to be explored. A total of 92 ampD mutants were obtained, representing four types of mutations: single nucleotide substitution (causing an amino acid substitution or antitermination of the enzyme), duplication, deletion, and IS element insertion. Duplication, which can go through reversion, was the most frequent type. Intriguingly, mutations in ampD led to the induction of two β-lactamases, AmpC and PenB. Coregulation of AmpC and PenB in B. cenocepacia, and likely also in many Bcc species with the same gene organization, poses a serious threat to human health. This resistance mechanism is of evolutionary optimization in that ampD is highly prone to mutations allowing rapid response to antibiotic challenge, and many of the mutations are reversible in order to resume cell wall recycling when the antibiotic challenge is relieved.