Aim: This study investigated the molecular mechanisms of resistance to carbapenems and cephalosporins in 24 consecutive, multidrug-resistant Acinetobacter baumannii (MDRAB) isolates collected between January and April 2015 by a private sector laboratory in Durban, South Africa.
Results: All isolates were resistant to all carbapenems tested. blaOXA-23 and blaOXA-51 genes were found in 23 isolates, while blaOXA-24, blaOXA-48, and blaOXA-58 were absent in all isolates. The most prevalent extended-spectrum β-lactamase was TEM-116 (92%). blaADC was present in 83.3% of isolates, of which two were new variants with three and five amino acid differences compared to Acinetobacter-derived cephalosporinase (ADC)-1, the first at positions 64E → K, 341N → T, and 342R → G and the second at positions 24G → D, 167S → P, 283R → F, 341N → T, and 342R → G, respectively. All isolates were negative for blaPER, blaCMY, blaGES, blaKPC, blaCTX-M, and blaSHV. Metallo-β-lactamase IMP and VIM were absent in all isolates, and NDM-1 was present in 1 isolate. ISAba1 was located upstream blaOXA-23 in all isolates and upstream blaADC (30, 78, 79, 87 and the ADC variants) in 54.2% of the ADC-carrying isolates. None of the isolates had ISAba1 inserted upstream blaOXA-51 gene. Four isolates were clonally related and showed two clusters (A and B), while 20 isolates remained unclustered. There was no direct relationship between the clusters and the hospitals they were isolated from.
Conclusions: This study reports the first NDM-1-producing carbapenem resistant Acinetobacter baumannii isolate in South Africa and highlights the presence of OXA-23, the known ADCs (ADC-30, ADC-78, ADC-79, and ADC-87), and two new ADC variants associated with ISAba1 from the private health sector in Durban, South Africa. The complexity and diversity of MDRAB severely limit treatment options.
Keywords: ADC; Acinetobacter baumannii; ISAba1.