Carbapenems are potent broad-spectrum β-lactam antibiotics reserved for the treatment of serious infections caused by multidrug-resistant Gram-negative bacteria such as Pseudomonas aeruginosa. The surge in P. aeruginosa resistant to carbapenems is an urgent threat as very few treatment options remain. Resistance to carbapenems is predominantly due to the presence of carbapenemases that deactivate the antibiotic.
Carbapenem resistant isolates of P. aeruginosa (32) were screened for carbapenem resistant genes using PCR. Carbapenemase encoding genes were detected in two isolates, the New-Delhi metallo beta-lactamase (blaNDM-1) and the Adelaide imipenemase (blaAIM-1) were detected in a clinical and a wastewater isolate, respectively. The sensitivity profile revealed that AIM-1 conferred much higher (>128 fold increased) levels of resistance to carbapenems when compared to NDM-1.
A further investigation using wastewater samples from various local healthcare and non-healthcare sources as well as water from a river, using probe-based qPCR revealed the presence of the blaAIM-1 gene in all the samples analysed. The widespread occurrence of blaAIM-1 throughout Adelaide hinted at a potential more widespread occurrence of this gene than originally thought. The possibility of global distributing of the blaAIM-1 gene was investigated. A BLAST search revealed the presence of the blaAIM-1 gene in Asia, America, and Europe. To elucidate the identity of the organism(s) carrying the gene and to assess the genomic arrangement of the blaAIM-1 gene, shotgun metagenomic sequencing was conducted on two healthcare wastewater samples and one non-healthcare wastewater sample to uncover the blaAIM-1 gene and surrounding features. Comparison of these nucleotide sequences with that from P. aeruginosa isolates revealed that, unlike the genetic environment and arrangement in P. aeruginosa, the blaAIM-1 gene was not carried as part of a transposon, or on any other mobile genetic elements. A phylogenetic tree assembled with the deduced amino acid sequences of AIM-1 suggested that the potential origin of the blaAIM-1 gene in P. aeruginosa may be the non-pathogenic ubiquitous environmental organism, Pseudoxanthomonas mexicana.