The present study included MDR-P. aeruginosa isolates from a national diagnostic microbiology laboratory in Qatar and are therefore representative of the whole country. Notably, MDR-P. aeruginosa in Qatar are highly resistant to b-lactam agents. The most active b-lactam antibiotics in this study were those in combination with b-lactamase inhibitors, ceftazidime-avibactam and ceftolozane-tazobactam, were not available for clinical use at the time of the study. Yet, less than half of the isolates were susceptible. Given their recent availability for patients in Qatar, the results reported demonstrate the importance of their appropriate clinical use to minimize further loss of activity .
This report included 20 (26.67%) isolates that possessed 21 MBL-encoding genes (16 blaVIM-2, 2 blaVIM-5, and 3 blaIMP-2) (Table S1 and Table 2, Additional File 1). This is consistent with the known predominance of Verona integron-encoded metallo-β-lactamases (VIM), and to a lesser extent imipenemases (IMP), in P. aeruginosa from the Middle East [6-8]. Unlike other geographic settings, New Delhi metallo-b-lactamases (NDM) have not been detected in P. aeruginosa from the Arabian Peninsula [7, 9].
Apart from areas with a high prevalence of MBL in P. aeruginosa, the presence of Class A ESBL b-lactamases can result in resistance to ceftolozane-tazobactam . Avibactam is an efficient inhibitor of Class A b-lactamases and hence ceftazidime-avibactam combination retains its activity in this situation but not ceftolozane-tazobactam [10, 11]. In a report from Spain of 24 extremely-drug resistant ST235 P. aeruginosa isolates, 13% were susceptible to ceftolozane-tazobactam and 58% to ceftazidime-avibactam and the predominant b-lactamases identified were VIM-2 (42%) and the Class A ESBL Guiana-Extended-Spectrum (GES)-5 (46%) . Consistent with this, five out of seven ceftolozane-tazobactam-resistant, ceftazidime-avibactam-susceptible MDR-P. aeruginosa isolates in our study possessed class A blaSHV-11 and ESBL-encoding genes such as blaVEB-9 and blaTEM-116. Interestingly, those 7 isolates belonged to seven different STs (Table S1, Additional File 1).
The b-lactamase blaVEB-9 (19, 25.33%), formerly known as blaVEB-1a, was the most frequent ESBL gene identified in the present study . blaVEB-1 is one of the most frequently reported ESBLs in P. aeruginosa from the Middle East including Kuwait, Saudi Arabia and Iran [13-15]. Though blaVEB-9 was reported from Thailand and Eastern Europe, to the best of our knowledge, it has not been previously reported from the Middle East [11, 16]. In this study, MDR-P. aeruginosa producing Vietnamese extended-spectrum beta-lactamase-9 (VEB-9) belonged to ST235 (8/16), ST357 (7/8), ST308 (1/3) and ST3022 (1/1) (Table S1 and Table S2, Additional File 1). This pattern suggests dissemination within specific P. aeruginosa STs in Qatar that may be different from neighboring countries.
An interesting observation in this study was that 16 (21.33%) MDR-P. aeruginosa isolates were susceptible to aztreonam but resistant to several other antipseudomonal b-lactams tested (Table S1, Additional File 1). Aztreonam is a weak inducer of Class C enzymes and is not a substrate for Class B and narrow-spectrum Class D b-lactamases . The retained aztreonam activity in these isolates despite resistance to other antipseudomonal b-lactams may be explained by the absence of Class A ESBL in those isolates. Therefore, aztreonam should be included in routine antimicrobial susceptibility testing of clinical P. aeruginosa isolates.
Most MDR-P. aeruginosa isolates included in this study belonged to five STs and had consistent b-lactamase genetic profiles and b-lactam susceptibility patterns (Table S2, Additional File 1). ST235, ST233, and ST357 are already known as high-risk clones in Qatar, Saudi Arabia, Bahrain, and the United Arab Emirates . These three STs are globally disseminated MDR-P. aeruginosa clones . Often, these strains cause regional or nationwide outbreaks, express MDR phenotypes, and are associated with high mortality [12, 18, 19]. VIM-producing ST1284 P. aeruginosa have been described from Brazil, and ST389 from cystic fibrosis patients in Italy [20, 21]. Both sequence types have otherwise limited geographic distribution.