Part I: Antibiotic Resistance (AMR) - Global and Local Epidemiology

1.7 Carbapenem-resistant Acinetobacter baumannii (CRAB)

The term Multidrug-resistant Acinetobacter baumannii (MRAB) is commonly used but lacks a standardised and precise definition internationally. [92,97] Resistance to carbapenems, a crucial class of antimicrobials for treating A. baumannii infections, is considered a significant event. [98–100] To enhance clarity and comparability of surveillance data across different centres, it is recommended to use the term CRAB (carbapenem-resistant Acinetobacter baumannii). The global increase in resistant A. baumannii strains is primarily attributed to the spread of strains carrying the Class D OXA type β-lactamase. [101–104] Therefore, for surveillance purposes, CRAB more accurately reflects the current scenario compared to MRAB. In 2024, the World Health Organization designated CRAB as a critical priority pathogen based on specific criteria including virulence, resistance, limited treatment options, and high mortality rates. [105]

Carbapenem resistance in A. baumannii can arise from enzymatic degradation and efflux pumps. Nonetheless, the prevailing increase in resistant A. baumannii strains is primarily attributed to those producing the Class D OXA type β-lactamase. [104,106] Among these enzymes, OXA-23, OXA-24, and OXA-58 are the most prevalent carbapenemases produced by A. baumannii, playing a significant role in global carbapenem resistance within this pathogen. [107]

Metallo-β-lactamases belong to class B β-lactamases and feature at least one zinc ion within their active sites (refer to Table 1.7). These enzymes are highly effective carbapenemases, capable of hydrolysing all β-lactams except for the monobactam aztreonam. [107] However, metallo-β-lactamases are less frequently observed in A. baumannii. The presence of multiple resistance determinants often harboured on integrons contributes to CRAB’s concurrent resistance to other antibiotic classes. [22]

A local survey conducted in 2010 on CRAB revealed that the majority of strains were attributed to the HKU1 and HKU2 clones. [99] OXA-23 was identified in all HKU1 isolates and was associated with a high level of carbapenem resistance. Additionally, OXA-51 was detected in both the HKU1 and HKU2 clones. The colonisation or infection of CRAB in chronic wounds was found to be linked, potentially serving as a reservoir for CRAB. This study underscored CRAB dissemination driven by two new clones. [101]

Carbapenem resistance was found to have a significant impact on the mortality of Acinetobacter bacteraemia [108] which is mainly accounted by the higher rate of discordant antimicrobial therapy. Acinetobacter resistant to carbapenem was also found to have a higher rate of resistance to other classes of antimicrobial agents.

The endemicity of CRAB is increasing in Hong Kong, with CRAB bacteraemia incidence rising from 0.27 per 100,000 patient-days in 2009 to 1.86 per 100,000 patient-days in 2013. The increase in the absolute number of CRAB bacteraemia better reflects the true burden to the healthcare system caused by CRAB. Risk factors include residing in an elderly home and recent use of carbapenems and β-lactam/β-lactamase inhibitor combinations within 90 days before admission. [109]

In a recent study conducted within a 3,200-bed healthcare network, 17,760 faecal specimens from 9,469 patients were screened for CRAB over a 7-month period. The results indicated that 2.6% of patients were carriers of CRAB. Use of fluoroquinolones 6 months before admission was the only significant factor associated with high bacterial load in culture swabs. [110]

An analysis of antibiogram data from private hospitals in Hong Kong revealed that the percentage of meropenem non-susceptibility among Acinetobacter ranged from 12% to 15% between 2014 and 2019. [111]