Co-existence of blaOXA-23 and blaVIM in Carbapenem-Resistant Acinetobacter baumannii Isolates Belonging to Global Complex 2 in a Chinese Teaching Hospital

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Co-existence of blaOXA-23 and blaVIM in Carbapenem-Resistant Acinetobacter baumannii Isolates Belonging to Global Complex 2 in a Chinese Teaching Hospital

Antibiotic resistance is one of the greatest threats to modern medicine—and carbapenem-resistant Acinetobacter baumannii (CRAB) is a leading culprit. This superbug resists last-line carbapenem antibiotics, making infections like pneumonia, bloodstream infections, and wound infections nearly untreatable. A 2019 study from a Chinese teaching hospital reveals a worrying trend: most CRAB isolates there carry two powerful resistance genes, highlighting how these bacteria spread and evade treatment.

The Threat of CRAB

Acinetobacter baumannii thrives in hospitals, where it preys on vulnerable patients—especially those in intensive care units (ICUs). When it becomes resistant to carbapenems (drugs used when all others fail), it’s called CRAB. Studies link CRAB infections to mortality rates as high as 76%—more than double the rate for carbapenem-susceptible strains. In China, CRAB rates have surged over the past two decades, putting millions of hospital patients at risk.

What the Study Investigated

Researchers from the Department of Clinical Laboratory at Xiangya Hospital (Central South University, Changsha, Hunan Province) analyzed 67 CRAB isolates collected from inpatients in 2016. Their goal was to answer three critical questions:

  1. How resistant are these CRAB isolates to common antibiotics?
  2. Which genes make them resistant to carbapenems?
  3. How do these superbugs spread (clonal dissemination)?

To find answers, the team:

  • Tested isolates against 10 antibiotics (including imipenem, tigecycline, and gentamicin) using standard lab methods.
  • Used polymerase chain reaction (PCR) and DNA sequencing to detect resistance genes (e.g., blaOXA-23, blaVIM).
  • Tracked clonal spread with two genetic tools: rep-PCR (DNA fingerprinting) and MLST (genetic typing of “housekeeping” genes to identify related bacteria).

Key Findings: Resistance and Spread

The results paint a stark picture of CRAB’s dominance in the hospital:

1. Nearly Untreatable with Most Antibiotics

All 67 isolates were 100% resistant to imipenem (the gold standard for carbapenem testing). They were also highly resistant to other frontline drugs:

  • 98.5% resistant to piperacillin/tazobactam (a combination antibiotic) and cefepime (a cephalosporin).
  • 86.6% resistant to gentamicin (an aminoglycoside).
  • 83.6% resistant to ciprofloxacin (a fluoroquinolone).

Only tigecycline (a last-resort antibiotic) worked for most: just 2 isolates were resistant, though 40% had “intermediate” resistance—meaning the drug may stop working soon.

2. Two Resistance Genes Drive Superbug Status

The team found three key carbapenemase genes:

  • blaOXA-23: Present in 94% of isolates. This is the most common gene for carbapenem resistance in CRAB worldwide.
  • blaVIM: Present in 80.6% of isolates. This gene produces a metallo-beta-lactamase (MBL), an enzyme that breaks down almost all beta-lactam antibiotics (except monobactams). MBLs are rare in CRAB—but their presence here is a red flag.
  • Co-harboring both genes: Shockingly, 74.6% of isolates carried both blaOXA-23 and blaVIM. This double resistance makes CRAB even harder to treat, as two separate mechanisms break down antibiotics.

3. Transposons Speed Up Resistance Spread

The team identified Tn2008—a “mobile” piece of DNA (transposon)—in 79.1% of isolates. Tn2008 carries the blaOXA-23 gene, allowing it to jump from one bacterium to another. This is a major driver of resistance spread: transposons turn single resistant bacteria into entire colonies of superbugs.

4. A Globally Spread Clone Dominates

Using MLST (multilocus sequence typing), the team found six “sequence types” (STs)—genetic signatures that reveal how related bacteria are. The most common was ST195 (41.8% of isolates), which belongs to clonal complex 208 (CC208) and global complex 2 (GC2). GC2 is a family of CRAB clones that spread worldwide—meaning this strain is part of a larger, international superbug network.

Why This Matters for Hospitals

These findings have three critical implications for infection control:

1. blaOXA-23 + blaVIM = a “Double Threat”

BlaOXA-23 breaks down carbapenems, while blaVIM resists even more drugs. Together, they create a CRAB strain that’s nearly impossible to treat with standard antibiotics. This is especially dangerous for ICU patients, who made up 70% of the study’s cases.

2. Transposons Fuel Rapid Spread

Tn2008’s presence means blaOXA-23 can jump between bacteria—even across species. This speeds up resistance spread, making it harder to contain outbreaks.

3. GC2 Is a Regional and Global Risk

ST195 (a GC2 clone) is dominant in Xiangya Hospital. Since GC2 spreads globally, this suggests the strain could be circulating in other hospitals in Hunan Province. Without action, it could become a regional epidemic.

Limitations and Next Steps

The study has two key gaps:

  1. No plasmid analysis: The team didn’t test which plasmids (small, mobile DNA molecules) carry resistance genes. Plasmids are a major way bacteria share resistance—understanding their role could help stop spread.
  2. Single-hospital focus: The study only included isolates from Xiangya Hospital. More research is needed to see if these trends hold in other Hunan hospitals.

What Hospitals Can Do

To slow CRAB’s spread, the authors recommend three urgent steps:

  1. Rapid testing for resistance genes: Identify isolates with blaOXA-23 and blaVIM early to isolate infected patients.
  2. Strengthen infection control: Improve hand hygiene for staff, disinfect surfaces (CRAB can live on objects for weeks), and limit equipment sharing.
  3. Monitor antibiotic use: Overuse of antibiotics drives resistance. Hospitals should track which drugs are prescribed and limit unnecessary use.

The Big Picture

This study is a warning: CRAB is evolving to be more resistant—and more widespread. For patients, it means a higher risk of deadly infections if they’re admitted to the ICU. For hospitals, it means investing in better testing, cleaner environments, and smarter antibiotic use.

The research was funded by the Hunan Provincial Natural Science Foundation (No. 2017JJ3478) and the National Natural Science Foundation of China (No. 81702068). The original study was published in the Chinese Medical Journal in 2019.

doi: 10.1097/CM9.0000000000000193

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