Is Achromobacter xylosoxidans a gram-negative bacterium, and is it susceptible to polymyxin (colistin)?

Achromobacter xylosoxidans and Polymyxin Coverage

Yes, Achromobacter xylosoxidans is a gram-negative bacterium, but polymyxin (colistin) susceptibility is highly variable and cannot be assumed—you must obtain antimicrobial susceptibility testing before relying on polymyxin for treatment.

Microbiological Classification

A. xylosoxidans is definitively a gram-negative, aerobic, non-fermenting bacillus 1, 2, 3, 4. It is a motile organism with peritrichous flagella (distinguishing it from Pseudomonas species) and is commonly found in aquatic environments 1, 5.

Polymyxin Susceptibility: Critical Variability

The susceptibility of A. xylosoxidans to polymyxins is inconsistent and strain-dependent:

Evidence of Susceptibility

• One case report documented an extended drug-resistant A. xylosoxidans isolate that was only sensitive to colistin, successfully treating complicated intra-abdominal infection 1
• This demonstrates that some strains retain colistin susceptibility even when resistant to multiple other agents

Evidence of Resistance

• Recent genomic analysis shows A. xylosoxidans exhibits high levels of antibiotic resistance to multiple drug classes 5
• The organism demonstrates intrinsic resistance mechanisms including multiple β-lactamases (blaOXA-114) and can acquire additional resistance genes 6
• Carbapenem-resistant strains carrying blaNDM-1 have been identified, indicating capacity for extensive resistance 6

Clinical Implications and Management Algorithm

Step 1: Always obtain cultures and susceptibility testing

• A. xylosoxidans is frequently multidrug-resistant 2, 3, 5
• Empirical therapy failure is significant without targeted treatment 2
• Never assume polymyxin coverage without laboratory confirmation

Step 2: Review susceptibility patterns

• Common susceptibilities reported include: meropenem, trimethoprim-sulfamethoxazole 2, ciprofloxacin, and amikacin 7
• Resistance patterns vary dramatically between isolates 5, 8
• Some isolates show resistance to ceftolozane/tazobactam, aztreonam, cefepime, and ciprofloxacin 5

Step 3: Select targeted therapy based on susceptibility results

• If colistin-susceptible: polymyxin can be used, preferably in combination therapy (see below)
• If colistin-resistant: alternative agents based on susceptibility (carbapenems, fluoroquinolones, aminoglycosides, or trimethoprim-sulfamethoxazole)

Polymyxin Use Considerations (If Susceptible)

While the provided guidelines focus on carbapenem-resistant gram-negative bacilli (CRAB, CRE, CRPA) rather than A. xylosoxidans specifically, the principles apply:

• Combination therapy is preferred over monotherapy for serious gram-negative infections requiring polymyxin 9
• Monitor renal function closely during treatment 9
• Avoid concurrent nephrotoxic or ototoxic drugs 9
• Consider therapeutic drug monitoring where available 9

Critical Pitfalls to Avoid

1. Never assume polymyxin coverage without susceptibility data—A. xylosoxidans resistance patterns are unpredictable
2. Do not dismiss A. xylosoxidans as a contaminant—it can cause fatal bacteremia even in immunocompetent patients 2
3. Recognize high-risk populations: immunocompromised patients, those with indwelling catheters, neutropenia, cystic fibrosis, or chronic lung disease 2, 3, 10
4. Be aware of intrahost evolution—isolates can develop increased resistance and altered virulence during treatment 5