Acinetobacter baumannii
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Direct answer
Acinetobacter baumannii is a Gram-negative, oxidase-negative coccobacillus and a non-fermenting hospital pathogen. It survives for weeks on dry surfaces, spreads through intensive care units, and causes ventilator-associated pneumonia and bloodstream infection. Carbapenem resistance, most often from OXA-type carbapenemases, is common and leaves colistin and sulbactam-based regimens as important options.
What you must remember
- Morphology and growth: non-motile Gram-negative coccobacillus, strictly aerobic, catalase positive and non-lactose fermenting on MacConkey agar.
- Environmental persistence: survives on dry surfaces, bedrails, mattresses and equipment for days to weeks, a key reason for ICU outbreaks.
- Carbapenem resistance: OXA-type carbapenemases, especially the OXA-23 group, are the dominant mechanism, often with reduced permeability and efflux.
- Intrinsic activity: sulbactam is intrinsically active against Acinetobacter baumannii, which is why sulbactam-based regimens are used.
- Clinical infections: ventilator-associated pneumonia, central line bloodstream infection, wound infection and catheter-associated urinary infection in critically ill patients.
- Last-resort agents: colistin and polymyxin B are reserved for multidrug-resistant infection, because nephrotoxicity limits their use.
- Indian context: isolates from Indian intensive care units are frequently carbapenem resistant, which makes infection control and stewardship central.
From an ICU isolate to a treatment choice
A ventilated patient on a medical ICU grows an oxidase-negative coccobacillus from endotracheal aspirate. The steps below show how the team confirms the isolate, reads the resistance pattern and limits spread.
- Confirm the isolate: the coccobacillus is identified on MALDI-TOF or an automated system, with growth on MacConkey agar supporting the diagnosis.
- Test the carbapenems: imipenem and meropenem susceptibility is reported, since carbapenem resistance is common in ICU isolates.
- Check last-line agents: colistin MIC and sulbactam susceptibility guide the options when resistance is extensive.
- Look for a cluster: several isolates from patients on the same ward with the same antibiogram signal an outbreak, and the infection control team is informed.
- Decontaminate the environment: sinks, ventilator circuits, bedrails and shared equipment are cleaned with a validated disinfectant.
- Review antibiotic exposure: prior carbapenem and broad-spectrum cephalosporin use is reviewed, because it selects resistant strains.
The same workflow applies on a general ward, but the intensive care unit carries the highest risk because ventilated and catheterised patients are most exposed. Reporting each cluster early lets the team act before the environment becomes a long-term reservoir.
Where the exam tries to catch you
Acinetobacter questions turn on the shape, the growth conditions and the drug of last resort.
- Trap: assuming an oxidase-negative Gram-negative rod must be an Enterobacterales member Correct: Acinetobacter is a strictly aerobic non-fermenting coccobacillus, distinct from the glucose-fermenting enteric group.
- Trap: treating every ICU colonisation as an infection Correct: colonisation with carbapenem-resistant Acinetobacter needs infection control and clinical review rather than automatic antibiotics.
- Trap: choosing colistin as first-line therapy Correct: colistin is a last-resort drug for multidrug-resistant infection, because nephrotoxicity and heteroresistance limit its use.
Frequently asked questions
Why is Acinetobacter baumannii so common in intensive care units?
Acinetobacter baumannii is common in intensive care units because it survives on dry surfaces for weeks and spreads on hands and equipment. Its frequent carbapenem resistance leaves infected patients with fewer effective antibiotic options.
Which enzyme causes carbapenem resistance in Acinetobacter baumannii?
OXA-type carbapenemases, especially the OXA-23 group, are the dominant cause of carbapenem resistance in Acinetobacter baumannii. Reduced membrane permeability and efflux pumps add to the resistance seen in many isolates.
Why is sulbactam used against Acinetobacter baumannii?
Sulbactam is used against Acinetobacter baumannii because it has intrinsic antibacterial activity against this organism, in addition to its beta-lactamase inhibition. Sulbactam-based regimens are therefore an important option when carbapenem resistance is present.
Which drugs are last-resort options for multidrug-resistant Acinetobacter?
Colistin and polymyxin B are last-resort options for multidrug-resistant Acinetobacter baumannii, usually given in combination with other agents. Their use is limited by nephrotoxicity, and the choice is guided by local resistance data and current guidance.
How is an Acinetobacter outbreak in an ICU controlled?
An Acinetobacter outbreak in an ICU is controlled by isolating infected and colonised patients, using dedicated equipment and enforcing hand hygiene. Environmental cleaning with a validated disinfectant and antibiotic stewardship also reduce transmission and the selection of resistance.