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Microbiology · Gram-negative aerobic rods (Pertussis, Brucella, Tularemia, Legionella, Coxiella/Q fever, Bartonella, Pseudomonas, Burkholderia, Acinetobacter, etc.)

Acinetobacter

Acinetobacter

Acinetobacter species are gram-negative bacteria that rank second among the causative agents of nosocomial infections. They are widespread in the environment and pose a serious threat to immunocompromised patients, causing wound infections, respiratory tract infections, and severe systemic complications.

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MorphologyShort gram-negative rods prone to polymorphism
Main threatNosocomial infections (especially A. baumannii)
RespirationStrict aerobes
TreatmentSensitivity to neomycin and polymyxin has been noted
4 min readFor medical studentsUpdated Sep 25, 2026
Material prepared by WhiteCoat Pro editorial team based on platform study guidesReviewed by medical editor: Aulov A. A., general practitioner · 09/24/2026

Morphology and cultural characteristics

The genus Acinetobacter is represented by gram-negative bacteria. Externally, they appear as short, rather thick rods ranging from 1.5 to 2.5 µm in length. The pathogen is characterized by marked polymorphism: coccoid or ovoid forms are often found in smears. The cells are arranged chaotically, in pairs, or form short chains.

The bacteria do not form spores and lack flagella, making them non-motile microorganisms. However, they possess fimbriae and, under certain conditions, are capable of forming a protective capsule.

Acinetobacter species are strict aerobes and have undemanding cultivation requirements. They multiply excellently on standard nutrient media at 30–35 °C and a neutral pH (around 7.0). Colonies grow small and shiny. When plated on blood agar, some strains may demonstrate a zone of $\beta$-hemolysis.

The biochemical activity of these bacteria is weakly expressed:

  • They do not split complex polysaccharides.
  • They do not produce indole or hydrogen sulfide, and are lysine-negative.
  • Only some species are capable of fermenting simple monosaccharides with acid production.

Pathogenicity and epidemiology

In nature, Acinetobacter species are ubiquitous, most commonly isolated from soil and water. They can also act as transient human microflora: the bacteria are frequently detected on the skin and nasopharyngeal mucosa of completely healthy individuals.

The pathogenic potential of these microorganisms is realized through several key factors:

  1. Capsule — possesses anti-phagocytic activity, protecting the microbe from immune system attacks.
  2. Adhesins — specialized structures that help bacteria firmly attach to epithelial cells.
  3. Lipopolysaccharide (LPS) of the cell wall — acts as a potent toxic factor when the bacterium is destroyed.

In clinical practice, the most significant species are A. baumannii (the primary pathogen), A. lwoffii, and A. haemolyticus, while A. calcoaceticus is considered the type species. The highest risk group for infection comprises infants, middle-aged individuals, and patients with severe immunodeficiencies.

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Clinical manifestations and diagnostics

Nosocomial (hospital-acquired) infections caused by Acinetobacter represent a major medical challenge. Within the structure of nosocomial infections, they confidently occupy second place, second only to pseudomonads.

Main localizations of the infectious process:

  • Infection of wound surfaces and deep burns.
  • Inflammatory lesions of the skin, urogenital, and respiratory tracts.
  • When the process becomes generalized, severe conditions develop: sepsis, endocarditis, and peritonitis.

To identify the pathogen, blood, pus, or discharge from infected wounds is sampled for analysis. Laboratory diagnostics is carried out in several stages:

  1. Microscopic method (evaluation of morphology and Gram stain).
  2. Bacteriological method (isolation of a pure culture on nutrient media).
  3. Identification of the isolated strain by its biochemical properties.

No specific vaccines have been developed to date for the prevention of infection. For treatment, susceptibility of the pathogens to antibacterial drugs such as neomycin and polymyxin has been noted.

Briefly: answers to topical questions

What modern antibiotic groups are used to treat Acinetobacter baumannii infections?

Carbapenems, fluoroquinolones, as well as polymyxin and neomycin are used to treat infections caused by Acinetobacter. Carbapenems are the drugs of choice for empirical therapy of severe infections.

  • Carbapenems — imipenem and meropenem (meropenem activity against Acinetobacter is 8–16 times higher).
  • Fluoroquinolones — used in therapy, although the problem of fluoroquinolone-resistant strains exists.
  • Polymyxin and neomycin — susceptibility of the pathogen to these drugs has been reported.

When multidrug-resistant, carbapenem-resistant, or pandrug-resistant Acinetobacter spp. strains are isolated, alternative drugs or a combination of 2–3 antibiotics at the maximum possible doses are prescribed to overcome resistance mechanisms.

What biological material is collected for testing when Acinetobacter infection is suspected?

When Acinetobacter infection is suspected, blood, pus, and wound discharge are collected for testing. Clinical specimen collection for microbiological testing should be performed prior to the initiation of antibacterial therapy.

How to memorize

To remember their biochemical inertness, imagine that Acinetobacter species are "lazy" microbes. They do not want to break down polysaccharides and produce neither indole, nor hydrogen sulfide, nor lysine.

Class and exam questions

Which Acinetobacter species is the most clinically significant?

The species of greatest clinical significance is A. baumannii, which is the leading causative agent of nosocomial infections in this group.

Are these bacteria motile?

No, Acinetobacter species are non-motile because they lack flagella. However, they possess fimbriae for tissue adhesion.

Are there vaccines against Acinetobacter?

No, specific prophylaxis (vaccination) against these pathogens has not been developed at this time.

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What else to study on this topic

  • Mechanisms of adhesion to epithelial cells via fimbriae
  • Features of β-hemolysis of Acinetobacter on blood agar
  • Differences in biochemical activity between A. baumannii and A. lwoffii species
  • Differential diagnosis with Pseudomonas aeruginosa (pseudomonads)

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