Morphology and Culture Characteristics
The genus Acinetobacter is represented by Gram-negative bacteria. Structurally, they appear as short, relatively thick bacilli measuring 1.5 to 2.5 µm in length. Pronounced polymorphism is characteristic: smears frequently reveal coccoid or ovoid forms. Cells are arranged randomly, in pairs, or in short chains.
These bacteria do not form spores and lack flagella, rendering them non-motile. However, they possess fimbriae and, under certain conditions, can form a protective capsule.
Acinetobacter species are strict aerobes and have minimal nutritional requirements. They grow well on standard nutrient media at 30–35 °C with a neutral pH (approximately 7.0). Colonies appear small and glistening. When cultured on blood agar, some strains may demonstrate a zone of $\beta$-hemolysis.
Their biochemical activity is relatively weak:
- They do not split complex polysaccharides.
- They do not produce indole or hydrogen sulfide, and are lysine-negative.
- Only a few species are capable of fermenting simple monosaccharides with acid production.
Pathogenicity and Epidemiology
In nature, Acinetobacter species are ubiquitous, frequently isolated from soil and water. They can also act as part of the human transient microflora, often found on the skin and nasopharyngeal mucosa of completely healthy individuals.
The pathogenic potential of these microorganisms is mediated by several key virulence factors:
- Capsule — provides anti-phagocytic activity, protecting the microbe from immune system attacks.
- Adhesins — specialized structures that help bacteria firmly attach to epithelial cells.
- Lipopolysaccharide (LPS) of the cell wall — acts as a potent toxic factor upon bacterial lysis.
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 groups for infection include infants, middle-aged adults, and patients with severe immunodeficiencies.
Clinical Manifestations and Diagnostics
Nosocomial infections caused by Acinetobacter species present a major medical challenge. They securely hold second place in the structure of hospital-acquired infections, second only to Pseudomonas.
Main localizations of the infectious process:
- Infection of wound surfaces and deep burns.
- Inflammatory lesions of the skin, urogenital, and respiratory tracts.
- Generalized processes lead to critical conditions: sepsis, endocarditis, and peritonitis.
To identify the pathogen, blood, pus, or discharge from infected wounds is collected. Laboratory diagnostics is performed in several stages:
- Microscopy (evaluation of morphology and Gram staining).
- Culture method (isolation of a pure culture on nutrient media).
- Identification of the isolated strain based on its biochemical properties.
Specific vaccines for infection prophylaxis have not yet been developed. In treatment, susceptibility of the pathogens to antibacterial agents such as neomycin and polymyxin has been observed.