General Characteristics and Morphology
Pasteurella species belong to the family Pasteurellaceae. These are classic bacterial zoonotic pathogens affecting wild animals, livestock, and birds. The genus includes approximately twenty species, among which Pasteurella multocida (the main pathogen) and Pasteurella pneumotropica hold key medical and veterinary significance.
Under the microscope, these microorganisms appear as small, spherical, or oval coccobacilli, although they can also take the form of typical rods. In smears, they are arranged singly, in pairs, or sometimes in short chains. The bacteria are Gram-negative and non-motile. Their striking morphological feature is bipolar staining: the poles of the cell stain significantly more intensely than the central part. This feature is particularly noticeable in microscopy of smears prepared directly from infected tissues.
Cultural and Biochemical Properties
The pathogen is undemanding regarding cultivation conditions. The bacteria are facultative anaerobes and grow well on standard nutrient media at a normal temperature of 37 °C.
Biochemical activity is quite distinct:
- They produce oxidase and catalase enzymes (oxidase- and catalase-positive).
- They are capable of fermenting glucose with acid production, but this process occurs without gas formation.
- They reduce nitrates to nitrites.
The differentiation of various species within the genus is based on this set of biochemical properties. The antigenic structure of Pasteurella has not yet been studied comprehensively, but the presence of somatic O-antigen has been established, which is typical for Gram-negative microorganisms.
Epidemiology and Pathogenicity Factors
Animals and birds serve as the natural reservoir of infection. The bacteria permanently inhabit the mucous membranes of their respiratory and digestive tracts. In animals, P. multocida can cause severe diseases: hemorrhagic septicemia (in swine, horses, and cattle), fowl cholera, and various pneumonias.
In humans, these microorganisms can occasionally be part of the normal microbiota, but more often they act as pathogens. Main routes of transmission to humans:
- Contact (wound) — the primary mechanism. Bacteria enter tissues via animal bites, most commonly from cats and dogs.
- Aerogenic — a rare route where infection can develop even without visible animal contact or skin trauma.
Within the host organism, the bacteria act as facultative intracellular parasites (in this trait, they resemble the causative agents of tularemia—Francisella—and plague—Yersinia). The main pathogenicity factor causing intoxication is the endotoxin they release.
Clinical Presentation and Diagnostic Principles
The incubation period is short, lasting from 1 to 5 days. The most typical form of the disease develops after an animal bite. The disease manifests acutely: within the first hours at the site of injury, intense pain, erythema, and edema appear. Shortly thereafter, regional lymphadenopathy and a mild fever develop. In cases of aerogenic transmission or compromised host immunity, rare generalized forms are possible, such as bacteremia and chronic respiratory infections.
The primary method for confirming the diagnosis is bacteriological. Blood, sputum, or wound discharge is used for testing.
- Diagnostic algorithm: inoculation of material onto nutrient media $\rightarrow$ cultivation $\rightarrow$ identification of the pure culture.
- Differential diagnosis: Pasteurella must be strictly differentiated from similar bacteria belonging to the genera Haemophilus, Aggregatibacter, Actinobacillus, Yersinia, and Francisella.
Antibiotic therapy is prescribed for treatment (most commonly tetracyclines). Specific prophylaxis (vaccination) has not been developed for humans, and post-infection immunity remains insufficiently studied to date.