The Pasteurellaceae family represents a group of highly significant bacteria in both human and veterinary medicine. This family includes several key genera: Mannheimia, Pasteurella, Avibacterium, and Bibersteinia. While many of these organisms are normal inhabitants of the upper respiratory and gastrointestinal tracts, they are notorious opportunistic pathogens. They often remain dormant until host factors—such as stress, viral infections, or environmental changes—allow them to invade deeper tissues and cause systemic or localized disease.
Morphological Characteristics
To identify members of the Pasteurellaceae family in a laboratory setting, microbiologists look for a specific set of physical traits. These organisms are classified as Gram-negative coccobacilli. This means they appear as small, oval-shaped rods that do not retain the crystal violet stain during the Gram staining process.
A defining feature of this family is their bipolar staining pattern. When specific staining techniques such as Wright’s, Giemsa, or Leishman stains are applied, the bacteria exhibit a characteristic appearance where only the tips of the cells appear stained, while the center remains lighter. This "safety-pin" appearance is a vital diagnostic clue for differentiating them from other Gram-negative bacteria.
Cultural and Growth Requirements
The metabolic profile of Pasteurellaceae is relatively consistent across the family. They are facultative anaerobes, meaning they possess the metabolic flexibility to grow in both aerobic (oxygen-rich) and anaerobic (oxygen-poor) environments.
Biochemically, they are identified by two primary reactions:
- Oxidase positive: They produce the enzyme cytochrome c oxidase.
- Nitrate reduction: They possess the ability to reduce nitrates to nitrites.
Growth in a culture medium is highly dependent on nutritional supplementation. Most species grow well on standard blood agar. However, a critical distinction must be made for V-factor-dependent species, such as Avibacterium paragallinarum. These organisms require Nicotinamide Adenine Dinucleotide (NAD), also known as V-factor, to grow. In the lab, this is often achieved by using chocolate agar or by employing a "satellite" growth method where a staphylococcal "nurse colony" is provided to secrete the necessary V-factor into the surrounding medium.
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