Order - Pasteurellales

The order Pasteurellales is a group within the class Gammaproteobacteria, known for its significant impact on both human and veterinary medicine. This order comprises several families, with Pasteurellaceae being the most prominent. Members of this order are typically Gram-negative, non-motile, and facultatively anaerobic bacteria. They are often associated with various diseases in animals and humans, making them a critical focus of microbiological and medical research.

Taxonomy and Classification

The order Pasteurellales includes several genera, with the most notable being Pasteurella, Haemophilus, and Actinobacillus. These genera are characterized by their small, rod-shaped or coccobacillary forms. The taxonomy of Pasteurellales is as follows:

• Domain: Bacteria
• Phylum: Proteobacteria
• Class: Gammaproteobacteria
• Order: Pasteurellales

Key Genera

1. Pasteurella: Known for causing diseases such as pasteurellosis in animals and humans. Pasteurella multocida is a well-known species within this genus.
2. Haemophilus: Includes species like Haemophilus influenzae, which can cause respiratory infections and meningitis in humans.
3. Actinobacillus: Contains species such as Actinobacillus pleuropneumoniae, a pathogen in pigs.

Morphology and Physiology

Members of the Pasteurellales order are generally small, ranging from 0.3 to 1.0 micrometers in width and 1.0 to 3.0 micrometers in length. They exhibit a variety of shapes, including rods and coccobacilli. These bacteria are Gram-negative, meaning they do not retain the crystal violet stain used in the Gram staining procedure. Instead, they appear pink due to the counterstain, typically safranin.

Growth Conditions

Pasteurellales bacteria are facultatively anaerobic, meaning they can grow in both the presence and absence of oxygen. They often require enriched media for optimal growth, such as blood or chocolate agar, which provide necessary growth factors like hemin and NAD (nicotinamide adenine dinucleotide).

Pathogenicity and Clinical Significance

The pathogenic members of the Pasteurellales order are of great concern in both human and veterinary medicine. They are responsible for a range of diseases, often characterized by respiratory infections, septicemia, and localized infections.

Human Infections

• Haemophilus influenzae: This species can cause a variety of infections, including otitis media, sinusitis, bronchitis, and invasive diseases like meningitis and septicemia, particularly in children.
• Pasteurella multocida: Commonly associated with animal bites and scratches, leading to wound infections, cellulitis, and in severe cases, septicemia.

Animal Infections

• Pasteurella multocida: Causes diseases such as fowl cholera in poultry, hemorrhagic septicemia in cattle and buffalo, and atrophic rhinitis in pigs.
• Actinobacillus pleuropneumoniae: A significant pathogen in pigs, causing pleuropneumonia, which can lead to severe economic losses in the swine industry.

Diagnosis and Treatment

Diagnosing infections caused by Pasteurellales involves isolating and identifying the bacteria from clinical specimens. This is typically done using culture techniques on enriched media, followed by biochemical tests and molecular methods such as PCR (polymerase chain reaction).

Treatment

The treatment of infections caused by Pasteurellales often involves the use of antibiotics. However, antibiotic resistance is a growing concern, necessitating the use of susceptibility testing to guide therapy. Commonly used antibiotics include:

• Beta-lactams: Such as penicillins and cephalosporins.
• Macrolides: Such as erythromycin and azithromycin.
• Tetracyclines: Such as doxycycline.

Prevention and Control

Preventing infections caused by Pasteurellales involves a combination of good hygiene practices, vaccination, and proper wound care. Vaccines are available for some animal diseases, such as those caused by Pasteurella multocida and Actinobacillus pleuropneumoniae. In humans, vaccination against Haemophilus influenzae type b (Hib) has significantly reduced the incidence of invasive diseases caused by this pathogen.