Classification and Epidemiological Significance
The genus Burkholderia belongs to the family Burkholderiaceae. Originally free-living microorganisms, they widely inhabit soil, water, and plants. They exhibit pronounced polypathogenicity, capable of causing diseases in plants, animals, and humans. From a medical standpoint, three species are of primary interest:
- Burkholderia mallei — the causative agent of glanders, a zooanthroponotic highly dangerous infection. The primary reservoir includes equines (horses, donkeys, mules) and camels. Humans are infected primarily via contact routes when caring for sick animals.
- Burkholderia pseudomallei — the causative agent of melioidosis (false glanders). Endemic to Southeast Asia. Humans are infected when damaged skin or mucous membranes come into contact with contaminated water and soil (e.g., in rice paddies).
- Burkholderia cepacia (a complex of 9 genomovars) — an opportunistic microorganism. It poses a serious problem for inpatient facilities because it survives exceptionally well on moist surfaces of medical equipment and causes severe hospital-acquired pneumonias, peritonitis, and meningitis.
Biological Properties
All representatives of the genus are small Gram-negative rods. Regarding their type of respiration, Burkholderia species are strict aerobes and do not require complex culture media for cultivation. They do not form spores or capsules.
Most of these bacteria are motile due to a polar tuft of flagella (lophotrichous). However, the glanders agent (B. mallei) is an exception—it lacks flagella and is completely non-motile.
A notable microscopic feature of the melioidosis agent (B. pseudomallei) is bipolar staining: the ends of the cell stain significantly more intensely than its center. Additionally, this species has a high temperature optimum and is capable of growing at 42 °C.
Pathogenicity and Virulence Factors
The aggressiveness of Burkholderia is driven by a complex array of molecules and structures:
- Aggression enzymes. Bacteria actively secrete lipases, metalloproteases, and serine proteases. In B. cepacia, these hydrolyze matrix proteins (collagen, fibronectin), damaging alveolar cells. The secretion of these enzymes is tightly controlled by a Quorum sensing system—release occurs only when a critical concentration of microbes is achieved in the tissue.
- Endotoxins. For the glanders agent, a key virulence factor is mallein. This is an endotoxin that affects smooth muscle cells, triggering severe fever and exhaustion (cachexia).
- Adhesion and nutrition structures. Pili and exopolysaccharides allow bacteria to form stable biofilms. To scavenge iron from host tissues, specialized iron-binding proteins—siderophores (salicylic acid, ornibactin, etc.)—are secreted.
Clinical Manifestations and Resistance
The pathogenesis of these infections translates to severe necrotic and septic processes:
- Glanders begins with the formation of a papule at the portal of entry (mucosa, skin), which transforms into a pustule and a deep ulcer. The process generalizes into bacteremia with 100% mortality in the acute form if left untreated.
- Melioidosis proceeds as an acute septicopyemia with the formation of multiple abscesses in internal organs.
- The B. cepacia complex provokes the development of "Cepacia syndrome" — rapidly progressive necrotizing pneumonia.
Therapy is extremely difficult due to antibiotic resistance. B. cepacia produces an inducible $\beta$-lactamase and surpasses even Pseudomonas aeruginosa in resistance. In the environment, the bacteria behave differently: while the glanders agent rapidly dies upon drying, B. pseudomallei survives for months even in dry secretions.