ARS | Publication request: Moraxella

Submitted to: Veterinary Microbiology
Publication Type: Book / Chapter
Publication Acceptance Date: September 10, 2012
Publication Date: N/A

Technical Abstract: Moraxellae are short, plump gram-negative rods, 11.5 µm X 1.5 to 2.5 mm, and are often arranged in pairs (“diplo¬ bacilli”) or short chains. The cell wall is typical of gram-negative bacteria being com¬ posed of lipopolysaccharide and protein. The lipo¬poly¬saccharide of Moraxellae does not contain O-repeat units, in contrast to many other gram-negative microorganisms (e.g., members of the family Entero¬ bac¬ teriaceae).The fimbrial adhesins (pili) of M. bovis are virulence determinants and can be lost in subculture (see below, “Variability”). Capsules may be present on fresh isolates. The role of adhesins, as in other microorganisms, is to allow the bacterium expressing them to adhere to cells lining a particular niche, as well as to the surface of so-called “target” cells prior to the initiation of disease (in some cases, niche and target cells may be the same). Moraxella bovis produces a type 4 pilus (fimbria) that adheres to conjunctival and corneal epithelial cells. This pilus is similar to those of Pseudomonas aeruginosa, Neis¬seria gonorrhoeae, Dicholobacter nodosus, Pasteurella multocida, and Vibrio cholerae. Mutants unable to produce this adhesin are avirulent. The capsule plays many roles, the most important of which are interference with phagocytosis (antiphagocytic), and protection of the outer membrane from the deposition of membrane attack complexes generated by activation of the complement system. The cell wall of the members of this genus is one typical of gram-negative bacteria (except for the absence of the O-repeat unit). The lipopolysaccharide (LPS) in the outer membrane is an important virulence determinant. LPS binds to lipopolysaccharide-binding protein (a serum protein), which transfers it to the blood-phase of CD14. The CD14-LPS complex binds to Toll-like receptor proteins on the surface of ma¬ cro¬ phage cells triggering the release of proinflammatory ¬ cytokines. The most noteworthy toxin produced by M. bovis, is an RTX (repeats in toxin, so called because of the common feature of repeats in glycine-rich sequences within the protein) type of cytotoxin. This ¬ cytotoxin, sometimes referred to as “hemolysin” due to its behavior on blood agar plates, has been termed Mbx (for Moraxella bovis toxin). Mbx is a pore-forming toxin with specificity for conjunctival and corneal epithelial cells, and neutrophils.