GRAM-NEGATIVE AEROBIC OR CAPNOPHILIC RODS AND COCCI

A General Introduction has been added to each disease chapter in an attempt to give a brief updated overview of the taxonomic, biological and other characteristics of the virus family or group of bacteria /protozoa that cause disease in livestock and, where relevant, involve wildlife. As the text of the three-volume book Infectious Diseases of Livestock is currently under revision the Editors are aware that there are inconsistencies between the updated introductions to chapters and the content of the chapters themselves. Once the chapters have been updated – a process that is currently underway – these inconsistencies will be removed.

Nearly all Gram-negative bacteria fall in the proposed taxon class Proteobacteria^{1, 5, 8} which is divided into five groups. Most of the bacteria of veterinary importance are included in the alpha, beta and gamma groups of Proteobacteria, and the bacteria discussed under this heading are found in all three groups. The alpha group contains Bartonella, Brucella, Ehrlichia and Cowdria; the beta group Bordetella, Burkholderia, Neisseria and Taylorella, and the gamma group Pseudomonas, Vibrio, Pasteurella and Enterobacteriaceae,⁵ among others.

Before the advent of modern molecular methods of bacterial taxonomy, bacteria were classified according to easily recognizable characteristics, such as shape, Gram staining and oxygen requirement. Modern polyphasic methods of taxonomy, using chemical analysis of cell constituents and the determination of 16S rRNA and rDNA sequences, have proved the validity of some characteristics such as the Gram stain, and has shown that characteristics such as shape and oxygen requirement are not specific, and should not be used in classification systems.⁵

As these proposed taxonomic changes are still evolving, the above groupings will be broadly followed for practical purposes.

Bacteria, classified as Gram-negative, aerobic rods and cocci, represent few genera that are significant in terms of diseases of livestock (Table 1). Most of these genera are environmental inhabitants.

The family Pseudomonadaceae, one of the most complex groups of Gram-negative bacteria, contains bacteria that are straight or curved, and which are motile as a result of having polar flagella. The type genus is Pseudomonas. Differentiation between the genera in this family is difficult and requires specialized techniques which are often beyond the capabilities of most diagnostic laboratories. Pseudomonas is the only genus in the family that is of veterinary importance. ⁶ More information on the genus Pseudomonas is given in the introduction to Pseudomonas spp.

Table 1 The most common diseases in livestock caused by this group

The family Moraxellaceae contains bacteria that are aerobic, non-motile and occur singly, in pairs, or in masses. They are coccal or rod-shaped, do not form endospores, and may be encapsulated. Although they are Gram-negative, they may be decolorized with difficulty, in which case they appear to be Gram-positive.² Two genera (Moraxella and Psychrobacter) in this family are important in livestock; the diseases with which they are associated are listed in Table 1.^{1, 5}

Bacteria of the genus Moraxella, many of which are avirulent or poorly pathogenic, occur on the mucous membranes of healthy humans and other warm-blooded animals. In cattle, M. bovis causes infectious bovine keratoconjunctivitis which may occasionally be economically important. Moraxella infections in the other livestock species are sporadic, and in some, such as infectious ovine ophthalmia, the primary role of the bacteria in the causation of the diseases for which they are held responsible is in doubt. Psychrobacter phenylpyruvica, formerly Moraxella phenylpyruvica, causes sporadic infections in livestock.⁷

Bacteria of the genus Bordetella are obligate parasites of humans and animals and commonly localize and multiply throughout the respiratory tract, between the cilia of the respiratory epithelial cells. Two species in this genus (particularly B. pertussis, the cause of whooping cough) are responsible for severe infections in humans. In livestock, B. bronchiseptica generally infects pigs, causing non-progressive atrophic rhinitis and progressive atrophic rhinitis, in which the infection occurs in combination with toxigenic strains of Pasteurella...