Animal Brucellosis

Klaus Nielsen

• Editorial: CRC Press
• Año: 2018
• ISBN:
  9781351086585
• Edición: 1
• Categoría: Medicine
• Categoría: Veterinary Medicine

This timely publication updates and standardizes currently used diagnostic procedures for this widespread, economically costly livestock disease. It includes state-of-the-art technology, now in limited use, which will replace the conventional methodology in the near future. The volume covers research done on improved diagnostic techniques, vaccines, taxonomy, epidemiology, pathology, and basic immunology. It is an important literature review for those more established in this field and serves as a guide to researchers or diagnosticians becoming involved with this disease.

Chapter 1
CURRENT CONCEPTS IN THE TAXONOMY OF THE GENUS BRUCELLA

Margaret E. Meyer

TABLE OF CONTENTS

• I. Development and Structure of the Genus
• II. Current Taxonomic Status of the Genus
• III. Differences Among the Species
  • A. Derivation of the Biotypes of Brucella Abortus
  • B. Possible Derivation of Brucella Melitensis
  • C. Derivation of Brucella Suis
  • D. Origin of Brucella Canis
  • E. Derivation of Brucella Neotomae
• IV. Mechanisms That Could Account for Alterations in Characteristics of Members of the Genus Brucella
  • A. Incomplete Reversion of L-Forms, Producing Strains With Altered Characteristics
  • B. Activity of Plasmids
  • C. Conventional Mutational Events Involving Discrete Characteristics
  • D. Activity of Bacteriophages
  • E. Further Use of Restriction Endonucleases
  • F. Activity of Porins
  • G. Use of Gas Chromatography to Ascertain Differences in Constituent Fatty Acids
  • H. Exploration of the Ribosomes
• V. Current Taxonomic Status of the Genus Brucella
  • A. What Course of Action?
  • B. Bringing Order Into the Taxonomy of the Genus BRUCELLA
  • C. Change in Nomenclature
• References

I. Development and Structure of the Genus

The isolation of the causal agent of Malta fever by Bruce¹ a century ago had a decisive and permanent impact on clinical medicine as well as on systematic bacteriology. Malta fever had been difficult to differentiate symptomatically and clinically from other fevers, i.e., the so called typhoid-malarial complex, then endemic in countries of the Mediterranean littoral. Thus, to clinicians, the isolation of these organisms separated and defined Malta (undulant) fever as a distinct clinical entity. Since we have the advantage of historical hindsight, we now know that when Bruce² later named the causal organisms Micrococcus melitensis, he created the first species of Brucella.

Interestingly, the realization that undulant fever of man and brucellosis of animals were different manifestations of the same infection led to the creation of the genus Brucella. Evans³ established that M. melitensis was, in fact, a small rod (coccobacilli) rather than a coccus and that it was morphologically, culturally, and biochemically essentially indistinguishable from Brucella abortus. Because these two organisms shared the distinctive in vivo capabilities of producing abortion in animals and undulant fever in man, Meyer and Shaw4 found unacceptable Evans¹ suggestion that they be classified in the genus Bacterium, which included the typhoid-dysentery group of intestinal organisms. To accommodate the distinctive features of M. melitensis and B. abortus and to commemorate the work of David Bruce, the researchers gave them separate rank as the genus Brucella.

From its formation in 1920 to 1963, an additional species, B. suis was incorporated into the genus,^5,6 as were several biotypes.^6-11 During these 43 years, there were various critical assessments as to the naming and numbering of biotypes and as to whether these were aberrant, atypical, and/or transitional strains of brucellae.^7,12-17 Nonetheless, the genus membership remained stabilized with the three species of B. abortus, B. suis, and B. melitensis, now frequently referred to as the three classical species.

Since 1966, three additional species have been added to the genus: B. neotomae, B. ovis, and B. canis, now appropriately referred to as the three new species. B. neotomae was accepted without controversy¹⁸ as it has an essentially smooth colonial morphology, fits other criteria by which Brucella organisms can be identified,¹⁹ and also has a distinctive metabolic pattern.²⁰

The flow of thought that prevailed concerning both the structure of the genus and the pedigree required for admission into it was abruptly interrupted with the descriptions of B. ovis²¹ and the accompanying suggestion that it was a Brucella organism, and that it should be considered a new species.²² In fact, for 18 years, doubt and controversy²³ reigned regarding the true identity of B. ovis before it was ultimately admitted into the genus Brucella. B. ovis had not previously been assigned to a taxonomic niche because it differed markedly from the existing criteria for generic recognition of brucellae and because the manifestations of infection it caused in individual animals, as well as in flocks of sheep, did not fit the classic disease pattern associated with brucellosis. Further, it contradicted the conventional wisdom that only smooth brucellae were virulent and could long maintain themselves in populations of host/reservoir animals. The same circumstances initially clouded the identity of B. canis.^24-27

Hoyer and McCullough²⁸ ushered in the “high tech” era in this genus in 1968 by being the first to explore species relatedness at the genome level. The results of their DNA-DNA hybridization experiments established that the then four accepted species (B. abortus, B. suis, B. melitensis, and B. neotomae) had 100% homology among them in their polynucleotide sequences, that B. ovis had 94% homology with the other species, and that the base composition of G + C of 56 to 58 mol% was the same in all five species. In a subsequent paper,²⁹ they established that B. canis had DNA homology with the three classical species and by reciprocal DNA-DNA hybridization established that the difference in B. ovis was not due to a rearrangement of 6% of the sequences, but that they were actually missing from the genome. On the basis of these results, they concluded that B. ovis was a deletion mutant of one of the classical species and that all the species are closely related.

Recently, Verger et al.³⁰ examined the DNA homologies of the polynucleotide sequences in 51 strains of Brucella which included representatives of the six species and several strains of biotypes within each of the classical species (there are no reported biotypes within the three new species). In DNA-DNA reassociation experiments using labeled DNA strands from B. melitensis 16M to determine its homology with the other 50 strains, they reported relative binding ratios (percent homology) of from 84 to 100%. In their results on reciprocal DNA-DNA relatedness, they reported percentages ranging 87 to 104%. Even though their 23% range in percentages of binding ratios and 17% range in reciprocal ratios considerably exceeded the reported standard error of 3% in DNA relatedness results,^31,32 they nonetheless denied the validity of Hoyer and McCullough’s finding concerning the 6% difference between B. ovis and the other species. However, by using a different molecular genetic technique, De Ley et al.³³ established with certainty the genetic similarity of the six species. These investigators previously had found that genome sizes (i.e., molecular complexes) are similar among different strains within a single, well-defined species (standard deviation of a group of averages is less than 14.5%). When the same techniques were applied to the six species of Brucella,³⁴ they found genome molecular complexities of 2.37 × 10⁹, with standard deviation of 8%, indicating an intimate genetic relationship. Their data on DNA ribosomal RNA hybridization also shows, via a similarity map, there to be but little measurable differences among the species. Thus, by all available molecular genetic techniques for ascertaining relatedness at the genome level, it is clear that the relationship among all brucellae is exquisitely close.

II. Current Taxonomic Status of the Genus

What, then, is the current taxonomic status of this genus? Taxonomy seems to mean different things to different people. However, in its purest sense, it means having a scheme of hierarchical classification that reflects and reveals the evolutionary relatedness of the organisms, ideally at all taxon levels, i.e., biotypes, species, genus, and family. As distinct from the evolutionary relatedness imbued in a taxonomy, the taxonomic process obviously includes a workable identification key and a system of nomenclature.

For many years the genus Brucella was sequestered in the family Brucellaceae,³⁵ which also included many other genera (i.e., Bordetella, Pasteurella, etc.). Through DNA-DNA hybridization studies and G + C base ratio determinations, the genus Brucella was found to be unrelated to the other family members and, in fact, most of the genera in the family were found to be unrelated to each other. In the most rece...