What are Brucella?

Definition

Brucella are gram-negative, nonmotile, non-spore-forming,
nonencapsulated, small coccobacilli of worldwide distribution. They are
pathogenic in animals, including humans.

Natural Habitat and Features

The taxonomy of the genus Brucella has gone through several changes. By 1986, there were six recognized species, but scientists believed that not enough genetic difference existed among them and thus suggested that all be combined as B. melitensis, with the old species names listed as a subspecies. In 2010, the International Committee on Systematics of Prokaryotes, subcommittee on the taxonomy of Brucella, unanimously agreed to reinstate the original six species and to add four additional species.

Each of the ten species has a different host range and has some metabolic
differences. Two species that have been most studied and whose DNA
(deoxyribonucleic acid) has been sequenced, melitensis and
abortus, have two circular chromosomes, one with just more
than 2 million base pairs and one with just more than 1million base pairs.

Brucella spp. are small, gram-negative coccobacilli, approximately 0.5 micrometers (m) in diameter and 0.6 to 1.5 m in length. With the exception of microti, they are slow-growing in culture, often taking several days to weeks to show growth. Nutritionally, they are considered to be fastidious and require several vitamins and amino acids.

Both abortus and melitensis grow best when
erythritol is added to the medium as a carbon source. They are considered
facultative intracellular parasites and can survive for long periods in soil and
water. They have worldwide distribution and reservoirs in several domestic and
nondomestic mammal species. Studies with microti suggest that, in
addition to its normal vole reservoir, soil could serve as a reservoir for this
species. In their animal reservoirs, there is often a high concentration of
bacteria in the reproductive organs; these bacteria are
transferred during sex, by licking of the external genitalia, and by contact with
the placenta and fluids released during birth. Carnivores can sometimes become
infected when they eat an infected animal.

Pathogenicity and Clinical Significance

Each Brucella species has a small group of mammals that can
serve as hosts. For example, abortus affects cattle, bison,
buffalo, elk, camels, and yaks; melitensis affects sheep, goats,
and camels; suis affects swine; and canis
infects dogs. Immunizations and the slaughter of infected animals have eradicated
or nearly eradicated brucellosis in many parts of the
industrialized world. In the United States, abortus infection in
domestic cattle has been almost extirpated; however, a reservoir of the bacteria
exist in bison and elk in and around Yellowstone National Park. Humans are
accidental hosts when they ingest infected meat or unpasteurized dairy products or
come in contact with infected body fluids from one of the usual host animals.

In the United States, human brucellosis is rare, causing fewer than one hundred cases per year. It is much more common in the countries bordering the Mediterranean, in countries of the Arabian Peninsula, in India, and in Latin America. In humans, abortus, melitensis, canis, and suis can all cause classical brucellosis, while ceti and pinnepedialis have been associated with neural brucellosis.

Brucellosis, also known as undulant fever, Mediterranean fever, Malta fever, and Crimean fever, has varied, nonspecific, flulike symptoms such as fever, malaise, joint pain, headache, and fatigue. In humans, melitensis infections are the most common and cause the most severe symptoms, which can, on rare occasions, lead to death. Suis infections are also quite severe and can lead to prolonged illness, often with pus-forming lesions. Abortus and canis infections are often mild and self-limiting.

Brucellosis can be acute or chronic, with chronic infection being associated
with severe debility and increased morbidity. In addition, relapsing brucellosis
is difficult to distinguish from reinfection. Subclinical infections are
also seen in high-risk occupations, such as veterinarians and workers on the
kill-floor of slaughterhouses. In these subclinical infections, the person is
asymptomatic but shows Brucella infection after serologic
screening.

Drug Susceptibility

Because of high relapse rates with single drug therapy, most treatment regimens are based on two drugs. For adults, recommended drugs are oral doxycycline or ciprofloxacin for six weeks, with either oral rifampin for six weeks or intramuscular streptomycin or gentamicin for three weeks. The streptomycin or gentamicin treatment leads to fewer relapses, and gentamicin has fewer side effects. For children under eight years of age, doxycycline is not recommended. Instead, a regimen using rifampin and trimethoprim-sulfamethoxazole is the standard. Although immunizations have been developed for several species of domestic animals, there is no human immunization against brucellosis.

Bibliography

Corbel, Michael J. Brucellosis in Humans and Animals. Geneva: World Health Organization Press, 2006. This volume covers both animal and human Brucella infections and includes their epidemiology, diagnoses, and prevention.

Krieg, Noel R., et al., eds. Bergey’s Manual of Systematic Bacteriology. 2d ed. New York: Springer, 2010. Volume 2 of this multivolume work describes the Proteobacteria in detail.

Lopez-Goni, Ignacio, and Ignacio Moriyon, eds. Brucella: Molecular and Cellular Biology. Wymondhan, England: Horizon Bioscience, 2004. A comprehensive review of the genus Brucella and its varied species.

Madigan, Michael T., and John M. Martinko. Brock Biology of Microorganisms. 12th ed. Upper Saddle River, N.J.: Pearson/Prentice Hall, 2010. This text outlines many common bacteria and describes their natural history, pathogenicity, and other characteristics.

Romich, Janet A. Understanding Zoonotic Diseases. Clifton Park, N.Y.: Thomson Delmar, 2008. A good introduction to zoonotic diseases, including those caused by Brucella species.