A Microbial Biorealm page on the genus Brucella abortus
Higher order taxa
Bacteria; Proteobacteria; Alphaproteobacteria; Rhizobiales; Brucellaceae NCBI
Description and significance
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Brucella abortus is a gram-negative bacterium that is found in cattle populations (1). This intracellular parasite is a blood borne pathogen that causes premature abortion of a cattle fetus. What makes this bacterium so dangerous is that it is zoonotic, meaning it can be transferred from an animal to a human host and still remain pathogenic (3). In humans this disease cause both acute and chronic symptoms, but can be treated with antibiotics. Because of this economic effect on the cattle business and the disease potential in humans, the US has spent close to $3.5 billion trying to vaccinate the cattle herds in the US (6). It is possible for B. abortus to be spread from wild populations of elk and bison into domestic cattle herds, this is why the US government continues to be vigilant in tracking potential cases within herds (10).
The B. abortus genome contains 2 circular DNA chromosomes. The first chromosome is 2,124,241 nucleotides long and codes for 2200 genes. The second chromosome is 1,162,204 nucleotides long and codes for 1156 genes. The genome has a GC content of 57%, and 81% of the genome is a coding region (10). This pathogen is different from many in that it does not contain any plasmids or genomic islands that relate to pathogenicity within its genome (5). In addition to lacking these two features, the genome also lack many other genes that code for common virulence factors including “capsules, fimbriae, exotoxins, cytolysins, resistance forms, antigenic variation, plasmids, or lysogenic phages” (1). The genes that do encode for virulence in B. abortus are being examined but they are not well enough understood to say for sure what the mode of virulence is for this intracellular pathogen (5).
Cell structure and metabolism
Brucella abortus is a Gram-negative rod shaped bacteria that does not have flagella or pili, nor do they create capsule slime. It also does not produce spores. This heterotrophic bacteria carries out either aerobic or anaerobic respiration because it is a facultative bacterium (5). This means that the bacteria can grow with or without oxygen present. In order to grow Brucella abortus a very complex media is required, this is because it is a fastidious bacteria, which requires most of essential nutrients to be imported into the cell from the host (4). Although it is a fastidious bacteria, Brucella abortus does have “all major biosynthetic pathways” (5) available to it. In its primary host, cattle, the metabolic pathway for the breakdown of erythritol is one that is most desirable, it is even used “preferentially to glucose” (4). This is a possible factor in the bacteria’s virulence because erythritol is found in bovine placenta.
Brucella abortus is am intracellular bacteria, which means that it does not replicate outside the host organism. This bacterium, as an intracellular pathogen, enters phagocytes, such as macrophages, in humans and in cows. It attaches to the endoplasmic reticulum of these cells (5). These smooth bacteria enter macrophages and then live in compartments of vacuolar space along the ER. The few cells that make it to these vacuolar spaces down regulate apoptosis genes within the macrophage and therefore cause the cell to resist self-death and these pathogens become resistant within these cells of the immune system. These resistant bacterium are what go on to cause chronic disease in human hosts (8).
In bovine species the bacteria also infects the trophoblast epithelial cells, which are the cells that provide nutrition to the embryo (8). After a number of rounds of cellular replication in the trophoblast the cells lyse, causing more bacteria cells to enter the blood stream of the developing embryo (1). These cells in the blood stream go on to colonize the placenta and fetus in pregnant female cows, and will go on to induce abortion of the fetus (7).
Though Brucella abortus is an intracellular bacterium it can remain alive outside the host without replicating. This bacterium can remain in the excrement of cattle and the aborted fetuses of the cattle for quite some time depending on the exact conditions; thought the average time is around 30 days. Outside the host the bacteria cells are affected by direct sunlight, the pathogen can be eliminated by pasteurization, and can be killed by disinfectants (5).
Brucella abortus causes a disease called brucellosis, which used to be referred to as Malta Fever because it was first discovered in soldiers who were living on the island of Malta by Dr. David Bruce, for who the pathogen gets its name(4). B. abortus is originally found in cattle and causes problems with fetus development and viability, but this pathogen can be passed to humans. It is uncommon in the US, most cases emerge from slaughterhouse workers, meat packers, or large animal veterinarians, but in the developing world the disease is much more common because their cattle herds are not vaccinated. In these cases the most common mode of transmission is through unpasteurized milk and cheese products because the bacteria is present it the milk glands of a female cow (3).
In humans the disease has both an acute and a chronic phase. The chronic phase will last as long as the host is alive without treatment. Acute symptoms include fever, chills, headache, backache, weakness, and weight loss. The chronic symptoms are usually reoccurring joint pain, fatigue, and headaches (2).
There is an antibiotic regiment for humans who come in contact with the disease that includes the antibiotics rifampin and doxycycline together (2).
Application to Biotechnology
Until 1969 the US ran a number of experiments with biological weapons. One of the bacterium used in this research was Brucella suis, that almost identical to Brucella abortus, except that its preferential host is pigs instead of cows. One of the reasons that the Brucella bacteria were targeted for development into a biological weapon was because of the length of time that it causes disease and the fact that if affects both humans and livestock. Although it does not kill human hosts, this pathogen can cause a long and lingering chronic illness that will cause a great loss in productivity of a nations workforce. Another reason this bacterium was targeted as a biological weapon is because humans consume many of the animals that it affects as food, such as pigs, cows, and goats (4). The final reason that this posed a great biological threat was that it can be spread through aerosols and therefore is easily dispersed, especially in an urban environment (6).
Due to the new heighten threat of bioterrorism in last few years there is current research being done by the Armed Forces Institute of Pathology in a screening mechanism for such threats as Brucella abortus. The new research is being conducted on an optical detection system for such threats that combines spectroscopy and digital imaging that form a library, which can be screened. One positive to this screening mechanism is that the pathogens can be detected in complex environments and do not require amplification. The results of the preliminary test show that this screening method does have a high level of specificity and does accurately detect pathogen and correctly identify them (11).
As stated earlier, there is not much known about the exact pathway that enables the Brucella abortus pathogen to evade the immune system and become lodged in the ER of host macrophages. This process does enable the bacterium to remain virulent while evading the immune system. There has been research done to try and determine the virulence factors that allows the bacteria to reside in macrophages without the common virulence factors associated with pathogenic bacteria. One study found that PrpA is a gene encoded for on the bacterial genome, which cause IL-10 secretion in macrophages, is required to establish a chronic infection in mouse macrophages (2).
Although the US domestic cattle herds have all been vaccinated for Brucella abortus, there is now fear that B. abortus from wild bison and elk can infect domestic herds. Because of this possibility, researchers are looking to test the vaccine administered to cows in elk and see if the same immune response is seen. If the same response was seen it was hypothesized that the vaccine could be given to wild populations to further stop transmission and further protect the cattle. In this study the elk did seem to amount an immune response initially because the level of antibody production was higher in those elk that received the vaccine. The problem was that this response did not proliferate, so though there seemed to be an initial immune reaction it did not last. This means that the vaccine for cattle would offer little protect for the elk against Brucella abortus (12).
Edited by Elizabeth Costa, a student of Rachel Larsen and Kit Pogliano