A Microbial Biorealm page on the genus Ehrlichia chaffeensis
Higher order taxa
Domain: Bacteria; Phylum: Proteobacteria; Class: Alphaproteobacteria; Order: Rickettsiales; family: Anaplasmataceae [Others may be used. Use NCBI link to find]
Description and significance
Ehrlichia chaffeensis causes a tick-borne disease affecting both animals and humans. The first incident of human ehrlichiosis (the infection caused by the Ehrlichia bacterium) was reported in Japan in 1954. An event of human ehrlichiosis was not reported in the United States until 1986. In 1991, E. chaffeensis was isolated from a military recruit stationed at Fort Chaffee, Arkansas, this was the first isolation from a human in the US.(3) Public attention peaked after this bacterium was found to be the causative agent of human monocytotropic ehrlichiosis (HME). According to the Center for Disease Control and Prevention (CDC), HME is one of the most frequent life-threatening tick-borne zoonoses (a disease that can be transmitted from an animal to a human) but frequently goes unreported in the United States since symptoms are similar to many other diseases or infections a person may get from insect bites, like Lyme disease from a tick. E. chaffeensis has been isolated in white-tailed deer and even dogs, with the latter being a possible carrier of the tick which can infect humans with HME. E. chaffeensis is also easily contracted in nature where bare skin is exposed and a tick carrying the bacteria can attach and infect. HME is mostly found in the southern states of the US but there are cases reported all over the country as well as in other parts of the world.(2)
The complete genome of Ehrlichia chaffeensis str. Arkansas has been sequenced. It has a circular chromosome with a length of 1,176,248 nt (nucleotides) and a Guanine-Cytosine(GC) content of 30.1%, thus the Adenosine-Tyrosine(AT) content is 69.9%. “While sequence polymorphisms in several genes among E. chaffeensis strains have been reported, global genomic divergence and biological differences among strains are unknown”.(8) Sequence polymorphisms are variations in a population's DNA sequences. These differences are most concentrated among genes predicted to encode cell envelope proteins for three different strains of the species Arkansas, Liberty and Wakulla, whose genomes have been sequenced. Of all open reading frames (ORF; section of an organism’s genome that has a sequence of bases which could encode for a protein), the greatest differences among these strains are seen in nine genes encoding hypothetical proteins; those with no experimental evidence of expression in organisms, two genes encoding ankyrin repeat proteins (sets of two α-helices separated by loops) and hemE (iron containing prosthetic group).(8) A big similarity for Ehrlichia species is that the “16S rRNA gene DNA sequences are highly conserved among strains”.(12) This means that the 16S rRNA sequence between all the strains in the Ehrlichia genus are similar in sequence with respect to that gene.
Cell structure and metabolism
Gram-negative bacteria gain structural strength by having a thin cell wall made of peptidoglycan, as well as an inner and outer membrane. Interestingly, Ehrlichia chaffeensis, a Gram-negative bacteria, lacks most genes responsible for the biosynthesis of peptidoglycan and lacks all genes for the biosynthesis of lipid A, a lipid which anchors lipopolysaccharide to the outer membrane of the bacterium. By not having these genes, E. chaffeensis is weakened structurally and must also depend on cholesterol for growth. This is the first report of a Gram-negative bacteria that can use cholesterol to survive. This information provides insight into the unique nature of E. chaffeensis’ parasitism.(5)
E. chaffeensis is an obligatory, intracellular bacterium that replicates in monocytes/macrophages that are generally equipped with powerful, innate antimicrobial defenses to ensure protection of the immune system. This shows the bacteria’s unusual behavior because they are utilizing the very cells that are supposed to kill them, as a location to grow.(3) This cocci (spherical) shaped, vector-borne pathogen “contains significant levels of membrane cholesterol, but lack genes for cholesterol biosynthesis or modification”.(5) The vector for E. chaffeensis is the tick, which like a mosquito, is an organism that does not cause disease itself, but spreads infection by conveying pathogens from one host to another. In experimentation, host cell-free bacteria had the ability to directly take up exogenous cholesterol (NBD-cholesterol; a fluorescent cholesterol derivative). When methyl-beta-cyclodextrin, a cholesterol extraction reagent, was used to treat the bacteria, it caused ultrastructural changes. Furthermore, pre-treatment of E. chaffeensis with methyl-beta-cyclodextrin or NBD-cholesterol, deprived them of the ability to infect white blood cells, thus killing the E. chaffeensis.(5) This new knowledge may prove useful in combating the harmful effects (see pathology) that E. chaffeensis has on both humans and animals.
Ehrlichia chaffeensis has two forms in mammalian cells: dense-cored cells (DC) which have a dense nucleoid, and reticulate cells (RC) with a nucleoid that is uniformly dispersed. It has been determined by electron microscopy that DC but not RC attach to and enter into the host cells because DC cells are smaller in size than RC cells. Oppositely, RC but not DC are able to multiply by binary fission (division of one cell into two) inside of host cells. With the pairing of these two events, the E. chaffeensis developmental cycle can take place. The cycle occurs when dense-cored cells attach to and enter into the host cells, thus transforming into RC which multiply for 48 hrs, followed by maturation into DC at 72 hrs.(13)
Ticks, particularly Amblyomma americanum, act as a vector for E. chaffeensis. Thus, it is actually mammals, like the white-tailed deer, who are the initial hosts of E. chaffeensis. The ticks are only infected with the bacterium after they feed off the blood of an infected deer.(6)
A recent study on children reported that the common clinical signs and symptoms of patients with human monocytotrophic ehrlichiosis (HME) include headache, rash, myalgia, nausea/vomiting, fever and altered mental status. HME is one of the most frequent life-threatening tick-borne zoonoses (a disease that can be transmitted from an animal to a human) but frequently goes unreported in the United States since symptoms are similar to many other diseases or infections a person may get from insect bites, like Lyme disease from a tick. About half of HME patients had the combination of fever, headache and rash. Common laboratory aberrations include elevated aspartate aminotransferase (a diagnostic indication of insufficient supply of blood to the heart resulting in severe chest pain), thrombocytopenia (an abnormal decrease in the number of blood platelets) and elevated alanine aminotransferase (a diagnostic indication of liver disease). The average number of days of illness before the initiation of therapy was six.(10)
Of all HME patients in this study, 22% were admitted to the intensive care unit with 12.5% of them requiring blood pressure and ventilatory support. There is only a limited knowledge of HME’s clinical course, even though it has been recognized for nearly 20 years. Only a few cases of HME are diagnosed each year, even among physicians practicing in endemic regions. More research is needed to understand the true difficulties of this disease and the natural history among asymptomatically (showing no evidence of disease) and symptomatically (showing evidence of disease) infected children.(10)
Research article: “Serologic evidence for Rickettsia typhi and an ehrlichial agent in Norway rats from Baltimore, Maryland, USA”
Researchers trapped 90 Norwegian rats in Baltimore, Maryland in order to determine whether these rodents carried antibodies against Rickettsia typhi and Ehrlichia chaffeensis. Their serum was screened and six rats tested positive for antibodies against R. typhi and four additional rats tested positive for antibodies against E. chaffeensis. Sera from these rat also tested positive antibodies against two other pathogens. These data indicate that the agent of typhus and ehrlichia agents exist in rats in Baltimore.(9)
Research article: “Degradation of p22phox and inhibition of superoxide generation by Ehrlichia chaffeensis in human monocytes”
Ehrlichia chaffeensis replicates in monocytes or macrophages, which are the primary producers of reactive oxygen species (ROS). ROS include oxygen ions, free radicals and peroxides both inorganic and organic which can result in significant damage to cell structures. The effects of ROS from infection of E. chaffeensis and whether E. chaffeensis alters ROS generations in host monocytes is unknown. Researchers showed that E. chaffeensis lost infectivity upon exposure to O2−. Upon incubation with human monocytes, E. chaffeensis did not induce O2− generation, but actively blocked it. E. chaffeensis does not infect neutrophils, another potent ROS generator, so the aforementioned affects were not seen in this cell type. These results lead to a greater understanding of the complicated and unique survival mechanism of E. chaffeensis, of ROS-sensitive bacteria and how they avoid the harmful internal conditions of monocytes. (4)
Research article: “Co-infection of white-tailed deer with multiple strains of Ehrlichia chaffeensis”
Investigations have been done by researchers on the effects of exposing deer to multiple strains of E. chaffeensis that differed in number of tandem repeats in the 120 kDa antigen gene or the variable-length PCR target (VLPT) gene. These two genes encode surface-expressed antigens of E. chaffeensis. Researchers hypothesized that infection with one strain would provide immunity to infection with other strains of E. chaffeensis. Experiments were done using three strains of E. chaffeensis which were called strain A, B and C. Deer were infected with different combinations of these strains, and they all showed multiple strain infections. (11)
Western blot analysis demonstrated that deer sera reacted differently to antigens from each exposed strain. A previous study showed that deer can become sequentially infected with up to three strains of E. chaffeensis. This suggests that competitive exclusion, where one strain prevents the infection of another strain, does not occur with E. chaffeensis, as this experiment showed again. (11)
Application to Biotechnology
It is still unknown whether or not E. chaffeensis produces any useful substances. Most major research on this bacterium has occurred in the past ten years, thus its biotechnological impact is still in its infancy.
(1) Dawson, J.E., Anderson, B.E., Fishbein, D.B., Sanchez, J.L., Goldsmith, C. S., Wilson, K. H. and C. W. Duntley. “Isolation and characterization of an Ehrlichia sp. from a patient diagnosed with human ehrlichiosis”. Journal of Clinical Microbiology. 1991. 29(12): p. 2741-2745.
(2) Dumler, J., Choi, Kyoung-Seong, Garcia-Garcia, Jose Carlos and Nicole S. Barat et. al. “Human Granulocytic Anaplasmosis and Anaplasma phagocytophilum” Emerging Infectious Diseases. Volume 11, Number 12, December 2005. p.1828-1834.
(3) Kumagai, Y., Cheng, Z., Lin, M. and Y. Rikihisa. "Biochemical Activities of Three Pairs of Ehrlichia chaffeensis Two-Component Regulatory System Proteins Involved in Inhibition of Lysosomal Fusion." Infect Immun. 2006 September; 74(9): 5014-5022.
(4) Lin, M. and Y. Rikihisa. “Degradation of p22phox and inhibition of superoxide generation by Ehrlichia chaffeensis in human monocytes”. Cell Microbiol. 2007 Apr;9(4):861-74.
(5) Lin, M. and Y. Rikihisa. “Ehrlichia chaffeensis and Anaplasma phagocytophilum lack genes for lipid A biosynthesis and incorporate cholesterol for their survival”. Infect Immun. 2003 Sep;71(9):5324-31.
(6) Lockhart, J. M., Davidson, W. R., Stallknecht, D. E., Dawson, J. E. and E. W. Howerth. “Isolation of Ehrlichia chaffeensis from wild white-tailed deer (Odocoileus virginianus) confirms their role as natural reservoir hosts”. J Clin Microbiol. 1997 July; 35(7): 1681–1686.
(7) McQuiston, J.H., Paddock, C.D., Holman, R.C. and J.E. Childs. “The human ehrlichioses in the United States”. Emerging Infectious Diseases. 1999 Sep-Oct;5(5):635-42.
(8) Miura, K and Y. Rikihisa. "Virulence Potential of Ehrlichia chaffeensis Strains of Distinct Genome Sequences." Infect Immuno. 2007. (in press)
(9) Reeves, W.K., Easterbrook, J.D., Loftis, A.D. and G.E. Glass. “Serologic evidence for Rickettsia typhi and an ehrlichial agent in Norway rats from Baltimore, Maryland, USA”. Vector Borne Zoonotic Dis. 2006 Fall;6(3):244-7.
(10) Schutze, G.E., Buckingham, S.C., Marshall, G.S. and C.R. Woods et al. “Human Monocytic Ehrlichiosis in Children”. Pediatr Infect Dis J. 2007 Jun;26(6):475-479. (in press)
(11) Varela-Stokes, A.S., Stokes, J.V., Davidson, W.R. and S. E. Little. “Co-infection of white-tailed deer with multiple strains of Ehrlichia chaffeensis”. Vector Borne Zoonotic Dis. 2006 Summer;6(2):140-51.
(12) Yu, X.J., McBride, J.W. and D.H. Walker. “Restriction and expansion of Ehrlichia strain diversity”. Vet Parasitol. 2007 Feb 28;143(3-4):337-46.
(13) Zhang, J.Z., Popov, V.L., Gao, S., Walker, D.H. and X.J. Yu. “The developmental cycle of Ehrlichia chaffeensis in vertebrate cells”. Cell Microbiol. 2007 Mar;9(3):610-8. Epub 2006 Sep 20.
Edited by Armen J. Sarkisian, student of Rachel Larsen and Kit Pogliano