Cultivation of Ehrlichia chaffeensis in mouse embryo, Vero, BGM, and L929 cells and study of Ehrlichia-induced cytopathic effect and plaque formation

Multiple Ehrlichia chaffeensis genes critical for persistent infection in a vertebrate host are identified as nonessential for its growth in the tick vector; Amblyomma americanum

Ehrlichia chaffeensis is a tick-transmitted monocytic ehrlichiosis agent primarily causing the disease in people and dogs. We recently described the development and characterization of 55 random mutations in E. chaffeensis, which aided in defining the critical nature of many bacterial genes for its growth in a physiologically relevant canine infection model. In the current study, we tested 45 of the mutants for their infectivity ability to the pathogen's tick vector; Amblyomma americanum. Four mutations resulted in the pathogen's replication deficiency in the tick, similar to the vertebrate host. Mutations causing growth defects in both vertebrate and tick hosts included in genes coding for a predicted alpha/beta hydrolase, a putative dicarboxylate amino acid:cation symporter, a T4SS protein, and predicted membrane-bound proteins. Three mutations caused the bacterial defective growth only in the tick vector, which represented putative membrane proteins. Ten mutations causing no growth defect in the canine host similarly grew well in the tick vector. Mutations in 28 genes/genomic locations causing E. chaffeensis growth attenuation in the canine host were recognized as non-essential for its growth in the tick vector. The tick non-essential genes included genes coding for many metabolic pathway- and outer membrane-associated proteins. This study documents novel vector- and host-specific differences in E. chaffeensis for its functional gene requirements.

Development of an immunofluorescence assay for detection of SARS-CoV-2

SARS-CoV-2, the etiologic agent of the COVID-19 pandemic, emerged as the cause of a global crisis in 2019. Currently, the main method for identification of SARS-CoV-2 is a reverse transcription (RT)-PCR assay designed to detect viral RNA in oropharyngeal (OP) or nasopharyngeal (NP) samples. While the PCR assay is considered highly specific and sensitive, this method cannot determine the infectivity of the sample, which may assist in evaluation of virus transmissibility from patients and breaking transmission chains. Thus, cell-culture-based approaches such as cytopathic effect (CPE) assays are routinely employed for the identification of infectious viruses in NP/OP samples. Despite their high sensitivity, CPE assays take several days and require additional diagnostic tests in order to verify the identity of the pathogen. We have therefore developed a rapid immunofluorescence assay (IFA) for the specific detection of SARS-CoV-2 in NP/OP samples following cell culture infection. Initially, IFA was carried out on Vero E6 cultures infected with SARS-CoV-2 at defined concentrations, and infection was monitored at different time points. This test was able to yield positive signals in cultures infected with 10 pfu/ml at 12 hours postinfection (PI). Increasing the incubation time to 24 hours reduced the detectable infective dose to 1 pfu/ml. These IFA signals occur before the development of CPE. When compared to the CPE test, IFA has the advantages of specificity, rapid detection, and sensitivity, as demonstrated in this work.

Multiple Ehrlichia chaffeensis Genes Critical for Its Persistent Infection in a Vertebrate Host Are Identified by Random Mutagenesis Coupled with In Vivo Infection Assessment

Ehrlichia chaffeensis, a tick-transmitted obligate intracellular rickettsial agent, causes human monocytic ehrlichiosis. In recent reports, we described substantial advances in developing random and targeted gene disruption methods to investigate the functions of E. chaffeensis genes. We reported earlier that the Himar1 transposon-based random mutagenesis is a valuable tool in defining E. chaffeensis genes critical for its persistent growth in vivo in reservoir and incidental hosts. The method also aided in extending studies focused on vaccine development and immunity. Here, we describe the generation and mapping of 55 new mutations. To define the critical nature of the bacterial genes, infection experiments were carried out in the canine host with pools of mutant organisms. Infection evaluation in the physiologically relevant host by molecular assays and by xenodiagnoses allowed the identification of many proteins critical for the pathogen's persistent in vivo growth. Genes encoding proteins involved in biotin biosynthesis, protein synthesis and fatty acid biosynthesis, DNA repair, electron transfer, and a component of a multidrug resistance (MDR) efflux pump were concluded to be essential for the pathogen's in vivo growth. Three known immunodominant membrane proteins, i.e., two 28-kDa outer membrane proteins (P28/OMP) and a 120-kDa surface protein, were also recognized as necessary for the pathogen's obligate intracellular life cycle. The discovery of many E. chaffeensis proteins crucial for its continuous in vivo growth will serve as a major resource for investigations aimed at defining pathogenesis and developing novel therapeutics for this and related pathogens of the rickettsial family Anaplasmataceae.

L-thyroxine modifies nephrotoxicity by regulating the apoptotic pathway: The possible role of CD38/ADP-ribosyl cyclase-mediated calcium mobilization

Thyroid hormones are well-established as a key regulator of many cellular metabolic pathways developed in various pathogeneses. Here, we dedicated the current work to investigate the role of thyroid hormone analogue (L-thyroxine, L-TH) in regulating the renal cytotoxicity using in vivo and in vitro models. Swiss mice were exposed to gamma radiation (IRR, 6Gy) or treated with cisplatin (CIS, 15 mg/kg, i.p.) for induction of nephrotoxicity. Remarkably, pretreatment with L-TH (1μg/kg) ameliorated the elevated kidney function biomarkers, oxidative stress and protected the renal tissue from the subsequent cellular damage. Likewise, L-TH inhibited the apoptotic cascade by down-regulating the extreme consumption of the cellular energy (ATP), the expression of caspase-3 and Bax, and the stimulation of cyclic ADP ribose (cADPR)/calcium mobilization. Moreover, incubation with L-TH (120nM/4h) significantly blocked the cytotoxicity of CIS on Vero cells and the depletion of NAD+ content as well as modified the ADP-ribose cyclase (CD38) enzymatic activity. High doses of L-TH (up to30 nM/4h) inversely increased the radiosensitivity of Vero cells towards IRR (up to 6Gy). On the other hand, L-TH did not interfere CIS-induced cytotoxicity of colorectal adenocarcinoma (Caco-2) cell line. In conclusion, pretreatment with L-TH could be a promising protective approach to the renal cellular damage induced during either CIS or IRR therapy by regulating the unbalanced oxidative status, the expression of pro-apoptotic biomarkers via modulation of cADPR mediated-calcium mobilization.

Current and past strategies for bacterial culture in clinical microbiology

A pure bacterial culture remains essential for the study of its virulence, its antibiotic susceptibility, and its genome sequence in order to facilitate the understanding and treatment of caused diseases. The first culture conditions empirically varied incubation time, nutrients, atmosphere, and temperature; culture was then gradually abandoned in favor of molecular methods. The rebirth of culture in clinical microbiology was prompted by microbiologists specializing in intracellular bacteria. The shell vial procedure allowed the culture of new species of Rickettsia. The design of axenic media for growing fastidious bacteria such as Tropheryma whipplei and Coxiella burnetii and the ability of amoebal coculture to discover new bacteria constituted major advances. Strong efforts associating optimized culture media, detection methods, and a microaerophilic atmosphere allowed a dramatic decrease of the time of Mycobacterium tuberculosis culture. The use of a new versatile medium allowed an extension of the repertoire of archaea. Finally, to optimize the culture of anaerobes in routine bacteriology laboratories, the addition of antioxidants in culture media under an aerobic atmosphere allowed the growth of strictly anaerobic species. Nevertheless, among usual bacterial pathogens, the development of axenic media for the culture of Treponema pallidum or Mycobacterium leprae remains an important challenge that the patience and innovations of cultivators will enable them to overcome.

Ehrlichia chaffeensis infection in the reservoir host (white-tailed deer) and in an incidental host (dog) is impacted by its prior growth in macrophage and tick cell environments

Ehrlichia chaffeensis, transmitted from Amblyomma americanum ticks, causes human monocytic ehrlichiosis. It also infects white-tailed deer, dogs and several other vertebrates. Deer are its reservoir hosts, while humans and dogs are incidental hosts. E. chaffeensis protein expression is influenced by its growth in macrophages and tick cells. We report here infection progression in deer or dogs infected intravenously with macrophage- or tick cell-grown E. chaffeensis or by tick transmission in deer. Deer and dogs developed mild fever and persistent rickettsemia; the infection was detected more frequently in the blood of infected animals with macrophage inoculum compared to tick cell inoculum or tick transmission. Tick cell inoculum and tick transmission caused a drop in tick infection acquisition rates compared to infection rates in ticks fed on deer receiving macrophage inoculum. Independent of deer or dogs, IgG antibody response was higher in animals receiving macrophage inoculum against macrophage-derived Ehrlichia antigens, while it was significantly lower in the same animals against tick cell-derived Ehrlichia antigens. Deer infected with tick cell inoculum and tick transmission caused a higher antibody response to tick cell cultured bacterial antigens compared to the antibody response for macrophage cultured antigens for the same animals. The data demonstrate that the host cell-specific E. chaffeensis protein expression influences rickettsemia in a host and its acquisition by ticks. The data also reveal that tick cell-derived inoculum is similar to tick transmission with reduced rickettsemia, IgG response and tick acquisition of E. chaffeensis.

Targeted and random mutagenesis of Ehrlichia chaffeensis for the identification of genes required for in vivo infection

Ehrlichia chaffeensis is a tick transmitted pathogen responsible for the disease human monocytic ehrlichiosis. Research to elucidate gene function in rickettsial pathogens is limited by the lack of genetic manipulation methods. Mutational analysis was performed, targeting to specific and random insertion sites within the bacterium's genome. Targeted mutagenesis at six genomic locations by homologous recombination and mobile group II intron-based methods led to the consistent identification of mutants in two genes and in one intergenic site; the mutants persisted in culture for 8 days. Three independent experiments using Himar1 transposon mutagenesis of E. chaffeensis resulted in the identification of multiple mutants; these mutants grew continuously in macrophage and tick cell lines. Nine mutations were confirmed by sequence analysis. Six insertions were located within non-coding regions and three were present in the coding regions of three transcriptionally active genes. The intragenic mutations prevented transcription of all three genes. Transposon mutants containing a pool of five different insertions were assessed for their ability to infect deer and subsequent acquisition by Amblyomma americanum ticks, the natural reservoir and vector, respectively. Three of the five mutants with insertions into non-coding regions grew well in deer. Transposition into a differentially expressed hypothetical gene, Ech_0379, and at 18 nucleotides downstream to Ech_0230 gene coding sequence resulted in the inhibition of growth in deer, which is further evidenced by their failed acquisition by ticks. Similarly, a mutation into the coding region of ECH_0660 gene inhibited the in vivo growth in deer. This is the first study evaluating targeted and random mutagenesis in E. chaffeensis, and the first to report the generation of stable mutants in this obligate intracellular bacterium. We further demonstrate that in vitro mutagenesis coupled with in vivo infection assessment is a successful strategy in identifying genomic regions required for the pathogen's in vivo growth.

The tick-transmitted bacterium, Ehrlichia chaffeensis, causes human monocytic ehrlichiosis, an acute febrile illness that can progress to a fatal outcome. This and other related pathogens have evolved to establish infections in vertebrate and tick hosts for completing their lifecycle. Our recent studies suggest that the pathogen's differential gene expression during growth in ticks and mammals is a major contributor for its dual host adaptation. However, the importance of the pathogen phenotype differences is best understood if we have methods to knock down protein expression from one or more genes. Creating mutations in obligate intracellular pathogens remain a challenge due to their limited survival in the extracellular environment. Here, we present evidence for multiple insertion mutations in the E. chaffeensis genome. Three of the nine mutations in the genome inhibiting gene expression prevented infection of deer, the natural host for the pathogen. This is the first study demonstrating the feasibility of creating mutations in an Ehrlichia species; and directly linking specific regions of the genome to in vivo infection. Methods described here allow for studies to define genes important for infectivity and ability to cause disease, and are equally important for initiating similar studies in other related emerging zoonotic pathogens.

Growth and maintenance of Vero cell lines

Vero cells are derived from the kidney of an African green monkey, and are one of the more commonly used mammalian continuous cell lines in microbiology and molecular and cell biology research. This unit includes protocols for the growth and maintenance of Vero cell lines in a research laboratory setting.

Propagation of arthropod-borne Rickettsia spp. in two mosquito cell lines

Rickettsiae are obligate intracellular alphaproteobacteria that include pathogenic species in the spotted fever, typhus, and transitional groups. The development of a standardized cell line in which diverse rickettsiae can be grown and compared would be highly advantageous to investigate the differences among and between pathogenic and nonpathogenic species of rickettsiae. Although several rickettsial species have been grown in tick cells, tick cells are more difficult to maintain and they grow more slowly than insect cells. Rickettsia-permissive arthropod cell lines that can be passaged rapidly are highly desirable for studies on arthropod-Rickettsia interactions. We used two cell lines (Aedes albopictus cell line Aa23 and Anopheles gambiae cell line Sua5B) that have not been used previously for the purpose of rickettsial propagation. We optimized the culture conditions to propagate one transitional-group rickettsial species (Rickettsia felis) and two spotted-fever-group rickettsial species (R. montanensis and R. peacockii) in each cell line. Both cell lines allowed the stable propagation of rickettsiae by weekly passaging regimens. Stable infections were confirmed by PCR, restriction digestion of rompA, sequencing, and the direct observation of bacteria by fluorescence in situ hybridization. These cell lines not only supported rickettsial growth but were also permissive toward the most fastidious species of the three, R. peacockii. The permissive nature of these cell lines suggests that they may potentially be used to isolate novel rickettsiae or other intracellular bacteria. Our results have important implications for the in vitro maintenance of uncultured rickettsiae, as well as providing insights into Rickettsia-arthropod interactions.

Differential clearance and immune responses to tick cell-derived versus macrophage culture-derived Ehrlichia chaffeensis in mice

Human monocytic ehrlichiosis is caused by a tick-transmitted rickettsia, Ehrlichia chaffeensis. We recently reported that E. chaffeensis grown in tick cells expresses different proteins than bacteria grown in macrophages. Therefore, we tested the hypothesis that immune responses against E. chaffeensis would be different if the mice are challenged with bacteria grown in macrophages or tick cells. We assessed the E. chaffeensis clearance from the peritoneum, spleen, and liver by C57BL/6J mice using a TaqMan-based real-time reverse transcription-PCR assay. Macrophage-grown E. chaffeensis was cleared in 2 weeks from the peritoneum, whereas the pathogen from tick cells persisted for nine additional days and included three relapses of increasing bacterial load separated by three-day intervals. Tick cell-grown bacteria also persisted in the livers and spleens with higher bacterial loads compared to macrophage-grown bacteria and fluctuated over a period of 35 days. Three-day periodic cycles were detected in T-cell CD62L/CD44 ratios in the spleen and bone marrow in response to infections with both tick cell- and macrophage-grown bacteria and were accompanied by similar periodic cycles of spleen cell cytokine secretions and nitric oxide and interleukin-6 by peritoneal macrophages. The E. chaffeensis-specific immunoglobulin G response was considerably higher and steadily increased in mice infected with the tick cell-derived E. chaffeensis compared to DH82-grown bacteria. In addition, antigens detected by the immunoglobulins were significantly different between mice infected with the E. chaffeensis originating from tick cells or macrophages. The differences in the immune response to tick cell-grown bacteria compared to macrophage-grown bacteria reflected a delay in the shift of gene expression from the tick cell-specific Omp 14 gene to the macrophage-specific Omp 19 gene. These data suggest that the host response to E. chaffeensis depends on the source of the bacteria and that this experimental model requires the most natural inoculum possible to allow for a realistic understanding of host resistance.

Enzyme-linked immunosorbent assay with conserved immunoreactive glycoproteins gp36 and gp19 has enhanced sensitivity and provides species-specific immunodiagnosis of Ehrlichia canis infection

Ehrlichia canis is the primary etiologic agent of canine monocytic ehrlichiosis, a globally distributed and potentially fatal disease of dogs. We previously reported on the identification of two conserved major immunoreactive antigens, gp36 and gp19, which are the first proteins to elicit an E. canis-specific antibody response, and gp200 and p28, which elicit strong antibody responses later in the acute phase of the infection. In this report, the sensitivities and specificities of five recombinant E. canis proteins for the immunodiagnosis of E. canis infection by an enzyme-linked immunosorbent assay (ELISA) were evaluated. Recombinant polypeptides gp36, gp19, and gp200 (N and C termini) exhibited 100% sensitivity and specificity for immunodiagnosis by the recombinant glycoprotein ELISA compared with the results obtained by an indirect fluorescent-antibody assay (IFA) for the detection of antibodies in dogs that were naturally infected with E. canis. Moreover, the enhanced sensitivities of gp36 and gp19 for immunodiagnosis by the recombinant glycoprotein ELISA compared to those obtained by IFA were demonstrated with dogs experimentally infected with E. canis, in which antibodies were detected as much as 2 weeks earlier, on day 14 postinoculation. gp36 and gp19 were not cross-reactive with antibodies in sera from E. chaffeensis-infected dogs and thus provided species-specific serologic discrimination between E. canis and E. chaffeensis infections. This is the first demonstration of the improved detection capability of the recombinant protein technology compared to the capability of the "gold standard" IFA and may eliminate the remaining obstacles associated with the immunodiagnosis of E. canis infections, including species-specific identification and the lack of sensitivity associated with low antibody titers early in the acute phase of the infection.

L-selectin and E-selectin expressed on monocytes mediating Ehrlichia chaffeensis attachment onto host cells

Ehrlichia chaffeensis, the agent of human monocytic ehrlichiosis, is an obligatory intracellular bacterium that exhibits monocytic host cell tropism. Ehrlichiae must enter the host cell, and then establish infection. The tropism of E. chaffeensis for monocytes suggests that the cell contains some specific surface components that mediate E. chaffeensis attachment and entry into host cells. In this study, host cell surface components that play a role in ehrlichial attachment were identified using a human monocyte/macrophage cell line, THP-1. E. chaffeensis attachment to THP-1 cells was partially blocked in the presence of antibodies to E-selectin and L-selectin, but not by antibodies to P-selectin, integrin alpham, integrin alphax, or normal mouse IgG as determined by real time polymerase chain reaction. Conversely, in HeLa cells that do not exhibit surface expression of E-selectin and L-selectin, antibodies to these cell surface proteins did not inhibit E. chaffeensis attachment. These findings indicate that E-selectin and L-selectin are cell surface proteins that might act as co-receptors and contribute to E. chaffeensis attachment and entry into THP-1.

Ehrlichia chaffeensis: a prototypical emerging pathogen

Ehrlichia chaffeensis is an obligately intracellular, tick-transmitted bacterium that is maintained in nature in a cycle involving at least one and perhaps several vertebrate reservoir hosts. The moderate to severe disease caused by E. chaffeensis in humans, first identified in 1986 and reported for more than 1,000 patients through 2000, represents a prototypical "emerging infection." Knowledge of the biology and natural history of E. chaffeensis, and of the epidemiology, clinical features, and laboratory diagnosis of the zoonotic disease it causes (commonly referred to as human monocytic ehrlichiosis [HME]) has expanded considerably in the period since its discovery. In this review, we summarize briefly the current understanding of the microbiology, pathogenesis, and clinical manifestations associated with this pathogen but focus primarily on discussing various ecological factors responsible for the recent recognition of this important and potentially life-threatening tick-borne disease. Perhaps the most pivotal element in the emergence of HME has been the staggering increases in white-tailed deer populations in the eastern United States during the 20th century. This animal serves as a keystone host for all life stages of the principal tick vector (Amblyomma americanum) and is perhaps the most important vertebrate reservoir host for E. chaffeensis. The contributions of other components, including expansion of susceptible human populations, growth and broadening geographical distributions of other potential reservoir species and A. americanum, and improvements in confirmatory diagnostic methods, are also explored.

Cultivation of an ovine strain of Ehrlichia phagocytophila in tick cell cultures

Ehrlichia phagocytophila (previously known as Cytoecetes phagocytophila) which causes tick-borne fever (TBF) in sheep and pasture fever in cattle in the UK and mainland Europe is transmitted by the temperate hard tick Ixodes ricinus. The disease in sheep is characterized by fever, leucopenia and immunosuppression. Studies on the pathogenesis and other aspects of the disease have been hampered because the organism has not been cultivated in continuous or primary cell culture systems. This paper describes the first successful cultivation of a European isolate of E. phagocytophila in two continuous cell lines, IDE8 and ISE6, derived from the temperate hard tick Ixodes scapularis. Once adapted to tick cell cultures the organism was serially sub-cultured in new cells by transferring small portions of infected cell suspension every 2 to 3 weeks. The identity of the organism was confirmed by polymerase chain reaction (PCR), with primers specific to the granulocytic ehrlichiae. Sequence analysis of the PCR products amplified from infected tick cells were shown to be identical with those amplified from the blood of sheep infected with the same strain of E. phagocytophila. A susceptible sheep inoculated with a third passage of the tick cell-adapted E. phagocytophila reacted with fever and rickettsiaemia 5 days later, thus satisfying Koch's postulates.

Cultivation of Ehrlichia canis in a continuous BALB/C mouse macrophage cell culture line

This report describes the successful adaptation of the Israeli isolate of Ehrlichia canis on a continuous mouse macrophage cell line (J774.A1). Successful infection of the J774.AI cells was first judged by the direct immunofluorescence antibody test using an anti-E. canis-IgG:FITC conjugate. A particular property of infected J774.A1 cells was the ability to reestablish after harvesting of the monolayer by scaping. Infected cells were used as antigen for immunofluorescence antibody tests (IFA), and the results compared well with those of DH82 cells. It was concluded that the J774.A1 continuous cell line could serve as an alternate propagation cell line for E. canis organisms.

Isolation and characterization of Ehrlichia chaffeensis strains from patients with fatal ehrlichiosis

Two new isolates of Ehrlichia chaffeensis (designated Jax and St. Vincent) were obtained from patients with fatal ehrlichial infections. Patients developed characteristic manifestations of severe disease due to E. chaffeensis, including marked thrombocytopenia, pulmonary insufficiency, and encephalopathy. Primary isolation was achieved in DH82 cells; the Jax and St. Vincent isolates were detected within 19 and 8 days postinoculation, respectively. The isolates were characterized by molecular evaluation of the 16S rRNA gene, the groESL heat shock operon, a 120-kDa immunodominant protein gene, and an incompletely characterized repetitive-motif sequence (variable-length PCR target [VLPT]). The sequences were compared with those of the corresponding molecular regions in the type isolate (Arkansas). St. Vincent contained one fewer repeat unit in both the 120-kDa protein gene and the VLPT compared with corresponding sequences of the Jax and Arkansas isolates. 16S rRNA gene sequences from the two new isolates had 100% identity to the corresponding sequences of the 91HE17 and Sapulpa isolates of E. chaffeensis, and to the corrected 16S rRNA gene sequence of the Arkansas isolate. The Jax isolate grew more slowly than the St. Vincent isolate in DH82 cells, and both of the new isolates grew more slowly than the extensively passaged Arkansas isolate. Although specific associations between ehrlichial pathogenicity and genotype were not identified from these comparisons, recovery of this organism from a spectrum of clinical presentations remains an integral step in understanding mechanisms of disease caused by E. chaffeensis.

Emergence of the ehrlichioses as human health problems

Ehrlichiae are small, gram-negative, obligately intracellular bacteria that reside within a phagosome. The first human ehrlichial infection was recognized in the United States in 1987. It was later shown to be caused by a new species, Ehrlichia chaffeensis. In 1994, an ehrlichial pathogen within neutrophils that is closely related to the known veterinary pathogens E. equi and E. phagocytophila was found to infect humans. Molecular methods were required to detect, characterize, and identify these fastidious and uncultivated bacteria. Subsequently, E. chaffeensis infection was documented in more than 400 patients in 30 states, Europe, and Africa. Likewise, approximately 170 cases of human granulocytic ehrlichiosis have been diagnosed, most since 1994, predominantly in the upper midwestern and northeastern states, but also in northern California. The disease caused by ehrlichiae is generally undifferentiated but is often associated with leukopenia, thrombocytopenia, and elevated serum hepatic transaminase levels in tick-exposed patients. Infection ranges from subclinical to fatal; tetracycline appears to be an effective therapy. The emergence of these two newly recognized tickborne infections as threats to human health is probably due to increased clinical cognizance, but as in other emerging tickborne infections, it is likely that the rapid increase in identified cases signals a true emergence of disease associated with a changing vector-host ecology.

Isolation and characterization of a new strain of Ehrlichia chaffeensis from a patient with nearly fatal monocytic ehrlichiosis

Ehrlichia chaffeensis is the causative agent of human monocytic ehrlichiosis, a disease that ranges in severity from asymptomatic infection to death. Only one isolate of E. chaffeensis has been made, the Arkansas strain, upon which all characterizations of the agent of human monocytic ehrlichiosis have been based. We report the isolation and characterization of a new strain of E. chaffeensis, the 91HE17 strain, which was cultivated from a patient with a nearly fatal illness. The new isolate grows best in culture with careful control of pH. The two isolates are nearly identical as determined by light and electron microscopy and have significant antigenic identity in fluorescent-antibody and immunoblot assays using polyclonal antisera and the E. chaffeensis-specific monoclonal antibody 1A9. Isolate 91HE17 had 99.9% nucleotide sequence identity with the Arkansas strain in the 16S rRNA gene. Parts of the Escherichia coli GroE operon homologs had identical restriction enzyme digestion patterns, and a 425-bp region of the GroEL gene had at least 99.8% sequence identity between the E. chaffeensis Arkansas and 91HE17 strains. Isolate 91HE17 lacked an epitope identified in E. chaffeensis Arkansas by the monoclonal antibody 6A1. This new E. chaffeensis isolate is very similar to the Arkansas strain and provides the opportunity to substantiate the existence of diversity among ehrlichiae which infect humans. Specific factors which differ among strains may then be compared to assess their potential contributions toward cellular pathogenicity and ultimately toward the development of disease in humans.