Electroporation is a rapid, broadly adopted method for introducing exogenous DNA into the genus Rickettsia. This protocol provides a useful electroporation method for the transformation of obligate intracellular bacteria in the genus Rickettsia.
Rickettsioses are caused by a broad range of obligate intracellular bacteria belonging to the genus Rickettsia that can be transmitted to vertebrate hosts through the bite of infected arthropod vectors. To date, emerging or re-emerging epidemic rickettsioses remain a public health risk due to the difficulty in diagnosis, as diagnostic methods are limited and not standardized or universally accessible. Misdiagnosis resulting from a lack of recognition of the signs and symptoms may result in delayed antibiotic treatment and poor health outcomes. A comprehensive understanding of Rickettsia characteristics would ultimately improve clinical diagnosis, assessment, and treatment with improved control and prevention of the disease.
Functional studies of rickettsial genes are crucial for understanding their role in pathogenesis. This paper describes a procedure for the electroporation of the Rickettsia parkeri strain Tate’s Hell with the shuttle vector pRAM18dSFA and the selection of transformed R. parkeri in tick cell culture with antibiotics (spectinomycin and streptomycin). A method is also described for the localization of transformed R. parkeri in tick cells using confocal immunofluorescence microscopy, a useful technique for checking transformation in vector cell lines. Similar approaches are also suitable for the transformation of other rickettsiae.
Rickettsioses are caused by a broad range of obligate intracellular bacteria that belong to the genus Rickettsia (family Rickettsiaceae, order Rickettsiales). The genus Rickettsia is classified into four major groups based on phylogenetic characteristics1,2: the spotted fever group (SFG), which contains those rickettsiae that cause the most severe and fatal tick-borne rickettsioses (e.g., Rickettsia rickettsii, the causative agent of Rocky Mountain Spotted Fever), the typhus group (TG, e.g., Rickettsia prowazekii, the agent of epidemic typhus), the transitional group (TRG, e.g., Rickettsia felis, the causative agent of flea-borne spotted fever), and the ancestral group (AG, e.g., Rickettsia bellii).
Among the oldest known vector-borne diseases, rickettsioses are mainly acquired following transmission of the pathogens through the bites of infected arthropod vectors, including ticks, fleas, lice, and mites3,4. Although the discovery of effective antibiotics improved treatment outcomes, emerging and re-emerging epidemic rickettsioses continue to challenge traditional prevention and control strategies. Thus, a comprehensive understanding of rickettsia/host/vector interactions would ultimately establish a strong foundation for developing new approaches to prevent and cure these ancient diseases.
In nature, horizontal gene transfer (HGT) in bacteria occurs through conjugation, transduction, and transformation5. In vitro bacterial transformation utilizes these HGT concepts, although the intracellular nature of rickettsiae presents some challenges. The restricted growth conditions and poorly understood conjugation and transduction systems in different species of rickettsiae have prevented the application of conjugation and transduction methods in rickettsiae6,7,8. Compared with other obligate intracellular bacterial genera (e.g., Chlamydia, Coxiella, Anaplasma, and Ehrlichia), the genus Rickettsia differs with regard to the growth and replication strategies within the cell cytoplasm, which imposes specific challenges to the genetic modification of rickettsiae due to their unique lifestyle features9.
The initial hurdle to overcome when attempting the genetic modification of rickettsiae is to achieve successful transformation. Thus, designing a feasible approach with high transformation efficiency would be extremely valuable for developing genetic tools for rickettsiae. Here, we focus on electroporation, a broadly recognized transformation method that has been used to introduce exogenous DNA successfully into several species of rickettsiae, including Rickettsia prowazekii, Rickettsia typhi, Rickettsia conorii, Rickettsia parkeri, Rickettsia montanensis, Rickettsia bellii, Rickettsia peacockii, and Rickettsia buchneri10,11,12,13,14,15,16.
This paper describes a procedure for the electroporation of the R. parkeri strain Tate's Hell (accession: GCA_000965145.1) with the shuttle vector pRAM18dSFA derived from the Rickettsia amblyommatis strain AaR/SC plasmid pRAM18 engineered to encode mKATE, a far-red fluorescent protein, and aadA, conferring spectinomycin and streptomycin resistance13,15,20. Transformed R. parkeri are viable and stably maintained under antibiotic selection in tick cell lines. In addition, we show that the localization of transformed R. parkeri in live tick cells via confocal microscopy can be used to assess the quality of transformation rates in vector cell lines.
1. Propagation and purification of R. parkeri from tick cell culture
NOTE: All cell culture procedures are to be performed in a class II biosafety cabinet.
2. Transformation of R. parkeri with the pRAM18dSFA plasmid
3. Observation of the transformed R. parkeri
NOTE: Use an epifluorescence microscope with rhodamine/TRITC filters to observe the flasks prepared in step 2.11 after 3-7 days. Once plaques are evident in the cultures (5-14 days), transformed R. parkeri can be seen that express the red fluorescent protein mKATE, encoded on the pRAM18dSFA plasmid.
The morphology of R. parkeri in ISE6 cells under a light microscope after Giemsa staining are shown in Figure 1. In Figure 2, transformed R. parkeri expressing red fluorescence protein in ISE6 cells are shown using confocal microscopy. There is a substantial increase in the infection rate of transformed R. parkeri (red) in ISE6 cells (blue, corresponds to the nuclei) from (A) day 7 to (B) day 10 of incubation.
Figure 1: Slides showing Giemsa-stained Rickettsia parkeri in ISE6 cells. ISE6 cells infected with wild-type R. parkeri at (A) low infection and (B) 90%-100% infection rates. The nuclei of the ISE6 cells stain to dark purple with Giemsa; the R. parkeri appear as dark purple rods. The red boxes show intracellular rickettsiae, and the red asterisks indicate extracellular rickettsiae. All images were taken on a light microscope with a 100x objective. Scale bars = 50 µm. Abbreviation: N = nuclei. Please click here to view a larger version of this figure.
Figure 2: Transformed Rickettsia parkeri expressing red fluorescence protein in ISE6 cells. pRAM18dSFA-transformed R. parkeri (red) in ISE6 cells (blue), stained with Hoechst 33342 and detected by confocal microscopy (A) 7 days and (B) 10 days after transformation. Hoechst 33342 stains the nuclei, and its excitation and emission wavelengths are similar to DAPI. The merged signals are a combination of DAPI and TRITC filter images. All images were taken on a confocal microscope with a 60x objective. Scale bars = 20 µm. Abbreviations: DAPI = 4',6-diamidino-2-phenylindole; TRITC = tetramethylrhodamine. Please click here to view a larger version of this figure.
Here, we demonstrate a method for introducing exogenous DNA encoded on the shuttle plasmid pRAM18dSFA into rickettsiae using electroporation. In this procedure, cell-free rickettsiae were purified from host cells, transformed with a rickettsial shuttle vector, and released onto tick cells for infection. Also described is a confocal immunofluorescence procedure to detect red fluorescence protein-expressing R. parkeri in tick cells. Similar methods are applicable to other Rickettsia species and with further modifications might be adaptable to other obligate intracellular bacteria that are capable of maintaining a plasmid.
A successful transformation in this protocol requires three components: exogenous DNA, Rickettsia spp. bacteria, and a suitable type of host cell22. Depending on the specific research goal, these components can be changed. First, the exogenous DNA was introduced via the pRAM18dSFA plasmid, which contains spectinomycin- and streptomycin-selectable markers and expresses mKATE (a far-red fluorescent protein). This plasmid also confers ampicillin resistance for growth in Escherichia coli. As shown in previous studies, it is possible to replace antibiotic-resistance markers and the genes for fluorescent proteins13. Second, ISE6 cells were selected to represent transformed cell lines as this cell line is extensively used in many laboratories and is an essential model for the study of vector-pathogen interactions17,18,19. Other species of tick16 or mammalian cells23,24 can also be used to grow rickettsiae for transformation. Finally, in this protocol, R. parkeri was used, which can be manipulated in a biosafety level 2 (BSL-2) laboratory using a class II biosafety cabinet with proper safety precautions. Other more pathogenic Rickettsia spp. (e.g., R. rickettsii; R. prowazekii) require a BSL-3 setting for manipulation; therefore, a more stringent biosafety standard protocol must be used when working with such agents.
Although this protocol can be used as a standard procedure for rickettsiae transformation, several key technical steps require particular attention. First, a successful purification step must ensure cell-free rickettsiae survival and infectivity9; hence, the most critical aspect of the transformation procedure is to isolate infectious, cell-free rickettsiae. Any residual salt in the purified rickettsiae will result in arcing and transformation failure. Therefore, washing the isolated cell-free rickettsiae with sucrose is required to remove all salts. The cold sucrose solution also protects the rickettsial membrane integrity in the extracellular condition.
When 90%-100% of the cells in a culture are infected, the rickettsiae should be isolated from the ISE6 cells, and electroporation should be performed without delay and without interruptions, as rickettsiae are intracellular organisms and do not survive well outside cells. Although rickettsial cultures can be stored at 4 °C, this type of material should not be used for electroporation. A culture that has been stored at 4 °C can be used to inoculate a fresh layer of ISE6 cells, which can then be used for electroporation once infection has reached 90%-100%.
Second, the electroporation settings, including voltage, resistance, capacitance, and time constant, are specific to different rickettsial species. For example, the field strength required during electroporation is dependent on the size of the rickettsiae and exogenous DNA7. Finally, it is important to maintain the transformed rickettsiae under antibiotic selection to prevent the loss of exogenous plasmids during multiple passages in host cells9. The concentration and type of antibiotic employed are dependent on the Rickettsia species being transformed, as previously described13,15,16,23, and it is important that the antibiotic marker selected should not be one that is applied in clinical treatments.
To follow this protocol, both spectinomycin and streptomycin should be used for selection until all the residual untransformed rickettsiae have been eliminated. Combined, the two antibiotics kill both intracellular and extracellular rickettsiae and reduce the probability of resistant wild-type rickettsiae emerging. In addition, using both of these antibiotics does not affect downstream experiments, such as the ability to use them separately to select for two different plasmids, since the aadA gene confers resistance to both spectinomycin and streptomycin. The two antibiotics used in this protocol do not confer resistance to antibiotics used in the clinical treatment of rickettsial disease.
The ISE6 tick cell line utilized in this study has unique advantages. First, ISE6 cells were isolated from the black-legged tick Ixodes scapularis Say (Acari: Ixodidae), the principal vector for seven human pathogens in the United States. Second, ISE6 is an extensively used tick cell line in many laboratories and has been successfully used to recover many tick-associated bacteria (Rickettsia, Anaplasma, and Ehrlichia) following electroporation. Third, some pathogenic rickettsiae can only be propagated in tick cells but not in mammalian cells17,18,19,24. However, tick cell lines are relatively fragile compared to mammalian cells and have more intensive culture requirements25,26,27,28. In addition, the growth rate of ISE6 cells is significantly slower than that of mammalian cell lines, even if seeded at the same initial density, although slow replication can be advantageous when the rickettsial transformants exhibit reduced growth.
This protocol also provides a method to evaluate the transformation efficiency of rickettsiae in live cells at critical experimental steps, which helps to optimize electroporation settings or in testing the efficiency of various cell lines for recovering transformants. Nevertheless, this protocol has limitations, especially for the quantification of the transformants obtained. Transformation efficiency as an indicator of a successful transformation could be demonstrated in a future study. Rickettsial viability could be assessed using a suitable staining kit and flow cytometry21 to quantify the transformants that are obtained. Two parameters would be used to determine transformation efficiency-the number of live rickettsiae used for transformation and the number of transformed rickettsiae that express mKATE.
In addition, image analysis with fluorescence signals could be used to compare the transformation rates of different plasmids. As the underlying mechanism of rickettsial plasmid maintenance is poorly understood, well-characterized transformation systems for introducing exogenous plasmids can be valuable tools for further investigation. Furthermore, the direct visualization of fluorescent protein-expressing rickettsiae in cells or tissues will improve our understanding of rickettsia/host/vector interactions. This will provide information for designing strategies to control and prevent rickettsioses.
Data availability:
All data underlying the results of this study are publicly available.
The authors have nothing to disclose.
We thank Timothy J. Kurtti and Benjamin Cull for their insightful discussions and suggestions. This study was financially supported by a grant to U.G.M. from the NIH (2R01AI049424) and a grant to U.G.M. from the Minnesota Agricultural Experiment Station (MIN-17-078).
0.1 cm gap gene pulser electroporation cuvette | Bio-Rad | 1652083 | |
2 μm pore size filter | GE Healthcare Life Sciences Whatman | 6783-2520 | |
5 mL Luer-lock syringe | BD | 309646 | |
60-90 silicon carbide grit | LORTONE, inc | 591-056 | |
absolute methanol | Fisher Scientific | A457-4 | |
Bacto tryptose phosphate broth | BD | 260300 | |
Cytospin centrifuge Cytospin4 | Thermo Fisher Scientific | A78300003 | The rotor is detatchable so the whole rotor can be put into the hood to load infectious samples |
EndoFree Plasmid Maxi Kit (10) | QIAGEN | 12362 | used to obtain endotoxin-free pRAM18dSFA plasmid |
extended fine tip transfer pipet | Perfector Scientific | TP03-5301 | |
fetal bovine serum | Gemini Bio | 900-108 | The FBS batch has to be tested to make sure ISE6 cells will grow well in it. |
Gene Pulser II electroporator with Pulse Controller PLUS | Bio-Rad | 165-2105 & 165-2110 | |
hemocytometer | Thermo Fisher Scientific | 267110 | |
HEPES | Millipore-Sigma | H4034 | |
ImageJ Fiji | National Institute of Health | raw image editing | |
KaryoMAX Giemsa stain | Gibco | 2021-10-30 | |
Leibovitz's L-15 medium | Gibco | 41300039 | |
lipoprotein concentrate | MP Biomedicals | 191476 | |
Nikon Diaphot | Nikon | epifluorescence microscope | |
NucBlue Live ReadyProbes Reagent | Thermo Fisher Scientific | R37605 | |
Olympus Disc Scanning Unit (DSU) confocal microscope | Olympus | ||
Petroff-Hausser Counting Chamber | Hausser Scientific | Chamber 3900 | |
sodium bicarbonate | Millipore-Sigma | S5761 | |
Vortex | Fisher Vortex Genie 2 | 12-812 |