Genetic and biochemical diversity among isolates of Paenibacillus alvei cultured from Australian honeybee (Apis mellifera) colonies

Twenty-five unique CfoI-generated whole-cell DNA profiles were identified in a study of 30 Paenibacillus alvei isolates cultured from honey and diseased larvae collected from honeybee (Apis mellifera) colonies in geographically diverse areas in Australia. The fingerprint patterns were highly variable and readily discernible from one another, which highlighted the potential of this method for tracing the movement of isolates in epidemiological studies. 16S rRNA gene fragments (length, 1,416 bp) for all 30 isolates were enzymatically amplified by PCR and subjected to restriction analysis with DraI, HinfI, CfoI, AluI, FokI, and RsaI. With each enzyme the restriction profiles of the 16S rRNA genes from all 30 isolates were identical (one restriction fragment length polymorphism [RFLP] was observed in the HinfI profile of the 16S rRNA gene from isolate 17), which confirmed that the isolates belonged to the same species. The restriction profiles generated by using DraI, FokI, and HinfI differentiated P. alvei from the phylogenetically closely related species Paenibacillus macerans and Paenibacillus macquariensis. Alveolysin gene fragments (length, 1, 555 bp) were enzymatically amplified from some of the P. alvei isolates (19 of 30 isolates), and RFLP were detected by using the enzymes CfoI, Sau3AI, and RsaI. Extrachromosomal DNA ranging in size from 1 to 10 kb was detected in 17 of 30 (57%) P. alvei whole-cell DNA profiles. Extensive biochemical heterogeneity was observed among the 28 P. alvei isolates examined with the API 50CHB system. All of these isolates were catalase, oxidase, and Voges-Proskauer positive and nitrate negative, and all produced acid when glycerol, esculin, and maltose were added. The isolates produced variable results for 16 of the 49 biochemical tests; negative reactions were recorded in the remaining 30 assays. The genetic and biochemical heterogeneity in P. alvei isolates may be a reflection of adaptation to the special habitats in which they originated.

Contribution of adjuvant to adaptive immune responses in mice against Actinobacillus pleuropneumoniae

The authors have previously demonstrated that adjuvant-mediated differences in early cellular responses to antigens significantly affect subsequent adaptive immune responses. To investigate further the contribution of adjuvant to adaptive immune responses, outer-membrane proteins (OMP) purified from the respiratory pathogen Actinobacillus pleuropneumoniae, given either alone (antigen group) or complexed with SAMA4 (vaccine group), were injected intradermally into groups of mice. Controls were given PBS. Inclusion of adjuvant did not significantly alter the kinetics of antibody responses against OMP in serum or respiratory tract washings (RTW) over 21 weeks. Re-exposure to OMP at 21 weeks also induced identical recall responses in both immunized groups. However, differences between the responses of the vaccine and antigen groups were apparent when sera and RTW were reacted against OMP and OMP-derived polysaccharide antigens (ODPA). Serum and RTW reactivity against protein antigens was stronger in the vaccine group than in the antigen group. Serum and RTW from the vaccine group also reacted against a greater number of proteins than did the antigen group. Although serum reactivity against ODPA was equivalent for both groups, RTW from the vaccine group reacted only faintly against ODPA compared with the antigen group. The results suggested that shifting of antibody reactivity away from polysaccharide antigens toward protein antigens was an adjuvant-mediated effect. The rapid death of controls following intranasal inoculation confirmed that protection was ultimately dependent on the presence of specific antibodies in the serum and respiratory tract. However, since both groups responded equally to intranasal infection with A. pleuropneumoniae, as seen by the rapid clearance of bacteria from the lungs, the biological significance of any differences between the groups was unclear. Knowledge of the effects of adjuvants may provide a rational basis for adjuvant selection and the ability to manipulate immunological outcomes more precisely.

Geographically diverse Australian isolates of Melissococcus pluton exhibit minimal genotypic diversity by restriction endonuclease analysis

Melissococcus pluton, the causative agent of European foulbrood is an economically significant disease of honey bees (Apis mellifera) across most regions of the world and is prevalent throughout most states of Australia. 49 Isolates of M. pluton recovered from diseased colonies or honey samples in New South Wales, Queensland, South Australia, Tasmania and Victoria were compared using SDS-PAGE, Western immunoblotting and restriction endonuclease analyses. DNA profiles of all 49 geographically diverse isolates showed remarkably similar AluI profiles although four isolates (one each from Queensland, South Australia, New South Wales and Victoria) displayed minor profile variations compared to AluI patterns of all other isolates. DNA from a subset of the 49 Australian and three isolates from the United Kingdom were digested separately with the restriction endonucleases CfoI, RsaI and DraI. Restriction endonuclease fragment patterns generated using these enzymes were also similar although minor variations were noted. SDS-PAGE of whole cell proteins from 13 of the 49 isolates from different states of Australia, including the four isolates which displayed minor profile variations (AluI) produced indistinguishable patterns. Major immunoreactive proteins of approximate molecular masses of 21, 24, 28, 30, 36, 40, 44, 56, 60, 71, 79 and 95 kDa were observed in immunoblots of whole cell lysates of 22 of the 49 isolates and reacted with rabbit hyperimmune antibodies raised against M. pluton whole cells. Neither SDS-PAGE or immunoblotting was capable of distinguishing differences between geographically diverse isolates of M. pluton. Collectively these data confirm that Australian isolates of M. pluton are genetically homogeneous and that this species may be clonal. Plasmid DNA was not detected in whole cell DNA profiles of any isolate resolved using agarose gel electrophoresis.

Detection of shiga-like toxin (stx1 and stx2), intimin (eaeA), and enterohemorrhagic Escherichia coli (EHEC) hemolysin (EHEC hlyA) genes in animal feces by multiplex PCR

A multiplex PCR was developed for the rapid detection of genes encoding Shiga toxins 1 and 2 (stx1 and stx2), intimin (eaeA), and enterohemolysin A (hlyA) in 444 fecal samples derived from healthy and clinically affected cattle, sheep, pigs, and goats. The method involved non-solvent-based extraction of nucleic acid from an aliquot of an overnight culture of feces in EC (modified) broth. The detection limit of the assay for both fecal samples and pure cultures was between 18 and 37 genome equivalents. stx1 and hlyA were the most commonly encountered virulence factors.

Demonstration that Australian Pasteurella multocida isolates from sporadic outbreaks of porcine pneumonia are non-toxigenic (toxA-) and display heterogeneous DNA restriction endonuclease profiles compared with toxigenic isolates from herds with progressive atrophic rhinitis

Capsular types A and D of Pasteurella multocida cause economic losses in swine because of their association with progressive atrophic rhinitis (PAR) and enzootic pneumonia. There have been no studies comparing whole-cell DNA profiles of isolates associated with these two porcine respiratory diseases. Twenty-two isolates of P. multocida from diseased pigs in different geographic localities within Australia were characterised genotypically by restriction endonuclease analysis (REA) with the enzyme CfoI. Seven of 12 P. multocida isolates from nasal swabs from pigs in herds where PAR was either present or suspected displayed a capsular type D phenotype. These were shown to possess the toxA gene by polymerase chain reaction (PCR) and Southern hybridisation, and further substantiated by production of cytotoxin in vitro. The CfoI profile of one of these seven isolates, which was from the initial outbreak of PAR in Australia (in Western Australia, WA), was identical with profiles of all six other toxigenic isolates from sporadic episodes in New South Wales (NSW). The evidence suggests that the strain involved in the initial outbreak was responsible for the spread of PAR to the eastern states of Australia. Another 10 isolates, representing both capsular types A and D, were isolated exclusively from porcine lung lesions after sporadic outbreaks of enzootic pneumonia in NSW and WA. CfoI restriction endonuclease profiles of these isolates revealed considerable genomic heterogeneity. Furthermore, none of these possessed the toxA gene. This suggests that P. multocida strains with the toxA gene do not have a competitive survival advantage in the lower respiratory tract or that toxin production does not play a role in the pathology of pneumonic lesions, or both. REA with polyacrylamide gel electrophoresis and silver staining was found to be a practical and discriminatory tool for epidemiological tracing of P. multocida outbreaks associated with PAR or pneumonia in pigs.

Reiterated repeat region variability in the ciliary adhesin gene of Mycoplasma hyopneumoniae

Mycoplasma hyopneumoniae is a highly prevalent pathogen which colonizes the ciliated epithelial lining of the porcine respiratory tract. Expression libraries constructed from genomic DNA of the non-pathogenic strain M. hyopneumoniae J were screened with porcine hyperimmune antiserum against M. hyopneumoniae. One clone expressed a 28 kDa protein which was also reactive with monospecific antiserum raised against a putative M. hyopneumoniae-specific 94 kDa antigen derived from strain J. Trypsin digestion of whole M. hyopneumoniae cells showed the 94 kDa antigen to be surface-accessible. DNA sequence analysis of the gene encoding the 94 kDa antigen revealed greater than 90% homology to two adhesin genes, encoding P97 and Mhp1, cloned from pathogenic strain 232 and strain P5722 of M. hyopneumoniae, respectively. Two regions of repetitive DNA sequence were identified in the gene encoding the 94 kDa antigen. The first encoded the deduced amino acid sequence A(T)-K-P-E(V)-A(T) arranged as nine tandem repeats (RR1). The second region of repetitive DNA sequence encoded the deduced amino acid sequence G-A(E,S)-P-N(S)-Q-G-K-K-A-E arranged as five tandem repeats (RR2). Comparison of the three M. hyopneumoniae adhesin genes revealed that the genes encoding P97 and Mhp1, and the strain J gene encoding the 94 kDa antigen contained 15, 12 and 9 tandem repeats, respectively, in RR1, and 4, 5 and 5 tandem repeats, respectively, in RR2. Southern hybridization analysis of EcoRI-digested genomic DNA probed with an 820 bp fragment spanning RR1 and RR2 identified a strongly hybridizing fragment ranging in size from 2.15 to 2.30 kb among seven geographically diverse strains of M. hyopneumoniae but failed to hybridize with DNA from four strains of Mycoplasma hyorhinis or Mycoplasma flocculare strain Ms42. PCR primers flanking the DNA sequence encoding RR1 and RR2 were used to amplify DNA from the seven strains of M. hyopneumoniae and DNA sequence analysis of the amplification products showed that the number of tandem amino acid repeats in RR1 varied considerably between strains. RR1 from M. hyopneumoniae strains YZ, Beaufort, Sue, OMZ407 and C1735/2 comprised 11, 15, 12, 15 and 8 tandem copies, respectively, of the 5-aa repeat whilst RR2 comprised 4, 3, 4, 3 and 4 tandem copies, respectively, of the 10-aa repeat. Two putative integrin binding sites (L-E-T and R-X-X-X-D) were identified in the 94 kDa ciliary adhesin. Variability in the number of amino acid repeats in RR1 amongst strains of M. hyopneumoniae may influence ciliary binding.

Flow cytometric analysis of cellular changes in mice after intradermal inoculation with a liposome-iscom adjuvanted vaccine

As it is not known what changes to leucocyte homeostasis are mandatory for effective adjuvant action, the biological relevance of systemic changes elicited by different vaccine formulations can only be interpreted in the context of the immunological outcomes. We used flow cytometry to quantify the changes in leucocyte subsets induced in mice intradermally immunized with SAMA4 (adjuvant group), outer membrane proteins (OMP) purified from Actinobacillus pleuropneumoniae (OMP antigen group), SAMA4 adjuvanted OMP (OMP vaccine group), or phosphate-buffered saline (PBS: control group). This approach allowed direct comparisons to be made between the effects of antigen, adjuvant or antigen-adjuvant complexes on immune effector cell populations. Antigens complexed with the liposome-iscom hybrid adjuvant, SAMA4, generated strong antibody responses and cytotoxic T-cell activity in animals immunized intradermally, reflecting remobilization and recruitment of specific cell populations. Splenomegaly, due to granulocytosis, monocytosis and megakaryocytosis, was most prominent in the OMP vaccine group. Histological examination of spleen sections confirmed that these changes were due primarily to splenic haematopoiesis. Circulating numbers of granulocytes and monocytes increased significantly (P < 0.05) in the blood of the OMP vaccine group, as did granulocyte numbers in the lungs (P < 0.05). No changes in T- and B-cell numbers were detected by flow cytometry in the spleens, lungs or blood over the 28-day period in any treatment group. Thymocyte numbers (predominantly CD4+CD8+ cells) in the OMP vaccine group fell by 95% within 3 days of immunization. Identical cellular responses were obtained when an innocuous antigen, ovalbumin, was complexed with SAMA4 instead of OMP, thus demonstrating that the adjuvant effects of SAMA4 were due to synergistic interaction between antigen and adjuvant and not due to the presence of toxic components. The association of strong adaptive immune responses with such complex changes in leucocyte homeostasis induced by complexing adjuvant and antigen suggested that the changes were important for effective vaccination and were not purely circumstantial.

Serum and mucosal antibody responses and protection in pigs vaccinated against Mycoplasma hyopneumoniae with vaccines containing a denatured membrane antigen pool and adjuvant

OBJECTIVE

To investigate the protective efficacy of a pool of denatured membrane protein antigens of Mycoplasma hyopneumoniae (J strain) in the molecular size range 70 to 85 kDa (F3 antigen) in combination with adjuvants for pigs challenged with M hyopneumoniae.

DESIGN

A vaccine efficacy experiment with assessment of serum and respiratory tract antibody responses.

PROCEDURE

F3 antigens were emulsified with five different adjuvants. To groups of three pigs per vaccine, four vaccines were given by intramuscular injection, and two vaccines, including one of those given intramuscularly, were given by intraperitoneal injection.

RESULTS

Compared to six unvaccinated pigs, animals vaccinated with F3 antigen displayed significantly reduced pneumonia (54% reduction in mean lung score) following experimental challenge. Analysis of post-vaccination, pre-challenge IgG and IgA ELISA antibody absorbances in serum and respiratory tract washings revealed no correlation with lung score. Six weeks after challenge, pigs previously vaccinated intramuscularly mostly demonstrated greater IgG and IgA responses in respiratory tract washings, and greater IgG serum antibody responses, than those vaccinated by intraperitoneal injection.

CONCLUSION

Pigs vaccinated with M hyponeumoniae antigens in the molecular size range of 70 to 85 kDa showed a significant reduction in lung lesions compared with unvaccinated control animals after experimental challenge. IgG and IgA antibody concentrations in serum and respiratory tract washings after vaccination do not provide a useful prognostic indicator of protection from enzootic pneumonia.

Oral immunization of mice with attenuated Salmonella typhimurium aroA expressing a recombinant Mycoplasma hyopneumoniae antigen (NrdF)

Mycoplasma hyopneumoniae is the etiological agent of porcine enzootic pneumonia, a commercially expensive respiratory disease of swine. Salmonella typhimurium SL3261 was used as a live carrier of plasmid pKF1, which encodes a 15-kDa recombinant M. hyopneumoniae protein. This expressed recombinant protein consists of the carboxy-terminal 11 kDa of a 42-kDa M. hyopneumoniae NrdF ribonucleotide reductase R2 subunit protein. Rabbit anti-15-kDa serum was able to inhibit the growth of viable M. hyopneumoniae J in vitro. When used as a live oral vaccine, S. typhimurium SL3261(pKF1) induced a significant secretory immunoglobulin A immune response in the lungs of mice orally immunized against the M. hyopneumoniae antigen. Utilization of live oral vaccines expressing potentially protective M. hyopneumoniae proteins, such as the NrdF antigen, which can stimulate a lung mucosal response against surface-accessible proteins may provide a cost-effective alternative to the present control strategies used for porcine enzootic pneumonia.

Identification of novel species-specific antigens of Mycoplasma hyopneumoniae by preparative SDS-PAGE ELISA profiling

Mycoplasma hyopneumoniae, M. hyorhinis and M. flocculare are commonly isolated from the respiratory tract of pigs and are phylogenetically related. The identification and characterization of antigens specific for M. hyopneumoniae is crucial for the development of serological reagents and for understanding the mechanisms of pathogenicity of this pathogen. Protein and antigen profiles of six strains of M. hyopneumoniae, four strains of M. hyorhinis and a type strain of M. flocculare were compared using SDS-PAGE and immunoblotting. Five strains of M. hyopneumoniae originally isolated from diverse geographical regions produced similar protein and antigen profiles. One strain, C1735/2, produced a unique protein profile and was poorly immunoreactive, suggesting that some strains of M. hyopneumoniae may possess a structurally modified repertoire of antigens. Major M. hyopneumoniae antigens with molecular masses of approximately 36, 43, 48, 52, 76, 78, 80, 82, 94, 106, 114 and 200 kDa were identified by immunoblotting using hyperimmune pig sera raised against both high and low passage strains of M. hyopneumoniae. Porcine hyperimmune sera raised against the GDL type strain of M. hyorhinis reacted strongly with all M. hyorhinis strains although the profiles displayed considerable variation. Major antigens of molecular mass 42, 49, 52, 78, 80 and 82 kDa were identified in type strains GDL and BTS-7 and field strain 2; however, field strain 1 produced a unique profile. A preparative SDS-PAGE profiling (PPP) technique was developed which enabled quantification of the immunoreactivity of denatured antigens with porcine serum by ELISA. PPP facilitated the rapid identification of species-specific and cross-reactive antigens among the three mycoplasma species. PPP studies revealed several strongly immunoreactive M. hyopneumoniae-specific antigens of 43, 76, 94, 114 and 200 kDa as well as antigens of molecular mass between 52 and 62 kDa which were not apparent in immunoblotting studies. Rabbit monospecific anti-42 kDa serum reacted specifically with a 43 kDa antigen in whole cell lysates of geographically diverse strains of M. hyopneumoniae and failed to cross-react with M. flocculare or M. hyorhinis whole cell lysates. This study has identified a number of M. hyopneumoniae-specific antigens which warrant further investigation to determine their potential as diagnostic reagents and the role they play, if any, in pathogenicity.

Molecular characterization of a ribonucleotide reductase (nrdF) gene fragment of Mycoplasma hyopneumoniae and assessment of the recombinant product as an experimental vaccine for enzootic pneumonia

A Mycoplasma hyopneumoniae clone bank was screened with hyperimmune pig serum. One clone exhibited sequence homology to the prokaryotic R2 subunit of ribonucleotide reductase and was expressed as an 11-kDa protein fused to beta-galactosidase. The vaccine potential of the fusion protein was assessed in pig trials. Following experimental challenge with a virulent isolate of M. hyopneumoniae, gross lung pathology (mean Goodwin lung score) of vaccinated animals, irrespective of adjuvant treatment, was significantly reduced compared with that of control unvaccinated pigs (P < 0.05).

An improved enzyme linked immunosorbent assay (ELISA) for the detection of porcine serum antibodies against Mycoplasma hyopneumoniae

An ELISA for the detection of serum antibodies to Mycoplasma hypopneumoniae in pigs and based on a 43 kDa purified protein derived from the cytoplasmic membrane of M. hyopneumoniae strain J is described. This ELISA (MHPP ELISA) was compared with another recently described (Auspharm ELISA, Sheldrake and Romalis 1992) that is based on column-purified sonicated proteins of strain J. Using sample to negative ELISA ratios of 3 and 4 as cutoffs for inconclusive and positive reactors respectively (compared to 2 and 3 for the Auspharm ELISA), the two tests had high specificity (MHPP 99.6%; Auspharm 100%) in 280 SPF pigs. In 176 pigs from commercial herds with endemic M. hyopneumoniae, the MHPP ELISA showed a higher sensitivity than the Auspharm ELISA in both high lung score (LS > or = 5) (85.5% vs. 69.9%) and low lung score (0 < LS < 5) (57.9% vs. 49%) pigs when the positive cutoff for each test was selected. The sensitivity when the inconclusive cutoff was selected was similar in both tests (85%; 85.7%) when low and high lung score pigs were pooled. Altough the MHPP also gave more positive reactors in 36 pigs from M. hyopneumoniae-infected herds with no lung pathology at slaughter than the Auspharm ELISA (11 vs. 4), the total number of inconclusive and positive reactors in these pigs was similar for both tests (18 vs. 14). The MHPP ELISA gave significantly higher ELISA ratios in infected pigs (up to 17.9) than the Auspharm ELISA (up to 9), and earlier seroconversion in naturally-infected (6-8 weeks vs. 9-10 weeks) and experimentally-infected pigs (2-4 weeks vs. 4-6 weeks post infection).

Rapid identification of some Leptospira isolates from cattle by random amplified polymorphic DNA fingerprinting

We compared random amplified polymorphic DNA (RAPD) fingerprinting with cross-absorption agglutination and restriction enzyme analysis for typing bovine leptospires. Using RAPD fingerprinting, we examined a number of Leptospira serovars, namely, hardjo genotypes bovis and prajitno, pomona, balcanica, tarassovi, swajizak, kremastos, australis, and zanoni, which are likely to be isolated from Australian cattle. Each serovar and genotype had a unique RAPD profile. Of 26 field isolates of Leptospira, 23 were identified as hardjo genotype bovis subtype A, 2 were identified as zanoni, and 1 was identified as pomona by RAPD fingerprinting, and their types were confirmed by cross-absorption agglutination and restriction enzyme analysis.

Induction of pathogenic-like responses in the legume Macroptilium atropurpureum by a transposon-induced mutant of the fast-growing, broad-host-range Rhizobium strain NGR234

Mutant strain ANU2861, a transposon Tn5 mutant of the fast-growing, broad-host-range Rhizobium strain ANU280 (NGR234 Smr Rfr) overproduces polysaccharide, is an ade auxotroph, and induces poorly developed nodules on Leucaena leucocephala and Lablab purpureus (H.C. Chen, M. Batley, J.W. Redmond, and B.G. Rolfe, J. Plant Physiol. 120:331-349, 1985). Strain ANU2861 cannot form nodules on Macroptilium atropurpureum Urb. (siratro) or on Desmodium intortum and D. uncinatum and the nonlegume Parasponia. The parent strain, ANU280, effectively nodulates all these legume species except Parasponia, on which it forms ineffective nodules. Ultrastructural examination of infection sites on the legume siratro showed that mutant strain ANU2861 caused root hair curling (Hac+ phenotype), some cortical cell division (Noi+), but no infection threads (Inf-). Localized cellular responses, known to occur in phytopathological interactions, were observed in electron micrographs of the epidermal tissue at or near the infection zone after inoculation with strain ANU2861 but not the wild-type parental strain. These include (i) the rapid (within 20 h) accumulation of osmiophilic droplets attached to membranes at potential sites of strain ANU2861 penetration and (after 48 h) in the epidermal cells in the immediate region of the curled root hairs, and (ii) localized cell death of the epidermal cells. In addition, strain ANU2861 can initiate a systemic response in split-root siratro plants which prevents the successful nodulation of strain ANU280. A 6.3-kilobase fragment of wild-type genomic DNA, which includes the site of Tn5 insertion in strain ANU2861, was cloned and introduced to strain ANU2861. All the phenotypic defects of the mutant strain were corrected by the introduction of this DNA fragment. This indicates that the original Tn5 insertion is responsible for the phenotype.

Nitrogen fixation ability of exopolysaccharide synthesis mutants of Rhizobium sp. strain NGR234 and Rhizobium trifolii is restored by the addition of homologous exopolysaccharides

Several transposon Tn5-induced mutants of the broad-host-range Rhizobium sp. strain NGR234 produce little or no detectable acidic exopolysaccharide (EPS) and are unable to induce nitrogen-fixing nodules on Leucaena leucocephala var. Peru or siratro plants. The ability of these Exo- mutants to induce functioning nodules on Leucaena plants was restored by coinoculation with a Sym plasmid-cured (Nod- Exo+) derivative of parent strain NGR234, purified EPS from the parent strain, or the oligosaccharide from the EPS. Coinoculation with EPS or related oligosaccharide also resulted in formation of nitrogen-fixing nodules on siratro plants. In addition, an Exo- mutant (ANU437) of Rhizobium trifolii ANU794 was able to form nitrogen-fixing nodules on white clover in the presence of added EPS or related oligosaccharide from R. trifolii ANU843. These results demonstrate that the absence of Rhizobium EPSs can result in failure of effective symbiosis with both temperate and subtropical legumes.

Effect of N-hydroxyparacetamol on cell cycle progression

N-hydroxyparacetamol treatment of rat kidney cells gave rise to a dose-dependent decrease in DNA synthesis. A concentration of 1.0 mM N-hydroxyparacetamol at pH 7.2 decreased the level of DNA synthesis to 13.0 +/- 2.3% of the control value after 1 hr incubation. This compound also caused a perturbation of cell cycle progression. A concentration of 0.44 mM N-hydroxyparacetamol induced G1/S and S phase blocks. These delays became evident at approximately 12 hr after treatment and persisted until about 15 hr when cells started to recover. It seems unlikely that N-hydroxyparacetamol inhibits DNA synthesis and perturbs cycle progression through alterations to DNA structure as such, since this compound failed to alter the migration pattern of naked plasmid DNA.