Similarly to Helicobacter pylori but unlike Vibrio cholerae O1/O139, Campylobacter jejuni is non-motile at 20°C but highly motile at ≥37°C. The bacterium C. jejuni has one of the highest swimming speeds reported (>100 μm/s), especially at 42°C. Straight and spiral bacterial shapes share the same motility. C. jejuni has a unique structure in the flagellate polar region, which is characterized by a cup-like structure (beneath the inner membrane), a funnel shape (opening onto the polar surface) and less dense space (cytoplasm). Other Campylobacter species (coli, fetus, and lari) have similar motility and flagellate polar structures, albeit with slight differences. This is especially true for Campylobacter fetus, which has a flagellum only at one pole and a cup-like structure composed of two membranes.
- E. coli
- H. pylori
- P. mirabilis
- S. enterica
multilocus sequence type
With the recently increasing consumption of poultry and poultry products [1-3], Campylobacter, mainly C. jejuni, are the leading cause of bacterial food poisoning in Japan and in many other countries. In Japan, eating of raw animal products such as chicken meat (“sasami”), chicken liver and cow liver is associated with Campylobacter infections. This organism is also one of the important causes of travelers' diarrhea .
C. jejuni infection commonly causes enteritis, which can manifest as watery diarrhea or bloody diarrhea with fever and abdominal cramps [5, 6]. It is also associated with systemic infections such as bacteremia and GBS [6, 7]. Death is rare . In contrast to humans, C. jejuni are part of the normal flora of the intestines of chickens (which have a higher body temperature, 42°C, than do humans) and are secreted into their stools. This organism almost never causes intestinal diseases in chickens . C. coli is also associated with human infection, accounting for 1–25% of them .
Campylobacter jejuni is spiral in shape, has a single flagellum at each pole and exhibits high motility, this last feature being required for its colonization of animal and human test subjects ; motility is also important for C. jejuni adherence and invasion in vitro . Over 40 genes are involved in biogenesis and assembly of C. jejuni flagella ; however, the bacterial polar structures responsible for their extremely high motility are not known. In this study, we examined the structures in the flagellate polar region of C. jejuni (and other Campylobacter species) by scanning and transmission electron microscopy to gain a better understanding of C. jejuni motility.
MATERIALS AND METHODS
Thirty strains of C. jejuni were isolated from patients with enteritis (food poisoning); of those, 14 strains belonged to ST21 (CC21), ST22 (CC22), ST42 (CC42), ST400 (CC353), ST407, ST545 (CC22), ST922, ST4052 (CC353), ST4060 (CC460), ST4063 (CC283) and ST4108 (CC607) . Seven strains of C. jejuni (serotype Penner HS:19) were from patients with GBS and belonged to ST22 (CC22), ST2140 (CC574), ST4049 (CC464), ST4051 (CC22), and ST4053 (CC353) . C. jejuni also included strain ATCC33560. Five strains of C. coli were isolated from patients with enteritis and belonged to ST860 (CC828), ST1068 (CC828), ST1593 (CC828), and ST4059 (CC828) . Four strains of C. fetus and two strains of C. lari, which were isolated from the feces of patients with food poisoning, were kindly provided by Dr Akemi Kai (Tokyo Metropolitan Institute of Public Health, Tokyo, Japan). V. cholerae O1 strain EO8 , V. cholerae O139 strain T16 , and H. pylori strain C7M  were also assessed. All bacterial strains were stored at −80°C in 3% skim milk (Difco; Becton Dickinson, Franklin Lakes, NJ, USA) supplemented with 5% glucose (Difco). The other bacterial strains used were from our laboratory stock.
Media and bacterial growth
For Campylobacter growth, blood-agar plates (trypticase soy agar supplemented with 5% sheep blood; Becton Dickinson, Tokyo, Japan) were inoculated and incubated for 1–2 days at 37°C in a microaerophilic atmosphere (6–12% O2 and 5–8% CO2; the remaining gases being mostly N2 from air). BHI broth (Difco) supplemented with 10% FBS (Gibco, Carlsbad, CA, USA) was used as a liquid medium (at 37°C in a microaerophilic atmosphere). Bacterial strains other than Campylobacter were also grown in BHI broth supplemented with 10% FBS (at 37°C).
Prior to motion analysis, test bacteria were grown in BHI containing 10% FBS at 37°C for approximately 3 hrs (to a log phase). Bacterial motility was then examined under an inverted, phase-contrast microscope with a Micro Warm plate (Kitazato, Tokyo, Japan) that regulated the temperature of the specimens. The motility speed (μm/s) was measured using a motion analysis system with the program C-Imaging C-MEN (Complix); the limit of resolution of swimming speeds was 100 μm/s. Bacterial swimming in a liquid layer of BHI broth containing 10% FBS (106 to 107 colony forming units/mL) between a glass slide and a glass cover (pre-coated with FBS) was continuously recorded 15 times in 0.05 s analysis segments (a total of 0.75 s) and the swimming speed (μm/s) of each bacterial cell in a specimen obtained, essentially as described previously . Pre-coating the glass surface with FBS is important because it prevents attachment of test bacteria to the glass surfaces. Measurements were performed in at least five different fields of each specimen, swimming speeds for approximately 300 bacterial cells being measured for each specimen (within a few mins), and the percentage of motile bacteria determined. Based on observations of heated or formalin-treated, non-motile bacteria, Brownian motion of bacteria was estimated to be 0.4 ± 0.3 μm/s. Accordingly, mean speeds of ≥4.0 μm/s (speeds 10 times or more faster than that of Brownian motion) were judged as indicating motility; bacterial motility was also judged by direct observation through a phase-contrast microscope. The data are presented as the mean ± SD of at least three trials.
Scanning electron microscopy
For analysis of bacterial shape, bacterial cells were grown on blood-agar plates for 12–18 hrs at 37°C and examined by scanning electron microscopy . For this, pieces of blood-agar-block on which colonies had developed were fixed with 2.5% glutaraldehyde in 75 mM PBS (pH 7.4) for 2 hrs at 4°C, washed with PBS, and subsequently postfixed in 1% osmium tetroxide for 2 hrs at 4°C. The fixed samples were dehydrated with 50%, 70%, 90% and 100% acetone for 2 hrs each at room temperature (around 18°C), and the samples in 3-methylbutyl (isoamyl) acetate were then critical-point dried. The dried samples were coated with gold–palladium and subjected to analysis using a scanning electron microscope.
Transmission electron microscopy
Campylobacter structures in the flagellate polar region were analyzed by transmission electron microscopy  and thin-section or negative-stain images obtained. For thin-section images, bacterial cells grown on blood-agar plates for 12–18 hrs at 37°C were carefully suspended in and fixed with 2.5% glutaraldehyde in PBS for 2 hrs at 4°C, followed by washing and postfixing with 1% osmium tetroxide, as described above. The fixed samples were dehydrated with 70%, 90%, 95% and 100% ethanol for 10 mins each at room temperature, and embedded in EPOK 812 (Oukenn, Tokyo, Japan). The embedded block was cut with an ultramicrotome (MT-500) with a diamond knife (producing 70 nm thin sections) and stained with 2% uranyl acetate and Sato's lead staining solution (containing lead citrate, lead nitrate and lead acetate). The stained thin sections were analyzed using a transmission electron microscope.
For negative-stain images, bacterial cells grown on blood-agar plates for 12–18 hrs at 37°C were carefully suspended in water. One drop of the bacterial suspension was applied to a collodion-coated grid screen (3 mm diameter), followed by addition of one drop of 1% uranyl acetate for 30–60 s (negative staining). The stained grids were analyzed using a transmission electron microscope.
Motility of Campylobacter jejuni and Campylobacter coli
Campylobacter jejuni was grown at 37°C and then examined for motility at various temperatures. As shown in Figure 1, the motility of C. jejuni is strictly regulated by temperature. C. jejuni is highly motile at 37–42°C, whereas motility is immediately lost when the temperature is lowered to room temperature range (<20°C). The motility of C. jejuni, which is lost at 20°C, immediately and completely recovers when the temperature is increased to 37–42°C. Reversibility was observed even in the presence of chloramphenicol (which inhibits protein synthesis) at 100 μg/mL, similarly to H. pylori.
This was in marked contrast to the data for V. cholerae O1/O139, which showed temperature-independent motility between 20 and 42°C, as shown in Figure 1. The difference between C. jejuni and H. pylori motility is clear at 42°C, at which temperature C. jejuni is still motile, but H. pylori motility has declined (Fig. 1).
The swimming speed at 37°C was the fastest for C. jejuni (>100 μm/s); the swimming speeds of H. pylori, V. cholerae O1 and V. cholerae O139 were 55.4 ± 6.6, 80.2 ± 8.6 and 75.6 ± 8.9 μm/s, respectively. The swimming speed of C. jejuni at 42°C was faster than at 37°C (>100 μm/s; the resolution limit of the assay system employed did not allow precise assessment of speed). The motility of C. coli was very similar to that of C. jejuni (Fig. 1); the swimming speeds at 37 or 42°C were >100 μm/s.
Next, correlations between bacterial shape and motility were examined for C. jejuni. C. jejuni enteritis strains (n = 30) and ATCC33560 all took the form of spiral rods with polar flagella at each pole and were highly motile, as shown in Figures 1 and 2a, d. Five of the C. jejuni GBS strains (n = 7) strains took the form of motile spiral rods (as shown in Figs. 1 and 2a, d), whereas one strain (KB3439; belonging to ST22) took the form of a straight rod with polar flagella at each pole (Fig. 2b, e). Interestingly, strain KB3439 was highly motile (Fig. 1), its swimming speed of >100 μm/s (at 37–42°C) being similar to that of a spiral rod, indicating that a spiral body shape is not essential for motility. As expected, the remaining strain (KB3449; belonging to ST4051), which took the form of a straight rod without flagella (Fig. 2c, f), showed no motility, (Fig. 1).
Structures in the flagellate polar region of Campylobacter jejuni and Campylobacter coli
As shown in Figure 3, all C. jejuni strains have cup-like structures (marked by closed arrowheads) at both ends of the bacterial spiral body, irrespective of their bacterial shapes or the presence or absence of flagella. These polar cup-like structures are located inside (and adjacent to) the inner membrane and are 33.8 ± 6.0 nm thick (including the inner membrane) and 206.4 ± 25.5 nm in length (n = 62), as shown in Figure 3a (inset panels in the right and middle lower corner). The space (cytoplasm) within the cup-like structures is less dense than the cytoplasm of the spiral (or straight) bodies (Fig. 3). Motile bacteria have a polar hollow for a flagellum (Fig. 3a, b; indicated by arrows), in contrast to non-motile bacteria (Fig. 3c).
Negative staining of C. jejuni cells (Fig. 4a, b) further demonstrated that inner tubular structures extend from the inner membrane and open into a funnel shape on the bacterial cell surface (diameter at the bacterial cell surface, 80.8 ± 10.1 nm [n = 31]); flagella expanding into a funnel shape toward the environment. The inner tubular structure is incompletely shown in Figure 4b, probably due to incomplete penetration of uranyl acetate into the tubular structure from the bacterial surface (funnel shape) side.
The funnel shape was also confirmed by thin sections of C. jejuni, as shown in Figure 4c. When the bacterial surface structure in the extreme polar region (the outer surface of the funnel shape) was examined by scanning electron microscopy, it looked smooth (as a ring structure), in contrast to the surface of the spiral body, which had a capsular wrinkle-like structure, as shown in Figure 4d.
A unique structure in the flagellate polar region was also observed for C. coli, which has polar cup-like structures (32.5 ± 5.8 nm thick [n = 42]) (Fig. 5a); these cup-like structures ares located inside (and adjacent to) the inner membrane, similarly to C. jejuni.
In the C. coli strain (M5) the pole structures spontaneously separate as small round particles with a flagellum from the bacterial spiral bodies (Fig. 5b); these small particles are 0.25 ± 0.05 μm (n = 32). They are distinct from coccoids (much larger round cells [0.63 ± 0.12 μm, n = 68]) with two flagella), which appear in the tip areas of bacterial colonies (Fig. 5c, d, f).
Structures in the flagellate polar region and motility of Campylobacter fetus and Campylobacter lari
In contrast to C. jejuni and C. coli (with a single flagellum at each pole), C. fetus has a single flagellum at only one pole, as shown in Figure 6a, although dividing (long) C. fetus cells have a single flagellum at each pole (Fig. 6a). C. fetus has, albeit rarely, two flagella at one pole (Fig. 6a).
In C. fetus, the cup-like structures appear to be composed of two parallel membranes (Fig. 6b); the cup-like structures are 31.0 ± 5.9 nm thick, including the inner membrane (n = 51). C. fetus has temperature-dependent motility, similar to the motility of C. jejuni (Fig. 6c); the swimming speed at 37 or 42°C being >100 μm/s.
Campylobacter lari is very similar to C. jejuni (and C. coli) in terms of polar flagellation, cup-like structures and high-speed and temperature-dependent motility (Fig. 6a–c); the cup-like structures are 29.8 ± 6.2 nm thick, including the inner membrane (n = 35) and the swimming speed at 37 or 42°C >100 μm/s.
In this study, we demonstrated that C. jejuni swims much faster at 37–42°C (>100 μm/s) than do curved rods, including H. pylori and V. cholerae, and non-curved rods, including V. parahaemolyticus, S. enterica, E. coli and P. mirabilis. C. jejuni is a motile bacterium with one of the highest swimming speeds (>100 μm/s) reported, to our knowledge.
The extremely high motility of C. jejuni might be associated with its structure in the flagellate polar region (characterized by cup-like structures, funnel shaped with tubular structures and less dense space) as shown in Figure 7. The bacterial polar structures occasionally separate from the bacterial spiral bodies, forming small round particles with a single flagellum.
By contrast, we found no polar cup-like structures in H. pylori (a spiral-shaped bacterium), V. cholerae O1 (biotypes Classical and El Tor) and O139 (comma-shaped bacteria), or non-curved rods such as V. parahaemolyticus, S. enterica, E. coli, and P. mirabilis (data not shown), indicating that these polar cup-like structures are unique to Campylobacter species. Further studies of isogenic mutants, in which, for example, the cup-like structures vary, are needed for clear-cut conclusions about the relationship between function and structure.
In a previous study, C. jejuni 11168-GS, whose genome has been completed , was shown to have the form of a straight rod with polar flagella and significantly impaired motility , whereas its original clinical isolate (11168-O) had a spiral body with polar flagella with high motility . However, in this study, C. jejuni KB3439, which is a straight rod with polar flagella, was highly motile, similarly to spiral C. jejuni with polar flagella, strongly suggesting that the spiral shape is not essential for high-speed motility in C. jejuni in vitro. Cup-like structures were present in C. jejuni non-motile strain KB3449, indicating other impaired steps related to flagella formation.
In this study, it was found that C. fetus, which grows at low temperatures (25°C) but not at higher temperatures (42°C), has a flagellum at only one pole (except for dividing [long] cells, which have flagella at each pole), unlike C. jejuni, C. coli, or C. lari. Nevertheless, C. fetus has high-speed motility that is strictly temperature dependent (similar to C. jejuni). However, the polar cup-like structures of C. fetus seem to be composed of two parallel membranes (an inner membrane and an inside [third] membrane, located immediately inside and parallel to the inner membrane). For three other Campylobacter (C. jejuni, C. coli, and C. lari), the inside structure (of their cup-like structures) remain uncertain.
During this study, Chen et al. described the flagellar motor architecture of C. jejuni . Their analysis by an electron cryotomographical survey focused on a small inner-outer membrane region, associated with the flagellar motor, and demonstrated two unique disk-like densities in the periplasm: the first disk (outer radius, 48 ± 9 nm) below the outer membrane (and connecting to the P-ring) and the second (radius, 32 ± 7 nm) beneath the first (probably connecting to the M/S-ring). These two disks may correspond to the funnel shape we identified in this study. The cup-like structures, located immediately beneath the inner membrane at the pole-side (over 200 nm in length), have not been analyzed by Chen et al. .
The molecular structure in the flagellate polar region, factors (other than temperatures) which affect motility speed (such as serum concentrations or origin of serum) and inhibitors of motility are under continuing investigation in our laboratory.
We thank Akemi Kai (Tokyo Metropolitan Institute of Public Health, Tokyo, Japan) for C. fetus and C. lari strains and Akihito Nishiyama (Niigata University Graduate School of Medical and Dental Sciences, Niigata, Japan) for discussion. This study was supported in part by a grant from the Japan Science and Technology Agency, Japan and a grant from the United States-Japan Cooperative Medical Science Program (Cholera and Other Bacterial Enteric Infections Panel).
None of the authors has any conflicts of interest associated with this study.