4. Spirochetes

Spirochetes are defined by their unique morphology (Figure 23-45) and mode of movement. Some, such as Borrelia burgdorferi and Treponema palladum, are obligate pathogens and these were discussed in Chapter 18. Others are free-living and either anaerobic or facultative and these are the focus of this section.

Spirochetes cluster together phylogentically into a large order that is distantly related to other organisms. In contrast to many other microorganisms the cellular morphology of these microbes is a good marker for their phylogeny. They can be divided into three distinct families, the Spirochaetaceae consist of the genera Borrelia, Brevinema, Cristispira, Spirochaeta, Spironema, and Treponema. The second family is the Sperpulinaceae and contains the genus Brachyspira and the third family is the Leptospiraceae consiting of the species Leptonema and Leptospira.

Despite their overall cell wall structure being that of gram-negative bacteria, spirochetes have a unique cell morphology. The outer member is typical of gram-negative bacteria, but the arrangement of structures inside this membrane is unique to the spirochetes. Inside of the outer membrane is a helical cell body containing the peptidoglycan-cell membrane complex, the cytoplasm and the nuclear region (Figure 23-46). The helical body wraps around a central axis that is filled by proteinaceous filaments, which resemble flagella and are called axial filaments or endoflagella. In most spirochetes the two sets of endoflagella are inserted into opposite poles of the helical cell and they overlap in the middle.

Figure 23-46 diagram showing cell morphology and axial filament construction] FIG TYPE 1L

Spirochetes are common in a wide range of aqueous environments: both marine and freshwater, high and low salt, and low to high temperatures. In general, free-living spirochetes tend to be found in environments rich in decaying plant material, where they probably ferment sugars released by the action of other microbes. There is some evidence that they even form cooperative arrangements with cellulolytic bacteria. Spirochetes are not apparently widely distributed in the deep sea, though they have been successfully isolated from some samples. Most are mesophiles, but a few species are able to grow at thermophilic temperatures. For example,Spirochaeta thermophila and Spirochaeta caldaria, both from thermal springs, have optimum growth temperatures of 67 and 50 °C, respectively.

Fig 23-45 [A few photomicrographs of the spirochetes] PHOTOS TYPE 1P

Spirochetes have a limited metabolism and can only ferment a few types of organic molecules. They ferment carbohydrates to acetate, ethanol, CO₂, and H₂ as major end products. All spirochetes so far examined use the Embden-Meyerhoff-Parnas pathway to take glucose to pyruvate. Under anaerobic conditions this is converted to acetate and ethanol using common fermentative pathways. Interestingly, the facultative anaerobes in the group use both oxidative phosphorylation and substrate level phosphorylation in the presence of air and seem to be dependent on at least some fermentation. The TCA cycle has not been detected in these microbes, and it is unclear how they get their ATP by oxidative phosphorylation.

The motility of these microorganisms is also unique and has been studied since their original isolation. Most observations have been made in Spirochaeta aurantia and Borrelia burgdorferi but due to the similarity within the group, what has been learned probably applies to other spirochetes. There are two types of motion observed, the microbe spins about its helical axis and this causes it to corkscrew through the solution. This drilling motion enables the microbe to move through liquids 10 times more viscous than can be handled by flagellated bacteria such as E. coli and Pseudomonas. This type of motility may also explain how the pathogenic spirochetes are capable of penetrating nearly every tissue of the host during growth. The second type of motion is a flexing or twitching motion, which causes the microbe to change direction. In solution the bacteria swim in nearly straight lines and then stop and either reverse rotation to move in the opposite direction or flex to reorient themselves and to swim in a new direction.

A mechanism for this motility is proposed in Figure 23-47. In this model, motors at the ends of the microbe drive rotation of the endoflagella, such that spinning the endoflagella in one direction causes the rotation of the cell body in the opposite direction. When the flagella at one end is spinning clockwise and the flagella at the other end is spinning counter-clockwise, the cell body rotates and causes straight swimming. If both flagella spin in the same direction, the forces oppose each other and cause the microbe to flex.

Figure 23-47 : Motility mechanisms of the spirochetes] FIG TYPE 1L

Chemotaxis toward various carbohydrates has been observed in Spirochaeta aurantia, as has chemotaxis toward rabbit serum in Borrelia burgdorferi. B. burgdorferi will also move away from ethanol and butanol. The genome sequence for B. burgdorferi has genes reminiscent of those that encode the chemotaxis system of E. coli, so the signaling methods might be similar.