Question

Spirillum is not classified as a spirochete because spirochetes a. do not cause disease. d. are prokaryotes. b. possess axial filaments. e. none of the above c. possess flagella.

Short answer

b. possess axial filaments.

Step-by-step solution

Understanding the Question

The question is asking for the characteristic that differentiates spirochetes from spirillum. We need to identify which option is true for spirochetes but not for spirillum.

Analyzing the Options

Let's break down each option: 1. a. "do not cause disease." - Spirochetes can cause diseases (e.g., Lyme disease), so this is incorrect. 2. b. "possess axial filaments." - Spirochetes are known for having axial filaments, which are a key feature not found in spirillum. 3. c. "possess flagella." - Spirillum possess flagella, just like spirochetes; this is not distinct. 4. d. "are prokaryotes." - Both spirillum and spirochetes are prokaryotes, so this does not differentiate them. 5. e. "none of the above." - This suggests all previous options are incorrect, which is not the case given option b.

Selecting the Correct Answer

Based on the analysis, option b (spirochetes "possess axial filaments") is a characteristic unique to spirochetes and not spirillum, making it the correct choice.

Key concepts

Spirochetes

Spirochetes are a unique group of bacteria that are characterized by their distinct spiral shape. These bacteria belong to the phylum Spirochaetota and are known for their helical or corkscrew-like structure. This peculiar shape aids them in movement through viscous environments, such as the mucous membranes of their hosts.
They are fascinating not only due to their shape but also due to their role in human disease. For example, the bacterium _Borrelia burgdorferi_, responsible for Lyme disease, belongs to this group. This demonstrates that spirochetes indeed can cause disease, contrary to some misconceptions. Unlike spirillum, spirochetes possess axial filaments, which are crucial for their motility.

Spirillum

Spirillum refers to a different type of spiral-shaped bacterium, distinguishable from spirochetes by a few key features. While they do share the spiral appearance, spirilla typically have a rigid structure as opposed to the flexible form seen in spirochetes. They belong to the class Betaproteobacteria.
Unlike spirochetes, spirilla move using conventional flagella, which are whip-like appendages that propel them forward in liquid environments. These flagella are located at either the ends or surrounding the body of the bacterium, facilitating locomotion in aquatic settings.
Spirillum species like _Spirillum volutans_, although not commonly known for causing disease in humans, are interesting subjects for studying microbial physiology due to their unique movement and structural properties.

Axial Filaments

Axial filaments are a defining feature of spirochetes, setting them apart from other types of spiral bacteria such as spirillum. These filaments are modified structures that resemble flagella but function differently.
Instead of protruding out from the cell in the manner of typical flagella, axial filaments are located within the periplasmic space, which is the region between the inner and outer membrane of the bacterium. Spirochetes usually have multiple axial filaments that wrap around the cell, allowing the entire bacterium to twist in a corkscrew motion.
This unique mode of motility is particularly advantageous for navigating environments with higher viscosity, such as human tissues. In the case of the disease-causing species, like _Treponema pallidum_, axial filaments enable effective penetration and movement through host tissues.

Flagella

Flagella are essential structures used by various bacteria for movement, existing as long, whip-like appendages. These structures are composed of a protein called flagellin and are powered by a motor mechanism located at the bacterial membrane.
In bacteria like spirillum, flagella are typically situated either at one or both ends of the cell or can sometimes cover the entire cell surface, termed as polar or peritrichous flagellation respectively.
Flagella allow the bacterium to swim through liquids by rotating in a propeller-like fashion. This mobility is crucial for bacteria in seeking nutrients or escaping harmful environments. While both spirochetes and spirilla utilize flagella for movement, the arrangement and function differ, with spirochetes having internal flagella in the form of axial filaments.

Prokaryotes

Prokaryotes represent one of the most diverse and fundamental domains of life. They are unicellular organisms that lack a defined nucleus, with their genetic material freely floating in the cytoplasm. This grouping includes both bacteria and archaea.
Both spirochetes and spirilla are classified as prokaryotes, placing them amongst a broad variety of organisms that inhabit environments ranging from extreme conditions to more common habitats such as soil and water.
Prokaryotes display vast metabolic versatility, enabling them to play numerous ecological roles, such as nitrogen fixation, decomposition, and as pathogens. Despite their simplicity, the prokaryotic structure is highly efficient and evolved, allowing bacteria like spirochetes and spirilla to survive and thrive in diverse settings.