University of Michigan - Ann Arbor

Individual cells of a specific prokaryotic organism typically exhibit a characteristic shape, known as cell morphology. While thousands of prokaryotic organisms exist, only a handful of morphologies are commonly seen microscopically. These standard shapes include spherical (coccus), rod-shaped (bacillus), curved comma-shaped (vibrio), short oval-like rods (coccobacillus), long rigid corkscrews (spirillum), and loose, flexible helical spirals (spirochete).

Cell Morphologies

The genus *Mycoplasma* consists of more than $${}100$$ species of bacteria that lack a cell wall and are classified as low G+C gram-positive bacteria, despite not staining with Gram-stain reagents. These bacteria are extremely small, with some diameters around $${}0.2$$ μm, which is smaller than certain large viruses. Due to the absence of a cell wall, *Mycoplasma* cells are pleomorphic, meaning they lack a fixed shape, can assume diverse forms, and may resemble tiny animal cells, making them challenging to identify.

Mycoplasmas

Pathogenic bacteria are bacteria that can cause infectious disease.

Pathogenic Bacteria

Which of the following bacterial morphologies is characterized by a spiral shape?

Which bacterial morphology is described as rod-shaped?

Which bacterial morphology is described as comma-shaped?

The term coccus (plural: cocci) is used to describe prokaryotic cells that possess a round, spherical shape.

Coccus

The term vibrio (plural: vibrios) is used to describe prokaryotic cells that are shaped like curved rods, closely resembling a long comma.

Vibrio

The term coccobacillus (plural: coccobacilli) refers to prokaryotic cells that appear as short rods, closely resembling ovals.

Coccobacillus

The term spirillum (plural: spirilla) describes prokaryotic cells that possess a long, spiral shape, visually resembling a corkscrew.

Spirillum

Spirochetes are gram-negative bacteria characterized by long, spiral-shaped bodies that can reach lengths up to $${}250$$ μm. Due to their extreme thinness, they are difficult to view under conventional brightfield microscopes, typically requiring darkfield fluorescent microscopy instead, and are highly difficult or impossible to culture in a laboratory.

Spirochete

Pleomorphism is the biological characteristic of certain cells or microorganisms, such as *Mycoplasma* species, that allows them to assume a variety of shapes rather than having a single characteristic morphology.

Pleomorphism

The term bacillus (plural: bacilli) refers to rod-shaped prokaryotic cells. In taxonomy, Bacillus is also a genus of low G+C gram-positive bacteria, many of which can form resilient endospores.

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