MICROBIOLOGY Study Guide - 2012

CHAPTER 3. BACTERIAL MORPHOLOGY

3.1. SHAPES AND SIZES OF BACTERIA

By the end of the 19th century, A wide variety of microorganisms with different shapes had been described. In 1896, K. Lehmann and R. Neumann proposed grouping microorganisms by shape into three main categories: spherical, rod-shaped, and convoluted (spiral).

Spherical Bacteria (cocci) resemble spheres with a diameter ranging from 0.7 to 1.5 µm (Fig. 2; 1–5). Depending on how Cells are arranged after division in one, two, or three mutually perpendicular planes, they are classified as follows:

✵ micrococci — cells occur singly;

✵ diplococci — after division in a single plane, the cells do not separate but remain paired;

✵ streptococci — cells divide in a single plane, but fail to separate afterward, forming chains of varying length;

✵ tetrads — cells divide in two mutually perpendicular planes, forming groups of four cocci;

✵ sarcinae — cells divide in three mutually perpendicular planes, forming cube-shaped packets of eight cocci;

✵ staphylococci — cells divide randomly in multiple planes, forming irregular clusters of cocci that resemble bunches of grapes.

Some spherical bacteria have an elongated shape. Pneumococci (the causative agents of Pneumonia) resemble a candle flame, with cells arranged in pairs with their broad bases facing inward. Meningococci (the causative agents of epidemic meningitis) and gonococci (the causative agents of Gonorrhea) are shaped like coffee beans joined together by their concave sides.

Rod-shaped bacteria vary in size, the shape of their Cell ends, and their mutual arrangement (Fig. 2; 7–9). The length of rod-shaped cells ranges from 1 to 10 µm, and their thickness from 0.3 to 2.0 µm. Rickettsiae are among the smallest rod-shaped bacteria. Other small rods, where the difference between length and width is negligible, look somewhat like cocci and are therefore called coccobacilli (such as the CAUSATIVE AGENT OF brucellosis).

The ends of the rods may be rounded, truncated, pointed, or club-shaped (swollen). Most rod-shaped bacteria separate after division and are arranged randomly. However, certain bacteria tend to form chains of rod-shaped cells. Rods are further subdivided into spore-forming and non-spore-forming species. Spore-forming rods are referred to as bacilli or clostridia.

In bacilli, the spores are located in the center of The Cell, and their diameter generally does not exceed that of the vegetative cell. In clostridia, the spore is offset from the center and has a diameter greater than that of the cell itself, giving these forms a spindle, tennis racket, drumstick, or lemon shape. Bacilli grow well in the presence of atmospheric oxygen and are termed aerobes, whereas clostridia are anaerobes and grow exclusively in the absence of oxygen.

Convoluted (spiral) microbial forms include curved or helically twisted cells. They are distinguished by the number of turns: vibrios are slightly Curved Rods resembling a comma; spirilla possess 3 to 5 large coils; and spirochetes are thin, long, spiral-shaped bacteria characterized by motility driven by undulating cell movements (Fig. 2; 10–13). They occupy an intermediate position between bacteria and Protozoa.

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Fig. 2. Bacterial Morphology (Vorobyov et al., 1994):

1 — micrococci; 2 — streptococci; 3 — sarcina; 4 — gonococci; 5 — pneumococci; 6 — pneumococcal capsule; 7 — Corynebacterium diphtheriae; 8 — clostridia; 9 — bacilli; 10 — vibrios; 11 — spirilla; 12 — treponema; 13 — borrelia; 14 — leptospira; 15 — actinomycetes; 16 — flagellar arrangement: a — monotrichous; b — lophotrichous; c — amphitrichous; d — peritrichous

Most spiral forms are pathogenic to humans and animals (such as the cholera vibrio, the Syphilis spirochete, the relapsing fever borrelia, and the causative agent of leptospirosis), although saprophytic species also exist that inhabit soil and Water and participate in the sulfur cycle (Vibrio desulfuricans, Spirillum volutans, etc.).

In addition to the bacteria described above, other forms are found in nature.

Branching bacteria belong to the family Actinomycetaceae. Bacteria of the genus Mycobacterium can form a fragile mycelium that, at Cytology/cytology/16.html">Early stages of growth, breaks down into thin, sometimes branching rods. They are typically characterized by cells positioned at an angle to one another. Bifidobacteria (genus Bifidobacterium) exhibit branching at one or both ends of the cells.

Corynebacteria (genus Corynebacterium) resemble mycobacteria but differ in having club-like swellings at the ends of the rods and cytoplasmic granules (Fig. 2; 7).

Actinomycete cells are characterized by particularly extensive branching (from the Greek actis — ray, myces — fungus). Some representatives form a well-developed, multiply branched mycelium with complex fructification structures (Fig. 2; 15). Saprophytic actinomycetes are widespread in soil, playing a key role in global biogeochemical cycles. Many of them are important producers of Antibiotics.

Recently, prokaryotes with unusual cell shapes have been discovered. Some form closed or open rings, others resemble a six-pointed star, and still others produce cellular appendages known as prosthecae.