MICROBIOLOGY - M.H. Serhiichuk - 2008

Chapter 2. MORPHOLOGY AND STRUCTURE OF THE BACTERIAL CELL

BACTERIAL MORPHOLOGY

Bacteria (Latin bacteria - rod) are unicellular prokaryotic microorganisms ranging in size from 0.1 to 28 µm (1 mm = 10³ µm = 10⁶ nm).

The most common bacterial shapes are spherical, rod-shaped, spiral, and filamentous. Numerous intermediate forms exist among them.

Spherical bacteria are called cocci (Latin coccus - berry, grain). Depending on the planes of Cell Division and the arrangement of bacterial Cells after division, cocci are classified into:

- micrococci (Latin micro - small). Cell division occurs in one or several planes. Cells are arranged singly or form irregular clusters (Fig. 2.6). These are typically Saprophytes and common inhabitants of Water, soil, and air. Representatives include: Micrococcus luteus, M. roseus, M. lysodeikticus, M. varians;

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Fig. 2.6. Micrococcus luteus:

a - Electron Microscopy; b - bright-field microscopy

- diplococci (Latin diplo - double). Cells divide in a single plane and appear in pairs in microscopic preparations (Fig. 2.7). Representatives include: Azotobacter chroococcum, Neisseria gonorrhoeae (the CAUSATIVE AGENT OF Gonorrhea and blennorrhea), N. meningitidis (the causative agent of meningitis);

Fig. 2.7. a - Azotobacter chroococcum; b - Neisseria gonorrhoeae in pus; c - Neisseria gonorrhoeae - electron microscopy

- streptococci (Latin streptos - chain). Cells divide in a single plane, forming chains of varying lengths (Fig. 2.8). Representatives include: Streptococcus pyogenes, S. faecalis, S. lactis, S. pneumoniae;

a b

Fig. 2.8. Streptococcus pyogenes:

a - scanning electron microscopy; b - smear from pus, bright-field microscopy

- tetrads (Greek tetra - four). Cells divide in two planes, forming tetrads (Fig. 2.9). Representatives include: Deinococcus proteolyticus, D. radiodurans, Stomatococcus mucilaginosus;

Fig. 2.9. Pediococcus damnosus:

a - electron microscopy; b - bright-field microscopy

- sarcinae (Latin sarcio - to bind, to patch). Cell division occurs in three mutually perpendicular planes, resulting in specific packet-like clusters of 8, 16, 32, or more cells (Fig. 2.10). No pathogenic species have been identified among sarcinae. Representative: Sarcina ventriculi;

Fig. 2.10. Sarcina ventriculi:

scanning electron microscopy

- staphylococci (from Lat. staphylos - a bunch of grapes). The cells divide irregularly in multiple planes, resulting in characteristic clusters resembling grape bunches (Fig. 2.11). Staphylococci include free-living saprophytes (found in dairy and meat products, soil, air, and on plant surfaces) as well as species pathogenic to humans and animals. Representative species include Staphylococcus aureus, S. epidermidis, S. saprophyticus, and S. haemolyticus.

Fig. 2.11. Staphylococcus aureus

The most numerous and diverse group of bacteria is represented by rod-shaped (cylindrical) forms. The length of cylindrical bacteria varies within a quite wide range—from 0.8 µm to 11 µm—whereas their width is a more stable parameter, measuring 0.5–1.0 µm. In addition to size, different species of rod-shaped bacteria vary in the shape of their cell extremities and their arrangement patterns observed under a Microscope. The Cell ends may be pointed, oval, sharply cut, or even slightly thickened. In microscope preparations, most spore-forming bacteria form chains of cells, whereas non-spore-forming bacteria do not. Rods may be arranged singly, in pairs forming X and V shapes, or in hieroglyphic patterns (characteristic of bifidobacteria). Morphologically, rod-shaped bacteria are divided into two forms: spore-forming and asporogenous (Fig. 2.12).

Fig. 2.12. Rod-shaped forms of bacteria:

a, b - asporogenous bacteria; c, d - spore-forming bacteria

Spore-forming bacteria include, for example, the following genera: Amphibacillus (A. xylanus), Bacillus (B. anthracis, B. subtilis, B. thuringiensis), Clostridium (C. botulinum, C. novyi, C. perfringens, C. septicum, С. tetani), Desulfotomaculum (D. nignficans, D. ruminis), Oscillospira (O. guilliermondii), Sporoha.loba.cter (S. lortetii, S. mahsmortui), Sporolactobacillus (S. inulinus), Sporosarcina (S. ureae, S. halophila), Syntrophospora (S. bryantii), and Sulfobacillus (S. thermosulfidooxidans).

Although spore-forming bacteria are generally considered better adapted to adverse environmental conditions, non-spore-forming (asporogenous) bacterial forms constitute a much more widespread group. The best-known representatives of non-spore-forming rods include:

- Escherichia coli - constantly isolated from human intestinal contents and feces, which is why it is commonly known as the colon bacillus.

- Serratia marcescens - formerly known as Bacterium prodigiosum. The appearance of this completely harmless microorganism on food, icons, and altars historically caused panic. The microbe synthesizes a Blood-red pigment, and the sudden appearance of icons "weeping" bloody tears or bread bearing blood-red spots was perceived by people as a terrifying omen. An ancient legend recounts that during the siege of Tyre, panic broke out in the army of Alexander the Great because "bloody" spots appeared on bread; however, a wise counselor reassured the soldiers by explaining that these spots presaged a bloody retribution against the enemy.

Among non-spore-forming rods, prominent Examples include bacteria causing such widespread diseases as dysentery (Shigella sonnei, S. dysenteriae, S. flexneri), typhoid fever (Salmonella typhi), paratyphoid fever (S. paratyphi), and salmonellosis (S. cholemesuis). Microorganisms of the genus Proteus (P. vulgaris, P. rettgeri) are isolated in cases of food toxicosis and wound suppuration.

Symptomatically or structurally, curved bacteria are divided into vibrios, spirilla, and spirochetes based on the number of turns (Fig. 2.13).

Fig. 2.13. Curved forms of bacteria:

a - vibrios; b - spirilla; c - spirochetes

Vibrios (from Lat. vibrio - to bend or vibrare - to quiver) are Curved Rods (0.3–1.3 × 1.4–5.0 µm) representing about 1/4 of a spiral, shaped like a comma. They are widespread in marine environments, river estuaries, on the body surfaces of marine animals, and within their intestinal contents. Certain species have also been detected in freshwater. Approximately 10 species are pathogenic to humans, and several cause diseases in marine vertebrates and invertebrates.

The most well-known human pathogens include Vibrio cholerae (the causative agent of cholera), V. parahaemolyticus (causing food poisoning often associated with infected fish or shellfish), and V. vulnificans (which causes septicemia with a high mortality rate).

Spirilla are mostly saprophytes inhabiting stagnant, polluted water bodies as well as decaying PLANT AND ANIMAL remains. Unlike vibrios, their cells are thicker, longer (1.4–1.7 × 14.0–60.0 µm), and twisted. Spirilla may have anywhere from 1–2 turns (resembling the Cyrillic letter С or the Latin letter S) up to 8–10 turns. Spirillum pleomorphum has been isolated from Arctic soil, with an optimal growth Temperature of 9 ⁰С, a maximum of 20 ⁰С, and a minimum below 0 ⁰С. S. volutans is found in stagnant fresh water. S. minus is the only human-pathogenic species, responsible for rat-bite fever (sodoku). This disease manifests with a body temperature rise to 39–41 ⁰С, fever, followed by rashes, Arthritis, and inflammation of the oral mucosa and conjunctiva.

Spirochetes (from Lat. spiro - corkscrew) are highly twisted rods with more than 10 turns (0.1–0.3 × 5.0–250.0 µm). Externally, they are covered by a thin outer membrane enclosing a protoplasmic cylinder—Cytoplasm with a nuclear region, surrounded by the cytoplasmic membrane. The spirally wound protoplasmic cylinder is encircled by periplasmic axial filaments that provide a gliding type of motility. Spirochetes include free-living forms found in fresh and saline lakes, as well as bottom sediments (Spirochaeta plicatilis, S. isovalenca). Some species are human pathogens, notably the causative agents of Syphilis (Treponema pallidum), relapsing fever (Borrelia recurrentis), and infectious jaundice (Leptospira interrogans). Human infection with leptospires occurs through water contaminated by infected animals. The disease begins abruptly after an incubation period lasting 5–6 days and is characterized by fever, general malaise, and severe headache and Muscle pain. A week later, leptospires concentrate in the Liver, Spleen, and Lymph Nodes. The disease typically concludes with recovery.

Filamentous bacteria (Fig. 2.14) comprise rod-shaped unicellular and Multicellular Organisms. Filaments are formed by multiple cells connected via sheaths, envelopes, plasmodesmata, etc. In filamentous cyanobacteria, the filaments are either simple or branched.

Filaments of trichome-forming bacteria can either float freely in water (cyanobacteria, Beggiatoa alba) or attach to a substrate (Thiothrix nivea).

Fig. 2.14. Filamentous forms of cyanobacteria

In addition to the aforementioned bacterial forms, there are bacteria with unusual shapes: ring-shaped, six-pointed star-shaped, Kidney-shaped, rectangular, triangular, and appendage-bearing, among others (Fig. 2.15).

Fig. 2.15. Unusual bacterial morphologies

Some bacterial genera exhibit pleomorphism (morphological Variability). During growth, cell shape can change significantly from cocci to rods (and vice versa), become irregular, or form branched filaments. Pleomorphism is characteristic of Mycoplasmas, mycobacteria, Azotobacter, and many others.

Actinomycetes are single-celled prokaryotic organisms that form long, branched filaments resembling fungal mycelium (Fig. 2.16). The Structure of the mycelium is uniform across various representatives of actinomycetes. The thickness of a mycelial filament ranges from 0.5-1.5 µm (most commonly 0.7-1.0 µm). It can be short or long, but the total length of the mycelium is immeasurable. Gravimetric analysis has revealed that a colony weighing 1 mg has a total mycelial length reaching up to 1,000 m. The length of the mycelium can only be determined During the first hours of growth, when a spore or a fragment of mycelium sprouts into filaments located in a single plane On the surface of an Agar medium. After 20-30 hours of development, the mycelium proliferates to such an extent that its entire mass is obscured by multilayering.

Fig. 2.16. Some forms of actinomycete spore-bearers

When actinomycetes grow on a solid nutrient medium, they form dense, leathery colonies that grow into the medium. Their size is determined by the species and cultivation conditions, ranging from small (0.5-2 mm) to large—up to 1 cm or more.

Three types of actinomycete mycelium are distinguished on solid media: substrate mycelium (filaments develop deep within the solid medium, through which nutrients are absorbed); aerial mycelium (creeping directly along The surface of the nutrient medium); and aerial mycelium that rises above the medium surface as a dense, fluffy, powdery, velvety, or leathery mass. The aerial mycelium may sometimes be underdeveloped.