Cocci: spherical forms
Spherical bacteria are classified depending on how the cells are arranged relative to each other after the completion of the division process. This process is influenced by the number of planes of division.
- Micrococci (Micrococcus): from the Greek word micros (small). Cells exist in isolation, located individually.
- Diplococci (Diplococcus): from the Greek diploos (double). They form stable pairs. They differ in shape: pneumococci (Streptococcus pneumoniae) are elongated like a candle flame (lanceolate) and joined at their blunt ends. At the same time, gonococci (Neisseria gonorrhoeae) and meningococci (Neisseria meningitidis) resemble coffee beans apposed by their concave surfaces.
- Streptococci (Streptococcus): divide strictly in one plane. Since the cells maintain physical contact at the division site, they line up in long chains (from Greek streptos).
- Tetrads: form characteristic groups of four cells.
- Sarcinae (Sarcina): division takes place in three mutually perpendicular planes. The result is the formation of volumetric packets or "bales" (from Latin sarcina) comprising eight or more structural units.
- Staphylococci (Staphylococcus): planes of division arise randomly. Because of this, irregular clusters form, resembling bunches of grapes (from Greek staphyle).
Rod-shaped bacteria
Rods differ in a wide variety of parameters. Their length varies from 1.0 to 10 µm, and their thickness from 0.5 to 2.0 µm. An important diagnostic criterion is the geometry of the ends of the bacterial cell:
- Straight (blunt): typical of the anthrax causative agent (Bacillus anthracis).
- Rounded: found in most regularly shaped microorganisms, for example, Escherichia coli.
- Tapered: characteristic of representatives of the Fusobacterium group.
- Club-shaped (thickened): a sign of irregularly shaped bacteria such as Corynebacterium diphtheriae.
The vast majority of rods separate after replication. However, some species remain joined by remnants of the cell wall. They can be arranged in a chain (anthrax streptobacilli) or in a V-shape at an acute angle (corynebacteria).
Curved and branched forms
Curved microorganisms differ in the degree and nature of coiling. If a rod is only slightly curved, it is classified as a vibrio (a striking example is the cholera vibrio). Campylobacters and helicobacters have a specific bend visually resembling the wings of a flying gull. Bacteria with a pronounced spiral structure are called spirilla, and the thinnest and most intensely coiled forms are called spirochetes.
A separate morphological group consists of microorganisms with the ability to branch. These include actinomycetes, which form long intertwining filaments (true mycelium), demonstrating true branching. There are also rods bifurcated only at the poles—bifidobacteria.
Morphology and Gram stain
The structural diversity of bacteria is closely related to their cell wall type:
- Gram-negative (G-) include kidney-shaped diplococci, small veillonellae, classical rods, as well as the entire spectrum of curved forms (from vibrios to spirochetes) and intracellular parasites (chlamydia, rickettsiae).
- The Gram-positive (G+) group includes staphylococci, streptococci, pneumococci, and branching microorganisms.
A special place among G+ bacteria is occupied by spore-forming rods—bacilli and clostridia. They are capable of synthesizing protective endospores, which can be located centrally, subterminally (displaced from the center), or terminally (at the very edge of the cell).
Briefly: answers to topical questions
What genera of pathogenic spirochetes exist and how do their coils differ?
Pathogenic spirochetes for humans are divided into three genera, which differ in the number and shape of coils.
- Treponemes (Treponema) — have 8–12 small, uniform coils.
- Borrelia (Borrelia) — are distinguished by 3–8 large, sparse coils.
- Leptospira (Leptospira) — have shallow and frequent coils (resembling a twisted rope), ends curved in the form of hooks with thickenings, and due to secondary coils, the cell takes the shape of the letters S or C.
How does the structure of the cell wall of Gram-positive and Gram-negative bacteria differ?
The structure of the cell wall of these groups of bacteria differs in the thickness of the peptidoglycan layer and the presence of additional structures.
| Sign | Gram-positive bacteria | Gram-negative bacteria |
|---|---|---|
| Wall thickness | Massive, thick | Thin |
| Peptidoglycan | Multiple layers | Thin layer in the periplasm |
| Unique components | Teichoic and lipoteichoic acids | Outer membrane with LPS and porins |
| Structure linkage | Tightly adheres to the membrane | Linked to peptidoglycan via lipoproteins |
What complex staining methods are used to identify endospores of bacilli and clostridia?
Special staining methods are used to identify endospores, allowing the spore to be differentiated from the vegetative cell.
- Aujeszky's method (Oezhko method) — reveals spores.
- Ziehl-Neelsen method — also used for spore staining.
When using these methods, the spore stains red, and the vegetative cell acquires a blue-violet color.
How are bacteria classified according to the number and arrangement of flagella?
According to sources, the following types of bacteria are listed by flagellar arrangement:
- Monotrichates — have a single flagellum; this type is indicated for vibrios and for curved bacteria described as having a single polar flagellum.
- Lophotrichates — have a tuft of flagella; this type of flagellation is indicated for selenomonads.
The complete classification by number and arrangement of flagella is not provided in these data sources.
How to remember the shape of diplococci: Pneumococcus — resembles a Candle flame (letter P in Russian). Gonococcus — like Coffee peas (letter G in Russian).
Class and exam questions
Why do cocci form different clusters (chains, grape-like clusters, packets)?
The shape of clusters directly depends on the number of planes in which cell division occurs and whether physical connection between cells is maintained after division.
What is the main morphological difference between staphylococci and streptococci?
Streptococci divide strictly in one plane and line up in chains. Staphylococci divide randomly in different planes, forming clusters in the form of bunches of grapes.
Which bacteria are capable of forming endospores?
Endospore formation is characteristic exclusively of Gram-positive rod-shaped microorganisms, mainly bacilli and clostridia.
How are the cells of Corynebacterium diphtheriae arranged?
After division, their cells do not separate completely, but remain connected by a section of the wall, causing them to be arranged at an angle to each other (V-shaped).
What else to study on this topic
- Features of the cell wall in Gram-positive and Gram-negative bacteria
- Structure and formation process of bacterial endospores
- Morphological differences of obligate intracellular parasites: Rickettsia and Chlamydia
- Specific features of mycelium structure in Actinomycetes
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