Let us make an in-depth study of the biological and morphological classification of microorganisms.
Biological Classification of Microorganisms:
Microorganisms may be defined as living creatures that are microscopic in size and relatively small, unicellular in structure. The diameter of the smallest body that can be resolved and seen clearly with the naked eye is about 100 µ (1 µ, or micron = 0.001 millimeter). All microorganisms are smaller than 100 µ.
Therefore, a microscope is necessary for the observation. Light microscope can resolve down to 0.2 n in diameter and thus includes all microbes except viruses which can be seen only under electron microscope whose limit of resolution is 0.0005 µ (i.e., 0.5 mµ = millimicron = 5 Angstrom Units).
One Angstrom Unit = 1/10th of 1 mµ. When microorganisms grow on solid or semi-solid media, their progenies accumulate locally to form masses or colonies which are visible to the naked eye. Microorganisms constitute a very antique group of living organisms which appeared on the Earth’s surface almost 1.5 billion years ago. Some scientists believe that the microbes were the first living microorganisms Of the Earth.
It is now regarded that the organisms evolved along the following lines:
Viruses containing Ribonucleic acid (RNA), Viruses containing Deoxyribonucleic acid (DNA), chlamydiae, rickettsiae, mycoplasma, bacteria, blue-green algae, lower and higher fungi, plants and animals. In order to decide to which species the microorganism under investigation should belong, it is necessary, first of all, to study its main properties and then to identify it by these properties, and at last to determine its position in the classification of microbes according to a key.
In microbiology, the binomial system of nomenclature is accepted where each species has a generic and a specific name. The generic name is written with a capital letter and the specific name with a small letter. For example, Staphylococcus aureus (the golden pus coccus), Bacillusanthracis (the anthrax bacillus), Corynebacterium diphtheria (the diphtheria bacillus), Mycobacterium tuberculosis (the tubercle bacillus).
International classification founded by an American bacteriologist, D. Bergey, is the most commonly used in microbiology. It is manual of determinative bacteriology. It includes descriptions of more than 1,500 species of microbes, though there are more than 3,000 known species of bacteria.
All lower organisms of the plant kingdom, Protophyta, are subdivided into three classes:
Schizomycetes (microorganisms lacking chlorophyll);
Microtatobiotes (rickettsiae, viruses, filterable microbes).
Schizophyceae, consists of a large group of blue-green algae which are not pathogenic to man and animals.
Schizomycetes comprises 10 orders of 5 of which (II, III, VI, VII, VIII) are non-pathogenic to man and animals. Certain families and genera of the orders I, IV, V, IX, X contain species pathogenic to man and animals.
Class II. Schizomycetes:
1. Order Pseudomonades (motile cells, with polar flagellation and non-motile cells).
7. Family Spirillaceae
1. Genus Vibrio
7. Genus Spirillum
2. Order Chlamydobacteriales (filamentous bacteria, motile and non-motile).
3. Order Hyphomicrobiales (bacteria multiplying mainly by budding).
4. Order Eubacteriales (true bacteria, rod- shaped, coccal-shaped, motile with flagella arranged peritrichously, and non-motile).
4. Family Enterobacteriaceae:
1. Genus Escherichia
2. Genus Aerobacter
3. Genus Klebsiella
8. Genus Proteus
9. Genus Salmonella
10. Genus Shigella
5. Family Bruceliaceae:
1. Genus Pasteur Ella
2. Genus Francis Ella
3. Genus Bordetella
4. Genus Brucella
5. Genus Haemophilus
6. Genus Actinobacillus
6. Family Bacteroidaceae:
1. Genus Bacteroiaes
2. Genus Fusibacterium
7. Family Micrococcaceae:
1. Genus Micrococcus
2. Genus Staphylococcus
3. Genus Kaffkya
4. Genus Sarcina
5. Genus Peptococcus
8. Family Neisseriaceae:
1. Genus Neisseria
10. Family Lactobacillaceae:
1. Genus Diplococcus
2. Genus Streptococcus
5. Genus Peptostreptococcus
12. Family Corynebacteriaceae:
1. Genus Corynebacterium
2. Genus Listeria
3. Genus Erysipelothrix
13. Family Bacillaceae:
1. Genus Bacillus
2. Genus Clostridium
5. Order Actinomycetales (actinomycetes, filamentous, branching cells).
1. Family Mycobacteriaceae
1. Genus Mycobacterium
2. Family Actinomycetaceae
1. Genus Nocardia
2. Genus Actinomyces
3. Family Streptomycetaceae
6. Order Caryophanales (large, non-motile, segmented cells, joined in filaments).
7. Order Beggiatoales (cells occur singly, in filaments sometimes spherical-shaped, sulphur globules and found in cells).
8. Order Myxobacteriales (slime bacteria, rod- shaped, non-rigid cells, motile, non-flagellate).
9. Order Spirochaetaies (spirochaetes and treponemas, cytoplasm in the form of spirals with at least one turn about a slender axial filament, non- rigid, non-motile).
1. Family Spirochaetaceae
2. Family Treponemataceae
1. Genus Borrelia
2. Genus Treponema
3. Genus Leptospira
10. Order Mycopiasmatales (small, pleomorphic,filterable microorganisms, pathogenic species cause pleuropneumonia in cattle, agalactia in goats and sheep, atypical pneumonia in humans).
Class III. Microtatobiotes:
1. Order Rickettsia’s:
1. Family Rickettsiaceae
1. Genus Rickettsia
2. Genus Coxiella
2. Family Chlamydiaceae:
1. Genus Chlamydia
2. Genus Miyagawanella
3. Family Bartoneliaceae:
1. Genus Bartonella
4. Family Anaplasmataceae
1. Genus Anaplasma
2. Type viruses (Vira) are divided into two subtypes:
Subtype Deoxyvira which includes viruses containing DNA and Subtype Ribovira which consists of viruses containing RNA. The new classification of viruses was discussed at the Ninth International Congress of Microbiology. This classification is based on the type of nucleic acid (DNA or RNA), anatomy of viruses, type of symmetry (spiral, cuboid, binary), the number of capsomeres in the viruses with a cuboid symmetry and on the presence of an external membrane.
All the viruses known (more than 3,000) are grouped into 5 classes, 8 orders, 21 families. The families are composed of genera which in their turn are made up of species designated in Latin, according to the binomial principle (e.g. family—Herpesviridae, genus — Herpes virus; species — Herpes simplex).
Viruses Containing Ribonucleic Acid (RNA):
Viruses of influenza, para influenza, epidemic parotitis, measles, rabies.
Virus of epidemic poliomyelitis, Coxackie and Enteric Cytopathogenic Human Orphan (ECHO) virus, rhinoviruses, foot and mouth disease virus and infectious hepatitis.
Viruses of tick-borne encephalitis, Japanese encephalitis, St. Louis encephalitis, haemorrhagic fever, yellow fever, Dengue fever, sand-fly fever.
Viruses Containing Deoxyribonucleic Acid (DNA):
Viruses of smallpox.
Viruses of human herpes, chicken pox, herpes zoster.
Viruses of tumours (oncogenic viruses).
This classification does not include fungi and unicellular Protozoa which belong to independent phyla and are studied separately from bacteria, actinomycetes, rickettsiae and viruses.
Morphological Classification of Microorganisms:
Size of Bacteria:
The unit measurement of bacteria is the micron (μ) which is equal to 1/1000th of a millimeter. The size of bacteria varies from 0.1 μ for Mycoplasma laidlawii, the smallest bacteria, to 2-3 by 15-20 μ. for Spirillum voiutans and even up to 1 mm for the giant sulphur bacteria. The majority of the pathogenic bacteria have dimensions from 0.2 to 7 0 µ. (Cocci—1 in diameter; bacilli — 3 in length, 0.3µ thick; spirochaetes — 4-16 µ in length and 0.1 – 0.6 µ thick).
The main groups of bacteria are classified by their morphology and Gram-staining reactions into:
I. Lower Bacteria—:
Simple, generally unicellular structures, never in the form of a mycelium or spherical filaments:
Straight rod shaped cells.
Curved rod-shaped (comma- shaped) cells.
Thin, spirally twisted, flexuous filaments.
Spirally twisted, non-flexuous rods (Table 1.1).
Cocci (Lat. Coccus – spherical):
According to the mode of cell grouping and the reaction to Gram- stain, cocci are distinguished into:
1. Diplococcus (Gr. diplos – double) — Cells mainly adherent in pairs and slightly elongated in axis of pair; Gram-positive (e.g., D. pneumonia).
2. Streptococcus (Gr. Streptos – Chain) — Cells mainly adherent in chains, due to successive cell divisions occurring in the same axis; Gram-positive (e.g., Streptococcus pyogenes).
3. Staphylococcus (Gr. Staphyle — cluster; bunch of grapes)—Cells mainly adherent in irregular clusters due to successive division occurring irregularly in different planes; Gram-positive (e.g., Staphylococcus aureus). The genus Micrococcus is similar to Staphylococcus, but it is motile, weakly Gram-positive and differs in size.
4. Gaffkya—Cells mainly adherent in plates of four (tetrads) due to division occurring successively in two planes at right angles; Gram-positive (e.g., Gaffkya tetragena — Producer of four).
5. Sarcina (Lat. Sarcio—to tie) divide in three planes at right angles to one another and resemble packets of 8, 16 or more cells. They are not virulent and are frequently found in the air.
6. Neisseria — Cells mainly adherent in pairs and slightly elongated at right angles to axis of pair; Gram-negative (e.g., Neisseria meningitidis).
On the basis of Gram and Ziehl-Neelsen staining reaction and spore formation, they are divided into:
1. Acid fast bacilli (AFB)—:
The genus Mycobacterium includes Mycobacterium tuberculosis (tubercle bacillus).
2. Gram-positive Spore forming bacilli—:
The genera Bacillus (aerobic) and Clostridium (anaerobic) are primarily Gram-positive. The size, shape and position of spores may assist recognition of the species, e.g., the tetanus bacillus is characterised by its bulging, spherical, terminal spore (drum-stick form).
3. Gram-positive non sporing bacilli—:
These include several genera of which Corynebacteriumts distinguished by its curving or club shape.
4. Gram negative bacilli—:
Include numerous genera belonging to the families Pseudomonadaceae, Achromobacteriaceae, Enterobacteriaceae, Brucellaceae and Bacteroidaceae. Pseudomonas is distinguished by its polar flagellation, whereas motile members of other families are peritrichously flagellate.
Vibrios and Spirilla:
Vibrios are recognised as short, non-flexuous, curved rods (e.g., Vibrio cholerae) and spirilla (Lat. Spira – Twist as non-flexuous spiral filaments e.g., Spirillum minus). They are Gram-negative and mostly motile having polar flagella and showing a very active, “darting” motility.
The spirochaetes are slender, flexuous, spiral filaments and are Gram-negative. They can be distinguished from spirilla because they are capable of active flexion of the cell body and are motile without flagella.
The pathogenic spirochaetes are classified into three genera:
1. Borrelia—are larger and more refractile than the other pathogenic spirochaetes and are more easily stained by ordinary methods. Their coils are large and open with a wave length of 2-3 µ. At least 8-12 fibrils of 0.02 µ thick are found twisted around the whole length of the protoplast under electron microscope (Borrelia recurrentis).
2. Treponema—are thin filaments with shorter wave length (e.g., 1.0 -1.5 µ) coils typically similar to regular “corkscrew” form. They are feebly refractile and difficult to stain except by silver impregnation method. Under electron microscope, 2-3 fibrils are seen wound round the protoplast of the cell wall (e.g., Treponema pallidum).
3. Leptospira—have very fine and close coils of wave length of 0.5 µ, which are clearly seen under electron microscope, winding round a single axial filament. Under dark ground microscope, one or both the extremities of the spirochaetes are seen “hooked or recurved, so that they may take the shape of walking stick, an S or C” (e.g., Leptospira icterohaemorrhagiae).
II. Higher Bacteria—:
Filamentous organisms sheathed or branching to form a mycelium e.g., Actinomycetes — simple branching filaments forming a mycelium; may form conidia; non-motile.
Actinomycetales (mycelial bacteria) include three main genera:
1. Actinomyces — have tendency to fragment into coccal and bacillary forms, do not form conidia; are anaerobic and Gram-positive, non-acid fast (e.g(, Actinomyces israeli).
2. Nocardia—are similar to Actinomyces, but aerobic and mostly acid-fast (e.g., Nocardia farcinica).
3. Streptomyces—their vegetative mycelium does not fragment into short forms; form aerial hyphae, conidia are formed in chains (e.g., Streptomyces griseus).