As per the morphological basis, bacteria are classified into six major groups which are depicted as follows:
Flowchart: Morphology of bacteria
1. True bacteria: They are a major group of
classification which is further divided into two sub-groups namely Cocci and
Bacilli. Cocci are of various types. Like Mono cocci (single cocci): Monococcus
species, Diplococci (Cocci in pairs): Streptococcus pneumonia, Staphylococci
(Cocci in a bunch form): Staphylococcus aureus, Streptococci (Cocci in chain
form): Streptococcus pyogenes, Tetrad (Cocci in the group in four): Micrococcus
species, Sarcina (Cocci in the group in eight, cube-like): Sarcina ventriculi (Fig.
1).
Fig. 1: Various cocci
Thereafter, Bacilli, which are rod-like structures based on the arrangement of organisms they are classified into: Diplobacilli
(Pairs), streptobacilli (Chais), palisades, coccobacilli (Oval), comma-shaped
bacilli (Vibrio), club rods (Corynebacteriaceae), enlarged rods (Fusobacterium)
(Fig. 2).
Fig. 2: Various Bacilli
2. Actinomycetes: They are rigid organisms but look
like fungi. They exhibit branching and tend to form filaments. Basically, they
are Gram-positive, but several species have complex cell wall structures that
make the Gram staining unsuitable (e.g. Mycobacteriaceae). They are widely
distributed in soil, compost, etc. They are heterotrophic, aerobic, and
mesophilic organisms. Some organisms are thermophilic growing at more than 55°C
like thermoactinomycetes, streptomyces, etc. (Fig. 3).
Fig. 3: Various Actinomycetes
3. Spirochaetes: They are double-membrane bacteria,
most of which have long, non-branched, helically coiled cells. They are Gram-negative
and are chemoheterotrophic in nature. The length varies between 3 and 500 µm
and diameters around 0.09 to 3 µm. Most spirochaetes are free-living and
anaerobic in nature. Example: Leptospira interrogans (Fig. 4).
Fig. 4: Spirochaete
4. Mycoplasmas: They are the smallest bacterial cells
and do not have rigid cell walls. They are indefinite shape and highly
pleomorphic (alter their shape or size in response to environmental conditions)
in nature. They can be parasitic or saprotrophic. Several species of
mycoplasmas are pathogenic in humans. Like Mycoplasma pneumoniae which causes
pneumonia and other respiratory disorders. M. genitalium which involves in
pelvic inflammatory diseases (Fig. 5).
Fig. 5: Structure of Mycoplasma
5. Rickettsia: They are non-motile, Gram-negative,
non-spore forming, small intracellular, and pleomorphic bacteria. They are also
present as cocci (0.1 µm in diameter), rods (1-4 µm long), or thread-like (10
µm long) forms. Rickettsia species are the pathogens responsible for typhus,
rickettsialpox, and Boutonneuse fever and are susceptible to antibiotics of the
tetracycline group. Examples: Rickettsia conorii, Rickettsia typhi, etc. (Fig. 6).
Fig. 6: Rickettsia typhi
6. Chlamydiae: They are ovoid in shape and Gram-negative
bacteria. Their cell wall consists of peptidoglycan and other proteins. They
are very small, obligate intracellular parasites. They multiply in the
cytoplasm of their host cell by a distinctive developmental cycle. They
commonly infect humans are eye disease, pneumonia, psittacosis, etc. Examples:
Chlamydia trachomatis (causes trachoma eye disease), Chlamydophila pneumonia
(causes pneumonia), Chlamydophila psittaci (causes psittacosis), etc. (Fig. 7).
Fig. 7: Chlamydia species