CLASSIFICATION OF BACTERIA

Microorganisms are very minute microscopic structures that are capable of free living. Some of the micro-organisms are non- pathogenic and live on the body of human beings like on skin, in nostrils and intestinal tract, etc. and they are called commensals. The organisms that are able to cause disease are called pathogenic organisms.

CLASSIFICATION OF MICRO-ORGANISMS

There are two groups depending upon the structure of cells:-
1. Prokaryotes
2. Eukaryotes

PROKARYOTES

This group includes those organisms that have a very simple cell structure. The nuclear material in them is in the form of a single chromosome that is not surrounded by a nuclear membrane. They divide by simple binary fission. Examples are bacteria, mycoplasmas, chlamydia and rickettsiae.

EUKARYOTES

These organisms have complete cell structures similar to the higher organisms. The nuclear material is bound by a nuclear membrane to form a nucleus. They have more than one chromosome. They have complete enzyme systems of their own and they divide by mitosis. Examples are fungi and protozoa.

THE CLASSIFICATION OF BACTERIA

Bacteria can be classified, depending upon:
Morphology
Gram staining
Requirement for oxygen
DNA homology(similarity)

A. MORPHOLOGICAL CLASSIFICATION
They are divided into the following groups, depending upon their shape in
1. Cocci; round or oval in shape
2. Bacilli; rod-shaped
3. Vibrios; comma-shaped
4. Spirochetes; spiral-like

Intermediate shapes like cocco-bacilli also exist.

B. CLASSIFICATION BASED ON GRAM STAINING
1. Gram positive -bacteria take
purple stain in a Gram-stained smear.
2. Gram negative – when bacteria stain pink in a Gram-stained smear. Gram-positive bacteria, when dead, may also stain red.
3. Gram variable – when the organism is Gram-positive but appears Gram-negative or the origin is negative but appears as Gram-positive on Gram staining.

C. CLASSIFICATION BASED ON OXYGEN REQUIREMENT

Strict Aerobes: They grow in the presence of oxygen
Anaerobes: They can be of two types-
i. Strict (Obligatory) Anaerobes: Theygrow only in the absence ofoxygen.
ii. Facultative Anaerobes:
They can grow both in the presence or absence of oxygen. Most of commonly isolated bacteria belong to this group.
c. Carboxyphilic: They require a high-percentage (10%) presence of carbon dioxide.
d. Microaerophilic: They require only small amounts of oxygen for their growth(2-10%). A high concentration of oxygen kills these organisms. Examples include Campylobacter spp.

IMPORTANT GROUPS OF BACTERIA

Gram-positive Cocci
i. Aerobes (Facultative anaerobes)
Staphylococcus species
Streptococcus species
Enterococcus species
ii. Anaerobes (obligatory)
Peptococcus species
Peptostreptococcus species
Ruminococcus species

Gram-positive Rods (bacilli)
i. Aerobes (Facultative anaerobes)
Corynebacterium species
Bacillus species
Listeria species
Lactobacillus species
Nocardia species
ii. Anaerobes (obligatory)
Clostridium species
Actinomyces species

c. Gram-negative Cocci
i. Aerobes (Facultative anaerobes)
Neisseria species
Moraxella species
ii. Anaerobes (obligatory)
Veillonella species
d. Gram-negative Rods (bacilli)
i. Aerobes (Facultative Anaerobes)
Escherichia coli
Klebsiella species
Proteus species
Shigella species
Salmonella species
Vibrio species
ii. Strict Aerobes
Aeromonas species
Plesiomonas species
Mycobacterium tuberculosis
Pseudomonas species
iii. Anaerobe (obligatory)
Bacteroides species
Fusobacterium species
iv. Micro-aerophilic
Campylobacter species
Helicobacter pylori
Prevotella species

e. Gram-negative Coccobacilli
i. Aerobes (facultative anaerobes)
Haemophilus species
Bordetella species
Brucella species
Legionella species
Franciscella species

f. Spirochaetes
i. Aerobe

Leptospira species
ii. Micro-aerophilic
Treponema species
Borellia species

INTRA-CELLULAR ORGANISMS

Bartonella bacilliformis
Chlamydia species
Rickettsiae species

CELL WALL-DEFICIENT ORGANISMS
Mycoplasma species
ā€˜Lā€™ form of bacteria

Bibliography
Microbiology Mentoring Package. American Society of Microbiology.
Greenwood, Barer, Slack, Irving. Medical Microbiology. Eighteenth Edition. Churchill Livingstone.
Warren Levinson. Review of Medical Microbiology and Immunology. Thirteenth Edition.Mc Graw Hill Education.

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