A DETAILED CLASSIFICATION OF BACTERIA

A BRIEF INTRODUCTION TO BACTERIA

⇒ Bacteria are a wide group of unicellular organisms that are ubiquitous in nature.

⇒ When cultures on agar, bacteria grow as colonies that contain many individual cells.

⇒ Bacteria are placed under the kingdom Protista and are prokaryotic cells.

⇒ The unit of measurement used in bacteriology is the micron (μ) or also called micrometer (μm).

⇒ 1 μm = 10^-6 or 0.000001 m



⇒ Largest known bacteria in the world is – Thiomargarita namibiensis

⇒ Smallest known bacteria in the world is – Mycoplasma genitalium

⇒ Longest known bacteria in the world is – Epulopiscium fishelsoni

⇒ There are different criteria for the classification of bacteria –

• Morphological
• Anatomical
• Staining
• Cultural characteristics
• Nutrition
• Environmental factors
• Biochemical reactions
• Antigenic structure

Morphological classification of bacteria – 

⇒ Cocci – These are spherical or oval cells. On the basis of arrangement of individual organisms they can be described as

• Monococci (Cocci in singles) – Monococcus
• Diplococci (Cocci in pairs) – Streptococcus pneumoniae
• Staphylococci (Cocci in grape-like clusters) – Staphylococcus aureus
• Streptococci (Cocci in chains) – Streptococcus pyogenes
• Tetrad (Cocci in group of four) – Micrococcus
• Octard (Cocci in group of eight) – Sarcinae

⇒ Bacilli – These are rod-shaped bacteria. On the basis of arrangement of organisms, they can be described as

• Diplobacilli
• Streptobacilli
• Palisades
• Chinese-letter form
• Coccobacilli
• Comma-shaped

⇒ Actinomycetes (actin- ray, mykes-fungus)

• These are rigid organisms like true bacteria but they resemble fungi in that they exhibit branching and tend to form filaments.
• They are termed such because of their resemblance to sun rays when seen in tissue sections.

⇒ Spirochetes

• These are relatively long, slender, non-branched microorganisms of the spiral shape having several coils.

⇒ Mycoplasma

• These bacteria lack in the rigid cell wall (cell wall lacking) and are highly pleomorphic and of indefinite shape.
• They occur in round or oval bodies and in interlacing filaments.

⇒ Rickettsiae and Chlamydiae

• These are very small, obligate parasites, and at one time were considered closely related to the viruses. Now, these are regarded as bacteria.

Check out the Morphology & Culture Characteristics of Escherichia coli

MORPHOLOGY AND CULTURE CHARACTERISTICS OF ESCHERICHIA COLI (E.COLI)

Anatomical classification of bacteria –

• Capsule
  • Capsulate– Streptococcus pneumoniae
  • Non-capsulate – Viridans streptococci
• Flagella
  • Flagellate –
    • Monotrichous
    • Lophotrichous
    • Amphitrichous
    • Peritrichous
  • Aflagellate – Shigella spp.
• Spore
  • Spore-forming – Bacillus spp.
  • Non-sporing – Escherichia coli

Classification of bacteria based on staining reaction –

• GRAM’S STAIN
  • Gram-positive cocci – Staphylococcus aureus
  • Gram-negative cocci – Nesseria gonorrhoeae
  • Gram-positive rods – Clostridium
  • Gram-negative rods – coli
• ACID FAST STAIN
  • Acid-fast bacilli – Mycobacterium tuberculosis
  • Non-acid-fast bacilli – Salmonella typhi

Classification of bacteria based on Cultural characteristics –

⇒ Extra growth factors requirements

• Fastidious – Hemophilus influenzae
• Non-fastidious – Escherichia coli

⇒ Hemolysis on Sheep Blood Agar

• Alpha-hemolysis – Streptococcus pneumoniae
• Beta-hemolysis – Streptococcus pyogenes
• Gamma–hemolysis –
• Target-hemolysis –
• Alpha-prime hemolysis –

⇒ Utilization of carbohydrates

• Oxidative – Micrococcus
• Fermentative – Escherichia coli

⇒ Growth rate

• Rapid growers– Vibrio cholerae
• Slow growers – Mycobacterium tuberculosis

⇒ Pigment production

• Pigment producer – Staphylococcus aureus
• Pigment non-producer – Escherichia col

Classification of bacteria based on Nutrition –

• Phototrophs – Those organisms which get energy from photochemical reactions are called as Phototrophs or phototrophic organisms.
• Chemotrophs – Those which get energy from chemical reactions are called as Chemotrophs or chemotropic organisms.
• Autotrophs – Those organisms which can synthesize essential metabolites from inorganic sources are called as Autotrophs or Autotrophic organisms.
• Heterotrophs – Those organisms which cannot synthesize their essential metabolites and depend upon external sources are Heterotrophs or heterotrophic organisms. Most of the pathogens are heterotrophic.

Check out the Morphology & Culture Characteristics of Salmonella typhi

MORPHOLOGY AND CULTURE CHARACTERISTICS OF SALMONELLA TYPHI (S. TYPHI)

Classification of bacteria based on environmental factors –

• Temperature
• Oxygen dependence
• pH
• Salt concentration

⇒ Temperature dependence –

• Psychrophiles (15-20⁰C) – Pseudomonas fluorescens
• Mesophiles (20-40⁰C) – Escherichia coli, Salmonella enterica, Staphylococcus aureus
• Thermophiles (50-60⁰C) – Bacillus stearothermophillus
• Extremely Thermophiles (as high as 250⁰C) –

⇒ Oxygen dependence –

• Aerobe – Those organisms that use oxygen for the growth and metabolism are called aerobes or aerobic organisms. They obtain energy from oxidative processes. E.g. – Staphylococcus spp., Streptococcus spp.
• Obligate aerobes – Those organisms that strictly require O₂ for their growth are called Obligate aerobes or Obligate aerobic organisms e.g. Pseudomonas aeruginosa.
• Microaerophilic – Those organisms that grow under reduced O₂, 5-10% and increased CO₂, 8-10% are called Microaerophilic organisms or microaerophiles e.g. – Campylobacter jejuni, Helicobacter pylori.
• Facultative anaerobe – Those organisms that are capable of growing either in presence or absence of O₂ are called facultative anaerobes or facultatively anaerobic organisms. – coli.
• Obligate anaerobe – Those organisms that strict absence of oxygen for the growth is called obligate anaerobe or obligate anaerobic organisms. e.g. – Clostridium
• Capnophilic – Those organisms that require increased concentration of CO₂, i.e., 5-10% for the growth is called Capnophilic organism or capnophile e.g. influenzae, N. gonorrhoeae.
• Aerotolerant – Those organisms which cannot use oxygen for growth, but tolerate its presence are called Aerotolerant organisms. e.g. – Streptococcus, Clostridium

⇒ pH dependence –

• Acidophiles – pH less than 3 – e.g. – Lactobacillus acidophilus
• Alkaliphiles – pH roughly 8.5–11 – e.g. – Vibrio
• Neutralophiles (pH 6.5-8) – e.g. – Streptococcus pneumoniae, Pseudomonas aeruginosa
• Majority of the medically important bacteria grow best at neutral or slightly alkaline reaction (pH 7.2-7.6).

⇒ Salt concentration –

• Non – Halophiles – Unable to grow in high salt concentration. e.g. – E.coli.
• Halotolerant – Tolerate low level of salt concentrations i.e. up to 8% salt concentration e.g. –
• Halophiles – those organisms that grow in high salt concentration are called Halophiles or halophilic (salt-loving) organisms. These are further classified as –

1.) Slightly halophilic – require 0.5-3% salt concentration for growth e.g. – Vibrio, Pseudomonas

2.) Moderately halophilic – requires 3-15% salt concentration for growth e.g. – Bacillus, Micrococcus

3.) Extremely halophilic – requires 15-30% salt concentration for growth e.g. – Halobacterium, Halococcus, Natranococcus spp.

SAHIL BATRA

Hi, I’m the Founder and Developer of Paramedics World, a blog truly devoted to Paramedics. I am a Medical Lab Tech, a Web Developer and Bibliophiliac. My greatest hobby is to teach and motivate other peoples to do whatever they wanna do in life.