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 –
- Cultural characteristics
- 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
- Chinese-letter form
⇒ 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.
- These are relatively long, slender, non-branched microorganisms of the spiral shape having several coils.
- 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 –
- Capsulate– Streptococcus pneumoniae
- Non-capsulate – Viridans streptococci
- Flagellate –
- Aflagellate – Shigella spp.
- Flagellate –
- 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 –
- Oxygen dependence
- 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.
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