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Bacteria: Classification

Based on shape cell wall structure and Gram stain cellular respiration Growth factors (Energy Source and Nutrient Source) Based on Temperature Based on PH
Classification: Placing organisms in groups of related species. Lists of characteristics of known organisms.

Classify based on Shape

ShapeCoccus – spherical, round, or ovoidBacillus – rod-shapedSpirillum – spiral-shapedVibrio – comma-shaped Spirillum bacterial classification

Coccus

Coccobacillus
Bacillus
Vibrio
Spirillum
Spirochete
Pleomorphic
Flexible
Stiff
Slightly curved rods



Some are:Round – cocciStreptococci – chainsStaphlococci – in groups or clustersDiplococci – in pairsMicrococci Strep
Staph


Some are:Rod– bacillus Streptobacilli – chainsStaphlobacilli – in groups or clustersDiplobacilli – in pairs

Cocci Summary

Rods: Straight
E. coli

Gram Negative Rod Summary

* Spirillum

Spiral Forms

* Spirochetes

Classify based on Gram stain
Gram positive or Gram negative The stain is named for H. C. J. Gram, a Danish physician who invented it in 1884.


The Gram stain, which divides most clinically significant bacteria into two main groups, is the first step in bacterial identification. Bacteria stained purple are Gram + - their cell walls have thick petidoglycan and teichoic acid. Bacteria stained pink are Gram – their cell walls have have thin peptidoglycan and lipopolysaccharides with no teichoic acid.

In Gram-positive bacteria, the purple crystal violet stain is trapped by the layer of peptidoglycan which forms the outer layer of the cell. In Gram-negative bacteria, the outer membrane of ipopolysaccharides prevents the stain from reaching the peptidoglycan layer. The outer membrane is then permeabilized by acetone treatment, and the pink safranin counterstain is trapped by the peptidoglycan layer.

Classification of Bacteria

Gram + cocci Gram - bacilli

Classification of Bacteria

Gram + bacilli

Examples of Gram Positive bacteria

Streptococcus pyogenes - causes strep throat Staphylococcus aureus - causes skin infections and may be responsible for boils

Examples of Gram Negative Bacteria

Treponema pallidum - causes syphilis Escherichia coli - may cause severe gastrointestinal problems

Atypical Cell Walls

Some bacterial groups lack typical cell wall structure Mycobacterium and Nocardia Gram-positive cell wall structure with lipid mycolic acid pathogenicity high degree of resistance to certain chemicals and dyes basis for acid-fast stain Some have no cell wall Mycoplasma cell wall is stabilized by sterols pleomorphic

Classifying Bacteria by Cellular Respiration

Ability to grow with / without airAerobic bacteria, or strict aerobes - require oxygenAnaerobic bacteria, or strict anaerobes - cannot tolerate oxygenFacultative anaerobics – are generally aerobes, but have the capacity to grow in the absence of oxygenExamples of Bacteria Classified by Cellular Respiration:Aerobic:Bacillus cereusAnaerobic:Clostridium spp. ( botulism, tetanus)Facultative anaerobes:Staphylococcus spp.

Classifying Bacteria by Growth Factors

Under this scheme, they are generally classified according to:Energy sourceNutrient sourceEnergy SourceChemotroph – chemical compounds as an energy source (most pathogenic bacteria are chemotrophs.)Phototroph - light as energy sourceNutrient SourceHeterotroph – derive carbon from preformed organic nutrients such as sugar (most pathogenic bacteria are heterotrophs.)Autotroph – derive carbon from inorganic sources such as carbon dioxide

1. Temperatures

Minimum temperature lowest temperature that permits a microbe’s growth and metabolismMaximum temperature highest temperature that permits a microbe’s growth and metabolismOptimum temperature promotes the fastest rate of growth and metabolism

Temperature Adaptation Groups

3. pH
The pH Scale Ranges from 0 - 14 pH below 7 is acidic [H+] > [OH-] pH above 7 is alkaline [OH-] > [H+] pH of 7 is neutral [H+] = [OH-]

3. pH

Acidophiles optimum pH is relatively to highly acidic Neutrophiles optimum pH ranges about pH 7 (plus or minus) Alkaphiles optimum pH is relatively to highly basic


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