Introduction to bacteria

Key features of a bacteria

Identifying bacteria

• Size, shape, color

• Culturing techniques

• Metabolic attributes

• DNA

Gram stain

• Usually the first test done to identify bacteria

• The Gram's stain differentiates between two major cell wall types.

• Gram positive and Gram negative

Gram positive• Gram positive bacteria have walls containing

relatively large amounts of peptidoglycan = a starch

– Staphylococcus epidermidis, Streptococcus pyogenes, Clostridium tetani, Bacillus anthacis (ANTHRAX)

Gram negative• Gram negative species have walls containing small

amounts of peptidoglycan and a lipopolysaccharide = a fat/sugar combo

– Escherichia coli, Salmonella typhi, Vibrio cholerae and Bordetella

– Gram negative bacteria are harder to control with antibiotics

Exceptions• Not all bacteria can be stained by

Gram's method • the best-known exceptions belong

to the genus Mycobacterium which have waxy cell wall. – These include Mycobacterium tuberculosis

(TB) and Mycobacterium leprae (leprosy).

Gram stain has four steps:

1. crystal violet, the primary stain

2. iodine, which acts as a mordant by forming a crystal violet-iodine complex

3. alcohol, which decolorizes

4. safranin, the counterstain.

G+ organisms are purple (or

bluish), G- organisms are red.

How does it work?

Cell envelope of Gram positive

Cell envelope of Gram negative

Summary

• Know how bacteria are different than eukaryotic cells

• Know key features of bacteria

• Know a key difference between Gram + and Gram - bacteria

• Know how the gram stain works and how we use it