Bacteria are unicellular organisms that lack organelles. They are classified by shape and appearance on gram stain. Shape:
- Cocci = spheres
- Bacilli = rods
- Spirochetes = coils
- Vibrios = comma shapes
Gram stain:
- Crystal violet stain is applied, fixed with iodine, then washed off with ethanol. Another counterstain is added
- Gram positive bacteria have exposed, thick peptidoglycan \(\to\) stain with crystal violet
- Gram negatives have an outer cell membrane, with a thin cell wall contained in the "periplasm" between this and their inner cytoplasmic membrane \(\to\) do not retain crystal violet \(\to\) stained by counterstain
Gram positive cocci
- Staphylococcus (incl S. Aureus)
- Streptococcus (incl. Pneumococcus)
- Enterococci
Gram positive rods
- Listeria
- Clostridium difficile
Gram negative cocci
- Neisseria gonorrhoeae
- Neisseria meningidites
Gram negative rods
- E coli
- Pseudomonas
- Klebsiella
Spirochetes (all gram negative, though stain poorly)
- Leptospira (causes leptospirosis)
- Trenonema Pallidum (causes syphilis)
Vibiros (all gram negative)
Mycobacteria are not visible on gram stain.