Note: The classification below, based primarily on morphologic and biochemical characteristics, deviates from those derived from strict taxonomic principles in two ways.

Only medically important bacteria organisms are included.

  1.  

Characteristics Genus Representative Diseases
I. Rigid, thick-walled cells    

A. Free-living (extracellular bacteria)

    

1. Gram-positive

    

a. Cocci

 Streptococcus Pneumonia, meningitis
  Staphylococcus Abscess of skin and other organs

b. Spore-forming rods

    

(1) Aerobic

 Bacillus Anthrax

(2) Anaerobic

 Clostridium Tetanus, gas gangrene, botulism

c. Non–spore-forming rods

    

(1) Nonfilamentous

 Corynebacterium Diphtheria
  Listeria Meningitis

(2) Filamentous

 Actinomyces Actinomycosis
  Nocardia Nocardiosis

2. Gram-negative

    

a. Cocci

 Neisseria Gonorrhea, meningitis

b. Rods

    

(1) Facultative

    

(a) Straight

    

(i) Respiratory organisms

 Haemophilus Meningitis, Chancroid
  Bordetella Whooping cough
  Legionella Pneumonia

(ii) Zoonotic organisms

 Brucella Brucellosis
  Francisella Tularemia
  Pasteurella Cellulitis
  Yersinia Plague

(iii) Enteric and related organisms

 Escherichia Urinary tract infection, diarrhea
  Enterobacter Urinary tract infection
  Serratia Pneumonia
  Klebsiella Pneumonia, urinary tract infection
  Klebsiella granulomatis Granuloma Inguinale
  Salmonella Enterocolitis, typhoid fever
  Shigella Enterocolitis
  Proteus Urinary tract infection

(b) Curved

 Campylobacter Enterocolitis
  Helicobacter Gastritis, peptic ulcer
  Vibrio Cholera
    (2) Aerobic Pseudomonas Pneumonia, urinary tract infection
    (3) Anaerobic Bacteroides Peritonitis
  3. Acid-fast Mycobacterium Tuberculosis, leprosy

B. Non–free-living (obligate intracellular parasites)

 Rickettsia Rocky Mountain spotted fever, typhus, Q fever
  Chlamyda Urethritis, trachoma, psittacosis
II. Flexible, thin-walled cells (spirochetes) Treponema Syphilis
  Borrelia Lyme disease
  Leptospira Leptospirosis
III. Wall-less cells Mycoplasma Pneumonia