1. Bergey's Phylogenetic Classification: @ In 1923 David Bergey published Bergey's Manual of Determinative Bacteriology. @ It arranged bacteria in 10 orders and 47 families. @ Also grouped bacteria into 19 "parts“ e.g. spirochetes, spiral and curved bacteria, gram-negative aerobic rods, and cocci. @ It contains a practical guide to identification of isolated bacterial types.

3. Molecular Classification @ The phylogenetic similarities of bacteria are estimated by the degree of DNA homology between the nucleic acid sequences. @ Also the identity of rRNA of organisms can be estimated from the identical spots seen following enzymatic separation of rRNA. @ In this way, the genetic similarity of the organisms is compared together. @ Hybridization technique detects homologous DNA sequences & identifies organisms.

5. Common Classification of Medical Bacteria: I. Rigid, thick–walled cell II. Flexible, thin-walled cells, III. Wall-less cells, I.Rigid, thick-walled cell: Divided into: A) Free-living outside cells, divided into: 1- Gram positive, divided into: a. Cocci, e.g.: Staphylococcus. b. Spore-forming rods, divided into: (i) Aerobic, e.g.: Bacillus. (ii) Anaerobic, e.g.: Clostridium.

7. c. Non-spore forming rods, divided into: (i) Non-filamentous, e.g.: Corynebacterium. (ii) Filamentous, e.g.: Actinomyces & Nocardia 2- Gram negative, divided into: a. Cocci, e.g.: Neisseria. b. Rods, divided into: (i) Facultative anaerobic, divided into: 1. Straight, divided into: a. Respiratory organisms, e.g.: Bordetella. b. Zoonotic organisms, e.g.: Brucella. c. Enteric and related organisms, e.g. E.coli.

9. 2. Curved e.g. Vibrio, Helicobacter. (ii) Aerobic, e.g.: Pseudomonas (iii) Anaerobic, e.g.: Bacteroides. c. Acid-fast, e.g. Mycobacterium. B) Non-free living inside cells, e.g.: Chlamydia. II. Flexible, thin-walled cells, e.g.: Treponema III. Wall-less cells, e.g.: Mycoplasma.

11. Bacterial pathogens: Bacterial pathogens may be divided into: 1- Readily gram-stained 2- Not readily gram-stained 1- Readily Gram-stained: Divided into: a) Gram positive cocci, e.g. Staph. & Strep. b) Gram negative cocci, e.g.: Neisseria. c) Gram positive rods, e.g.: Corynebacterium, Bacillus, Clostridia, Nocardia, Actinomyces

13. d) Gram negative rods, divided into: (i) Gastro-intestinal tract organisms, e. g. 1- Pathogenic inside & outside GIT: E.coli, Salmonella. 2- Pathogenic inside GIT only, e.g. Shigella, Vibrio, Campylobacter, Helicobacter. 3- Pathogenic outside GIT, e.g.: Klebsiella, Pseudomonas, Proteus, Bacteroides.

15. (ii) Respiratory tract organisms, e.g. Legionella, Haemophilus, Bordetella. (iii) Zoonotic bacteria, e.g. Brucella, Pasteurella Yersinia, Francisella. 2- Not readily gram-stained: divided into: a) Obligate intracellular pathogens, e.g. Chlamydia, Rickettsia. b) Not obligate intracellular parasites, e.g. Treponema, Mycobacteria, Mycoplasma, Leptospira.