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

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

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.

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.

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.

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

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.

(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.