CHAPTER 2 Prokaryotic microorganisms

1. CHAPTER 2Prokaryotic microorganisms

2. Overview of microorganisms • The Earth is 4.6 billion years old and microbial life is thought to have first appeared between 3.8 and 3.9 billion years ago; in fact, 80% of Earth's history was exclusively microbial life. Microbial life is still the dominant life form on Earth. It has been estimated that the total number of microbial cells on Earth on the order of 2.5 × 1030 cells, making it the major fraction of biomass on the planet. • There are various hypotheses as to the origin of prokaryotic and eukaryotic cells. Because all cells are similar in nature, it is generally thought that all cells came from a common ancestor cell termed the last universal common ancestor (LUCA). These LUCAs eventually evolved into three different cell types, each representing a domain. The three domains are the Archaea, the Bacteria, and the Eukarya.

3. Overview of microorganisms • In any event, it is accepted today that there are three distinct domains of organisms in nature: Bacteria, Archaea, and Eukarya.

4. 2.1 Bacteria and Archaea • Bacteria are prokaryotic cells. Like the Eukarya, they have membranes composed of unbranched fatty acid chains attached to glycerol by ester linkages. The cell walls of Bacteria, unlike the Archaea and the Eukarya, contain peptidoglycan. • Bacteria are sensitive to traditional antibacterial antibiotics but are resistant to most antibiotics that affect Eukarya. • Bacteria contain rRNA that is unique to the Bacteria as indicated by the presence molecular regions distinctly different from the rRNA of Archaea and Eukarya. • Bacteria include mycoplasmas, cyanobacteria, Gram-positive bacteria, and Gram-negative bacteria.

5. Membrane Lipids of Archaea, Bacteria, and Eukarya • The Bacteria and the Eukarya have membranes composed of unbranched fatty acid chains attached to glycerol by ester linkages. • The Archaea have membranes composed of branched hydrocarbon chains attached to glycerol by ether linkages.

6. Bacteria and Archaea • Archaea are prokaryotic cells. Unlike the Bacteria and the Eukarya, the Archaea have membranes composed of branched hydrocarbon chains (many also containing rings within the hydrocarbon chains) attached to glycerol by ether linkages. The cell walls of Archaea contain no peptidoglycan. Archaea are not sensitive to some antibiotics that affect the Bacteria, but are sensitive to some antibiotics that affect the Eukarya. • Archaea contain rRNA that is unique to the Archaea as indicated by the presence molecular regions distinctly different from the rRNA of Bacteria and Eukarya. • Archaea often live in extreme environments and include methanogens, extreme halophiles, and hyperthermophiles. One reason for this is that the ether-containing linkages in the Archaea membranes is more stabile than the ester-containing linkages in the Bacteria and Eukarya and are better able to withstand higher temperatures and stronger acid concentrations.

7. 2.1.1 Sizes, Shapes, and Arrangements of Bacteria • Bacterial cell shape is determined primarily by a protein called MreB. MreB forms a spiral band – a simple cytoskeleton – around the interior of the cell just under the cytoplasmic membrane. It is thought to define shape by recruiting additional protens that then direct the specific pattern of bacterial cell growth. For example, bacillus-shaped bacteria that have an inactivated MreB gene become coccoid shaped, and coccus-shaped bacteria naturally lack the MreB gene. Gram Stain of Staphylococcus aureus Gram Stain of Escherichia coli

8. Sizes, Shapes, and Arrangements of Bacteria • Most bacteria come in one of three basic shapes: coccus, rod or bacillus, and spiral. • Exceptions to the above shapes: Trichome-forming, sheathed, stalked, filamentous, square, star-shaped, spindle-shaped, lobed, and pleomorphic.

9. Sizes, Shapes, and Arrangements of Bacteria • Coccus: The cocci are spherical or oval bacteria having one of several distinct arrangements based on their planes of division. Diplococcus, tetrad, sarcina, staphylococcus. • An average coccus is about 0.5-1.0 micrometer (µm) in diameter.

10. Sizes, Shapes, and Arrangements of Bacteria • Bacillus: Bacilli are rod-shaped bacteria. Bacilli all divide in one plane producing a bacillus, streptobacillus, or coccobacillus arrangement. • An average bacillus is 0.5-1.0 µm wide by 1.0-4.0 µm long.

11. Sizes, Shapes, and Arrangements of Bacteria • Spirals: Spirals come in one of three forms, a vibrio, a spirillum, or a spirochete. • Spirals range in size from 1 µm to over 100 µm in length.

12. Vibrio • Curved or comma-shaped rod • A vibrio appears as a curved bacillus (arrows).

13. Spirillum • Thick, rigid spiral.

14. Spirochaeta • thin, flexible spiral. • The spirochete Borrelia (arrows) in a blood smear.

15. Scanning Electron Micrograph of Leptospira interrogans

16. Exceptions to the above shapes • Trichome -forming, sheathed, stalked, filamentous, square, star-shaped, spindle-shaped, lobed, and pleomorphic. Filamentous Bacterium Alysiella filiformis

17. 2.1.2 Cell Structure of Bacteria • The mycoplasmas are the only bacteria that naturally lack a cell wall. Mycoplasmas maintain a nearly even pressure between the outside environment and the cytoplasm by actively pumping out sodium ions. Their cytoplasmic membranes also contain sterols that most likely provide added strength. • All other bacteria have a cell wall. The Bacteria, with the exception of the Chlamydias, have a semirigid cell wall containing peptidoglycan. Peptidoglycan prevents osmotic lysis. • The Archaea, that are often found growing in extreme environments, also have a semirigid cell wall but it is composed of chemicals distinct from peptidoglycan such as protein or pseudomurein.

18. The Gram-Positive Cell Wall • Prokaryotic Cell (Bacillus megaterium).

19. Electron Micrograph of a Gram-Positive Cell Wall

20. Structure of a Gram-Positive Cell Wall • The Gram-positive cell wall appears as dense layer typically composed of numerous rows of peptidoglycan, and molecules of lipoteichoic acid, wall teichoic acid and surface proteins.

21. The Gram-Negative Cell Wall • Freeze-Facture of a Gram-Negative Bacterium Showing the Various Layers of the Cell Wall and Cytoplasmic Membrane (View from the Top).

22. The Gram-Negative Cell Wall • Freeze-Facture of a Gram-Negative Bacterium Showing the Various Layers of the Cell Wall and Cytoplasmic Membrane (Views from the Inside).

23. Electron Micrograph of a Gram-Negative Cell Wall

24. Structure of a Gram-Negative Cell Wall • The Gram-negative cell wall is composed of a thin, inner layer of peptidoglycan and an outer membrane consisting of molecules of phospholipids, lipopolysaccharides (LPS), lipoproteins and sutface proteins. The lipopolysaccharide consists of lipid A and O polysaccharide.

25. Gram-Positive Cell Wall & Gram-Negative Cell Wall

26. Structure of an Acid-Fast Cell Wall • In addition to peptidoglycan, the acid-fast cell wall of Mycobacterium contains a large amount of glycolipids such as mycolic acid, arabinogalactan-lipid comlex, and lipoarabinomannan.

27. A Peptidoglycan Monomer • The peptidoglycan monomer in E. coli, most gram-negative bacteria, and many gram-positive bacteria. These monomers join together to form chains and the chains are then joined by cross-links between the tetrapeptides to provide strength.

28. Peptidoglycan of Staphylococcus aureus

29. Peptidoglycan of Escherichia coli

30. Structure of Peptidoglycan • Peptidoglycan is composed of chains of peptidoglycan monomers (NAG-NAM-tetrapeptide). These monomers join together to form chains and the chains are then joined by cross-links between the tetrapeptides to provide strength.

31. Specific staining • Most bacteria can be placed into one of three groups based on their color after specific staining procedures are performed: Gram-positive, Gram-negative, Acid-fast.

32. Gram-positive • Retain the initial dye crystal violet during the Gram stain procedure and appear PURPLE when observed through the microscope. • Common gram-positive bacteria of medical importance include Streptococcus pyogenes, Streptococcus pneumoniae, Staphylococcus aureus, Enterococcus faecalis, and Clostridium species.

33. Gram Stain of Staphylococcus aureus • Note gram-positive (purple) cocci in clusters.

34. Gram-negative • Decolorize during the Gram stain procedure, pick up the counterstain safranin, and appear pink when observed through the microscope. • Common Gram-negative bacteria of medical importance include Salmonella species, Shigella species, Neisseria gonorrhoeae, Neisseria meningitidis, Hemophilus influenzae, Escherichia coli, Klebsiella pneumoniae, Proteus species, and Pseudomonas aeruginosa.

35. Gram Stain of Escherichia coli • Note gram-negative (pink) bacilli.

36. A Gram Stain of a Mixture of Ggram-Positive and Gram-Negative Bacteria • Note gram-negative (pink) bacilli and gram-positive (purple) cocci.

37. Acid-fast • Acid-fast bacteria stain poorly with the Gram stain procedue, appearing weakly gram-positive or gram-variable. They are usually characterized using the acid-fast staining procedure. • Bacteria with an acid-fast cell wall resist decolorization with an acid-alcohol mixture during the acid-fast staining procedure, retain the initial dye carbol fuchsin and appear red. • Common acid-fast bacteria of medical importance include Mycobacterium tuberculosis, Mycobacterium leprae, Mycobacterium avium-intracellulare complex and Nocardia species.

38. Acid-fast stain of Mycobacterium tuberculosis • Note redish acid-fast bacilli (arrows).

39. Teichoic acid • Nature: • poly-glycerol-phosphates and poly-ribitol-phosphates with variable side-groups. • Staphylococcus aureus has poly-ribitol-phosphate substituted with N-acetyl glucosamine. • Staphylococcus epidermidis has poly-glycerol-phosphate substituted with glucose. • Location: • teichoic acids are covalently linked to the peptidoglycan molecule and dispersed throughout the wall.

40. Teichoic acid

41. Lipopolysaccharide

42. Lipid A structure

43. Cell membrane

44. Cytoplasm and nuclear body

45. Inclusion body • Gas vacuoles

46. Capsule India Ink Capsule Stain of Klebsiella pneumoniae showing white capsules (Glycocalyx) surrounding purple cells Capsule stain of Enterobacter aerogenes

47. Vibrio parahaemolyticus E. coli Flagellum

48. Bacterial Flagella Arrangements

49. Monotrichous Flagellum of Vibrio cholerae • a single flagellum, usually at one pole

50. Spirillum with Lophotrichous Arrangement of Flagella • Note tuft of flagella at arrow.