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Bacteria – Morphology & Classification II MBBS Dr Ekta Chourasia Lecturer, Microbiology.

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Presentation on theme: "Bacteria – Morphology & Classification II MBBS Dr Ekta Chourasia Lecturer, Microbiology."— Presentation transcript:

1 Bacteria – Morphology & Classification II MBBS Dr Ekta Chourasia Lecturer, Microbiology

2 Dr Ekta Chourasia Learning Objectives After completing this section you should be able to perform the following objectives:  Define a prokaryotic cell and list the differences between prokaryotic and eukaryotic cell  Describe the structure of a bacterial cell and explain the function of its components  Explain why cell wall forms the basis for classification of bacteria  Explain the structural modifications (flagella) of the cell and their functional importance

3 Dr Ekta Chourasia Introduction: Microorganisms – several classes of living beings Based on the organization of their cellular structures, all living cells can be divided into two groups: eukaryotic and prokaryotic  Eukaryotic cell types - Animals, plants, fungi, protozoans, and algae  Prokaryotic cell types - bacteria & blue green algae

4 Dr Ekta Chourasia Schematic of typical animal (eukaryotic) cell, showing subcellular components. Organelles: (1) nucleolus (2) nucleus (3) ribosome (4) vesicle (5) rough endoplasmic reticulum (ER) (6) Golgi apparatus (7) Cytoskeleton (8) smooth ER (9) mitochondria (10) vacuole (11) cytoplasm (12) lysosome (13) centrioles

5 Dr Ekta Chourasia Prokaryotic Cells much smaller (microns) and more simple than eukaryotes prokaryotes are molecules surrounded by a membrane and cell wall. they lack a true nucleus and don’t have membrane bound organelles like mitochondria, etc. large surface-to-volume ratio : nutrients can easily and rapidly reach any part of the cells interior

6 Dr Ekta Chourasia Size of Bacteria Unit of measurement in bacteriology is the micron (micrometre, µm) Bacteria of medical importance  0.2 – 1.5 µm in diameter  3 – 5 µm in length

7 Dr Ekta Chourasia Shape of Bacteria Cocci – spherical/ oval shaped major groups Bacilli – rod shaped Vibrios – comma shaped Spirilla – rigid spiral forms Spirochetes – flexible spiral forms Actinomycetes – branching filamentous bacteria Mycoplasmas – lack cell wall

8 Dr Ekta Chourasia Arrangement of bacteria: Cocci Cocci in pair – Diplococcus Sarcina – groups of eight Tetrad – groups of four Cocci in chain - Streptococci Cocci in cluster - Staphylococci Coccus

9 Dr Ekta Chourasia Arrangement of bacteria: Bacilli

10 Dr Ekta Chourasia Other shapes of bacteria Comma shaped Spirochetes Spirilla

11 Dr Ekta Chourasia Anatomy of a Bacterial Cell

12 Dr Ekta Chourasia Anatomy of A Bacterial Cell Outer layer – two components: 1. Rigid cell wall 2. Cytoplasmic (Cell/ Plasma) membrane – present beneath cell wall Cytoplasm – cytoplasmic inclusions, ribosomes, mesosomes and nucleus Additional structures – plasmid, slime layer, capsule, flagella, fimbriae (pili), spores

13 Structure & Function of Cell Components

14 Dr Ekta Chourasia CELL WALL Outermost layer, encloses cytoplasm 1. Confers shape and rigidity nm thick 3. Composed of complex polysaccharides (peptidoglycan/ mucopeptide) - formed by N acetyl glucosamine (NAG) & N acetyl muramic acid (NAM) alternating in chains, held by peptide chains.

15 Dr Ekta Chourasia Cell Wall Cell wall – 4. Carries bacterial antigens – important in virulence & immunity 5. Chemical nature of the cell wall helps to divide bacteria into two broad groups – Gram positive & Gram negative 6. Gram +ve bacteria have simpler chemical nature than Gram –ve bacteria. 7. Several antibiotics may interfere with cell wall synthesis e.g. Penicillin, Cephalosporins

16 Dr Ekta Chourasia Gram positive cell wall The Gram-positive cell wall is composed of a thick, multilayered peptidoglycan sheath outside of the cytoplasmic membrane. Teichoic acids are linked to and embedded in the peptidoglycan, and lipoteichoic acids extend into the cytoplasmic membrane

17 Dr Ekta Chourasia Gram negative cell wall The Gram-negative cell wall is composed of an outer membrane linked to thin, mainly single-layered peptidoglycan by lipoproteins. The peptidoglycan is located within the periplasmic space that is created between the outer and inner membranes. The outer membrane includes porins, which allow the passage of small hydrophilic molecules across the membrane, and lipopolysaccharide molecules that extend into extracellular space.

18 Dr Ekta Chourasia Cell Wall

19 Dr Ekta Chourasia Summary of the differences between Gram positive & Gram negative bacteria Property of bacteriaGram Positive Gram Negative Thickness of wall20-80 nm10 nm Number of layers in wall12 Peptidoglycan content>50%10-20% Teichoic acid in wall+- Lipid & lipoprotein content0-3%58% Protein content0%9% Lipopolysaccharide013% Sensitive to penicillinYesLess sensitive Digested by lysozymeYesWeakly

20 Dr Ekta Chourasia Cytoplasmic (Plasma) membrane Thin layer 5-10 nm, separates cell wall from cytoplasm Acts as a semipermeable membrane: controls the inflow and outflow of metabolites Composed of lipoproteins with small amounts of carbohydrates

21 Dr Ekta Chourasia Other Cytoplasmic Components Ribosomes – protein synthesis Mesosomes – 1. Multilaminated structures formed as invaginations of plasma membrane 2. Principal sites of respiratory enzymes 3. Coordinate nuclear & cytoplasmic division during binary fission 4. More prominent in Gram +ve bacteria Intracytoplasmic inclusions – reserve of energy & phosphate for cell metabolism e.g. Metachromatic granules in diphtheria bacilli

22 Dr Ekta Chourasia Nucleus No nucleolus No nuclear membrane Genome –  single, circular double stranded DNA.  Haploid  Divides by binary fission

23 Dr Ekta Chourasia Additional Organelles 1. Plasmid –  Extranuclear genetic elements consisting of DNA  Transmitted to daughter cells during binary fission  May be transferred from one bacterium to another  Not essential for life of the cell  Confer certain properties e.g. drug resistance, toxicity

24 Dr Ekta Chourasia Additional Organelles 2. Capsule & Slime layer –  Viscous layer secreted around the cell wall.  Polysaccharide / polypeptide in nature a) Capsule – sharply defined structure, antigenic in nature Protects bacteria from lytic enzymes Inhibits phagocytosis Stained by negative staining using India Ink Can be demonstrated by Quellung reaction (capsule swelling reaction) b) Slime layer – loose undemarcated secretion

25 Dr Ekta Chourasia Additional Organelles 3. Flagella –  Long (3 to 12 µm), filamentous surface appendages  Organs of locomotion  Chemically, composed of proteins called flagellins  The number and distribution of flagella on the bacterial surface are characteristic for a given species - hence are useful in identifying and classifying bacteria  Flagella may serve as antigenic determinants (e.g. the H antigens of Gram-negative enteric bacteria)  Presence shown by motility e.g. hanging drop preparation

26 Dr Ekta Chourasia Types of flagellar arrangement Polar/ Monotrichous – single flagellum at one pole Lophotrichous – tuft of flagella at one pole Peritrichous – flagella all over Amphitrichous – flagella at both poles Amphilophotrichous – tuft of flagella at both ends

27 Dr Ekta Chourasia Additional Organelles 4. Fimbriae/ Pili –  Thin, hairlike appendages on the surface of many Gram- negative bacteria  10-20µ long, acts as organs of adhesion (attachment) - allowing bacteria to colonize environmental surfaces or cells and resist flushing  Made up of proteins called pilins.  Pili can be of two types –  Common pili – short & abundant  Sex pili - small number (one to six), very long pili, helps in conjugation (process of transfer of DNA)

28 Dr Ekta Chourasia Additional Organelles 5. Spores –  Highly resistant resting stages formed during adverse environment (depletion of nutrients)  Formed inside the parent cell, hence called Endospores  Very resistant to heat, radiation and drying and can remain dormant for hundreds of years.  Formed by bacteria like Clostridia, bacillus

29 Dr Ekta Chourasia The cycle of spore formation and germination At the beginning of spore formation, a septum forms, separating the nascent spore from the rest of the cell and all of the genetic material of the cell is copied into the newly- forming cell. The spore contents are dehydrated and the protective outer coatings are laid down. Once the spore is matured it is released from the cell. On germination, the spore contents rehydrate and a new bacterium emerges and multiplies.

30 Dr Ekta Chourasia Shape & position of bacterial spore Oval central Spherical central Oval sub terminal Oval terminal Spherical terminal Free spore Non bulging Bulging

31 Dr Ekta Chourasia P leomorphism & Involution forms Pleomorphism – great variation in shape & size of individual cells e.g. Proteus species Involution forms – swollen & aberrant forms in ageing cultures, especially in the presence of high salt concentration e.g. plague bacillus Cause – defective cell wall synthesis

32 Differences between prokaryotic & eukaryotic cells CharacterProkaryotesEukaryotes NucleusNuclear membrane AbsentPresent NucleolusAbsentPresent ChromosomeOne circularOne or more paired and linear Cell divisionBinary fissionMitosis Cytoplasmic membrane Structure and Composition fluid phospholipid bilayer, lacks sterols fluid phospholipid bilayer containing sterols FunctionIncapable of endocytosis (phagocytosis and pinocytosis) and exocytosis Capable of endocytosis and exocytosis

33 Dr Ekta Chourasia Differences between prokaryotic & eukaryotic cells CharacterProkaryotesEukaryotes CytoplasmMitochondriaAbsentPresent LysosomesAbsentPresent Golgi apparatus AbsentPresent Endoplasmic reticulum AbsentPresent VacuolesAbsentPresent Ribosomes70 S80 S

34 Dr Ekta Chourasia Differences between prokaryotic & eukaryotic cells CharacterProkaryotesEukaryotes Cell WallPresentAnimals & Protozoans – Absent Plants, Fungi & Algae - Present CompositionPeptidoglycan – complex carbohydrate Cellulose or chitin Locomotor organelles FlagellaFlagella/ Cilia

35 Dr Ekta Chourasia Bacterial Taxonomy Includes three components: 1. Classification : orderly arrangement 2. Identification of an unknown unit 3. Nomenclature : naming the units

36 Dr Ekta Chourasia Bacterial Taxonomy: Classification Orderly arrangement : Kingdom – Division – Class – Order – Family – Tribe – Genus – Species  Phylogenetic classification – represents a branching tree like arrangement. One characteristic being used for division at each branch or level  Molecular or Genetic classification – based on the degree of genetic relatedness of different organisms  Intraspecies classification – based on biochemical properties (biotypes), antigenic features (serotypes), bacteriophage susceptibility (phage types)

37 Dr Ekta Chourasia

38 Dr Ekta Chourasia Bacterial Taxonomy: Nomenclature Two kinds of name are given to bacteria  Casual / common name – for local use, varies from country to country e.g. “typhoid bacillus”  Scientific / International Name – same all over world, consists of two words (in Italics) e.g. Salmonella typhi, Staphylococcus aureus


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