Morphology and structure of bacteria

Prof. Marianna Murdjeva, MD, PhD

Department of Microbiology and ImmunologyMedical University-Plovdiv

Lecture course – micro 2013/2014winter term

Bacterial Morphology

• Shape

• Size

• Cell arangement

• Gram staining

Bacterial shape• Round (cocci)

– staphylococci, streptococci

– diplococci (pneumococci, meningococci)

• Rod-shaped (rods)– non-spore forming (M. tuberculosis,

C. diphtheriae)

– spore-forming (bacilli) - clostridia, B. anthracis

• Spiral (spirilli)

– vibrios (V. cholerae) – 1 curve

– spirilli (Helicobacter) – 2 curves

– spirochaetes: Treponema, Leprospira, Borrelia – many curves

Cell arrangement • Single – monococci

• Pairs– diplococci (pneumococci,

meningococci)

– diplobacteria (Klebsiella)

• Tetrads (sarcina)

• Chains (streptococci, B. anthracis)

• Clusters (staphylococci)

Bacterial size

Measurement – in micrometers (m)

• Small (0.2-0.3 m).

haemophillus, brucella

• Medium (0.5-2 m).

staphylococci, streptococci, E. coli

• Large (3-10 m).

Clostridia, B. anthracis

Procaryotic cell structures

Bacterial structure• Cell envelope = cell wall (CW)+cytoplasmic membrane (CM)

• Cytoplasmic components:

- core material (nucleoid) –N

- ribosomes (Ri)

- inclusions

• External structures

- capsule

- flagella and pilli

- spores

• Essential (obligatory) organels - CW, CM, N, Ri

• Non-essential (additional) organels – capsule, flagella, pilli, spores

Cell envelope =CW+ CM

Cell wall (CW)

Peptidoglycan:

1. glycan part:

N-acetyl glucosamine

N-acetyl muramic acid

-1,4 glycoside link

2. peptide part

CW in Gram (+) bacteria• Thicker PG layer

• Presence of teichoic acids

Variety of aminoacids

• Lack or small amount of lipids

• Surface proteins– protein А (S. aureus)

– M protein (S. pyogenes)

CW in Gram (-) bacteria

• Thinner

• Absence of teichoic acids

• More lipids

• Less aminoacids

• Presence of outer membrane (OM) – О Ag, LPS and core oligosacharide.

– LPS – Ag and toxic properties. Lipid А (endotoxin). Endotoxic shock.

– Periplasmic space. Porins.

– S and R (w/o О Аg) colonies.

The three major, covalently linked regions

that form the typical LPS

Function of CW in B

• Protection – protects CM from destruction (the high intracellular pressure)

• Gives the shape (skeletal structure) of B – due to PG

• Osmotic barrier

• Ag and pathogenic properties

• Target for lyzozyme, penicillins and cephalosporins

Atypical CW

• Some bacterial groups lack typical cell wall structure i.e. Mycobacterium and Nocardia

– Gram-positive cell wall structure with lipid mycolic acid (cord factor)

• pathogenicity and high degree of resistance to certain chemicals and dyes

• basis for acid-fast stain used for diagnosis of infections caused by these microorganisms

• Some have no cell wall i.e. Mycoplasma– cell structure is stabilized by sterols

– pleomorphic

Damage to Cell Wall• Lysozyme digests

disaccharide in peptidoglycan.

• Penicillin inhibits peptide bridges in peptidoglycan.

Cytoplasmic membrane (CM ) of B

• Three-layered

• Target for lipid-dissolving agents and some antibiotics (Polymyxin and Nistatin)

• Lack of inner membranes: no endoplasmic reticulum, around lysosomes and mitochondria, Goldi apparatus

Function of bacterial CM

• Respiratory (mitochondrial) equivalent

• Selective permeability

• Particpation in peptidoglycan synthesis and formation of penicillin-binding proteins, necessary for linkage with some antibiotics

• Participation in chromosome replication and large plasmids

Bacterial mesosomes

• Organels, formed by CM invagination

• Functions:

– respiratory (mitochondrial) equivalent

– coordination of core material division and cytoplasm during binary fission of bacteria

Cytoplasm and cytoplasmiccomponents of B

• Ribosomes – difference with Eu

• Inclusions:– volutine (diphtherial bacteria)

– glycogen (enteric bacteria)

– others

• Core (nucleoid)- a single DNA molecule

• Extra-chromosomal genetic elements (plasmids, bacteriophages)

Non-essential structures

• Capsule

• Flagella

• Pilli

• Spores

Capsule

• Real (S. рneumoniae, B. anthracis), slime (S. mutans), microcapsule (S. typhi)

• Structure

– polysaccharide (S. pneumoniae)

– polypeptide (B. anthracis)

• Staining (Klett, Neuffeld)

• Function:

– protection (virulence factor)

– Ag properties (K Ag)

Flagella• Structure – 3 parts:

- filament – long, thin, helical structure composed of protein flagellin

- hook- curved sheath

- basal body – stack of rings firmly anchored in cell wall

• Composition – protein (flagellin)

• Function

– motion

– virulence factor

– antigenic

– receptor

Types of flagella

a

a one-polar flagella (lophotrichous)

b

b whole surface (peritrichous)

c bipolar flagella (amphitrichous)c d

d single flagella (monotrichous)

Pilli (fimbriae)

• Types

– common (adhesins)

– sex (participate in conjugation)

Composition - protein (pillin=fimbrillin)

• Function:

– adhesion to cells (gonococci, E. coli)

– participation in conjugative transfer

– Ag properties (F Ag)

Bacterial spores

• Formation – at high temperature or dehydration

• Structure and composition – less water, thicker wall

• Types:

– according to location in the cell: central (B. anthracis), terminal(С. tetani), subterminal (C. perfringens)

– according to shape: round or oval

– according to their capacity to deform the cell: deforming(Clostridium) and non-deforming (B. anthracis)

Sporulation

Methods for studying bacterial structure

Microscopic

• Native

• Staining

– simple (Loffler, Pfeiffer)

– complex (Gram, Neisser, Zhiel-Neelsen, Moller, Klett)

Hans Gram(1853 – 1923)

Pharmacologist and pathologistCopenhagen

Acid-fast Cell Walls

• Genus Mycobacterium and Nocardia

• mycolic acid (waxy lipid) covers thin peptidoglycan layer

• Do not stain well with Gram stain use acid-fast stain

Brightfield Microscopy

• Simplest of all the optical microscopy illumination. techniques

• Dark objects are visible against a bright background.

Darkfield Microscopy• Light objects visible

against dark background.

• used to enhance the contrast in unstained samples.

• Instrument of choice for spirochetes

Microscopy: The Instruments

Electron Microscopy: Detailed Images of Cell Parts

Uses electrons, electromagnetic lenses, and fluorescent screens

Electron wavelength ~ 100,000 x smaller than visible light wavelength

Specimens may be stained with heavy metal salts

Two types of EMs:?

Recommended literature

• Todar’s Online Textbook of Bacteriology

site: textbookofbacteriology.net

• Medical Microbiology

Edited by Samuel Baron

http://www.ncbi.nlm.nih.gov/books/NBK7627/

• Microbiology and Immunology On-line

http://www.microbiologybook.org