Bacterial capsule

The outer red layer in this diagram is the capsule, which is distinct from the cell envelope. This bacterium is gram-positive, as its cell envelope comprises a single cell membrane (orange) and a thick peptidoglycan containing cell wall (purple).
A bacterial capsule has a semi-rigid border that follows the contour of the cell. The capsule excludes India Ink when dyed. A slime layer is a non-rigid matrix that is easily deformed and is not able to exclude India Ink. Biofilms are composed of many cells and their outer barriers. The primary functions of both capsules and slime layers are for protection and adhesion.

Large structure common to many bacteria.

- Bacterial capsule
The outer red layer in this diagram is the capsule, which is distinct from the cell envelope. This bacterium is gram-positive, as its cell envelope comprises a single cell membrane (orange) and a thick peptidoglycan containing cell wall (purple).

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Bacteria

Bacteria (singular bacterium, common noun bacteria) are ubiquitous, mostly free-living organisms often consisting of one biological cell.

Bacteria (singular bacterium, common noun bacteria) are ubiquitous, mostly free-living organisms often consisting of one biological cell.

Rod-shaped Bacillus subtilis
Phylogenetic tree of Bacteria, Archaea and Eucarya. The vertical line at bottom represents the last universal common ancestor.
Bacteria display many cell morphologies and arrangements
The range of sizes shown by prokaryotes (Bacteria), relative to those of other organisms and biomolecules.
Structure and contents of a typical Gram-positive bacterial cell (seen by the fact that only one cell membrane is present).
An electron micrograph of Halothiobacillus neapolitanus cells with carboxysomes inside, with arrows highlighting visible carboxysomes. Scale bars indicate 100 nm.
Helicobacter pylori electron micrograph, showing multiple flagella on the cell surface
Bacillus anthracis (stained purple) growing in cerebrospinal fluid
Many bacteria reproduce through binary fission, which is compared to mitosis and meiosis in this image.
A culture of ''Salmonella
A colony of Escherichia coli
Helium ion microscopy image showing T4 phage infecting E. coli. Some of the attached phage have contracted tails indicating that they have injected their DNA into the host. The bacterial cells are ~ 0.5 µm wide.
Transmission electron micrograph of Desulfovibrio vulgaris showing a single flagellum at one end of the cell. Scale bar is 0.5 micrometers long.
The different arrangements of bacterial flagella: A-Monotrichous; B-Lophotrichous; C-Amphitrichous; D-Peritrichous
Streptococcus mutans visualised with a Gram stain.
Phylogenetic tree showing the diversity of bacteria, compared to other organisms. Here bacteria are represented by three main supergroups: the CPR ultramicrobacterias, Terrabacteria and Gracilicutes according to recent genomic analyzes (2019).
Overview of bacterial infections and main species involved.
Colour-enhanced scanning electron micrograph showing Salmonella typhimurium (red) invading cultured human cells
In bacterial vaginosis, beneficial bacteria in the vagina (top) are displaced by pathogens (bottom). Gram stain.
Antonie van Leeuwenhoek, the first microbiologist and the first person to observe bacteria using a microscope.

Glycocalyx is produced by many bacteria to surround their cells, and varies in structural complexity: ranging from a disorganised slime layer of extracellular polymeric substances to a highly structured capsule.

Rod-shaped gram-positive Bacillus anthracis bacteria in a cerebrospinal fluid sample stand out from round white blood cells, which also accept the crystal violet stain.

Gram-positive bacteria

Traditionally used to quickly classify bacteria into two broad categories according to their type of cell wall.

Traditionally used to quickly classify bacteria into two broad categories according to their type of cell wall.

Rod-shaped gram-positive Bacillus anthracis bacteria in a cerebrospinal fluid sample stand out from round white blood cells, which also accept the crystal violet stain.
Violet-stained gram-positive cocci and pink-stained gram-negative bacilli
Gram-positive and gram-negative cell wall structure
Structure of gram-positive cell wall
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The structure of peptidoglycan, composed of N-acetylglucosamine and N-acetylmuramic acid
Colonies of a gram-positive pathogen of the oral cavity, Actinomyces sp.

Only some species have a capsule, usually consisting of polysaccharides.

TEM micrograph of a B. subtilis bacterium, with the hair-like glycocalyx visible surrounding the cell membrane (scale bar = 200 nm)

Glycocalyx

Glycoprotein and glycolipid covering that surrounds the cell membranes of bacteria, epithelial cells, and other cells.

Glycoprotein and glycolipid covering that surrounds the cell membranes of bacteria, epithelial cells, and other cells.

TEM micrograph of a B. subtilis bacterium, with the hair-like glycocalyx visible surrounding the cell membrane (scale bar = 200 nm)
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A distinct, gelatinous glycocalyx is called a capsule, whereas an irregular, diffuse layer is called a slime layer.

Slime layer

Easily removable , unorganized layer of extracellular material that surrounds bacteria cells.

Easily removable , unorganized layer of extracellular material that surrounds bacteria cells.

A bacterial capsule is similar, but is more rigid than the slime layer.

Streptococcus pneumoniae

Gram-positive, spherical bacteria, alpha-hemolytic or beta-hemolytic (under anaerobic conditions), aerotolerant anaerobic member of the genus Streptococcus.

Gram-positive, spherical bacteria, alpha-hemolytic or beta-hemolytic (under anaerobic conditions), aerotolerant anaerobic member of the genus Streptococcus.

Optochin sensitivity in a culture of Streptococcus pneumoniae (white disk)

The encapsulated, Gram-positive, coccoid bacteria have a distinctive morphology on Gram stain, lancet-shaped diplococci.

Pseudomonas aeruginosa

Pseudomonas aeruginosa in Petri dish.
Pigment production, growth on cetrimide agar, the oxidase test, plaque formation and Gram stain.
A culture dish with Pseudomonas
Gram-stained P. aeruginosa bacteria (pink-red rods)
Phagocytosis of P. aeruginosa by neutrophil in patient with bloodstream infection (Gram stain)
Production of pyocyanin, water-soluble green pigment of P. aeruginosa (left tube)
Pseudomonas aeruginosa fluorescence under UV illumination
The antibiogram of P. aeruginosa on Mueller–Hinton agar

Pseudomonas aeruginosa is a common encapsulated, Gram-negative, strict aerobic (although can grow anaerobically in the presence of nitrate), Rod-shaped bacterium that can cause disease in plants and animals, including humans.

Bacillus anthracis

Gram-positive and rod-shaped bacterium that causes anthrax, a deadly disease to livestock and, occasionally, to humans.

Gram-positive and rod-shaped bacterium that causes anthrax, a deadly disease to livestock and, occasionally, to humans.

B. anthracis belongs to the B. cereus group of strains.
Structure of B. anthracis
CD14 and Mac-1 work together to increase the macrophage internalization of B. anthracis spores. (a) CD14 and TLR2 work in close proximity to propagate the activation signal. Mac-1 starts in a closed confirmation which keeps the integrin from binding the spores, keeping them out of the cell. (b) CD14 binds the rhamnose residues on the BclA of the exosporium leading to TLR2-mediated activation of PI3K. (c) Activation of PI3K pushes Mac-1 into its open conformation so the spore can be bound and internalized (information from Oliva et al. 2009)
CapD protein crystal structure of B. anthracis

The protein capsule (poly-D-gamma-glutamic acid) is key to evasion of the immune response.

Streptococcus pyogenes

Species of Gram-positive, aerotolerant bacteria in the genus Streptococcus.

Species of Gram-positive, aerotolerant bacteria in the genus Streptococcus.

A carbohydrate-based bacterial capsule composed of hyaluronic acid surrounds the bacterium, protecting it from phagocytosis by neutrophils.

Klebsiella pneumoniae

Streptomycin(Aminoglycoside)
Cephalosporin (core structure)
Multidrug-resistant KP
Growth of Klebsiella pneumoniae CRE from positive blood culture on MacConkey agar in Tuscany, where an outbreak was reported starting in November 2018 of strains producing NDM carbapenemase

Klebsiella pneumoniae is a Gram-negative, non-motile, encapsulated, lactose-fermenting, facultative anaerobic, rod-shaped bacterium.

Schematic of typical gram-positive cell wall showing arrangement of N-Acetylglucosamine and N-Acetylmuramic acid; Teichoic acids not shown.

Cell envelope

The cell envelope comprises the inner cell membrane and the cell wall of a bacterium.

The cell envelope comprises the inner cell membrane and the cell wall of a bacterium.

Schematic of typical gram-positive cell wall showing arrangement of N-Acetylglucosamine and N-Acetylmuramic acid; Teichoic acids not shown.
Schematic of typical gram-negative cell wall showing arrangement of N-Acetylglucosamine and N-Acetylmuramic acid and the outer membrane containing lipopolysaccharide.

Either type may have an enclosing capsule of polysaccharides for extra protection.