1       Structure of bacterial cells

 

Bacteria are unicellular organisms and usually occur in simple shapes such as rods, cocci, spiral forms and occasionally as branching filaments. They typically have rigid cell walls containing a peptidoglycan layer and multiply by binary fission. Bacteria are smaller and less complex than eukaryotic cells and do not contain membrane-bound organelles. Genetic information essential for organism survival, the core genome, is usually contained in a single circular chromosome; a nuclear membrane and a nucleolus are absent. Some bacteria have more than one chromosome and chromosomes in certain bacteria are linear. The accessory genome encodes non-essential cell functions and may include plasmids and bacteriophages (see Chapter 3). Despite their morphological diversity, most bacteria are between 0.5 and 5 μm in length. Motile bacteria possess flagella by which they can move through liquids in vivo and in vitro.

Most bacteria found in nature are not harmful to humans, animals or plants. Some bacteria make an important contribution to the utilization of nutrients in soil, in water and in the digestive tracts of animals. Bacteria which cause disease in animals or humans are referred to as pathogenic bacteria.

A typical bacterial cell is composed of a capsule, cell wall, cell membrane, cytoplasm (containing nuclear material) and appendages such as flagella and pili. Some species of bacteria can produce dormant forms termed spores or endospores, structures which are resistant to environmental influences. The principal structural components of bacterial cells are presented in Table 1.1. Some bacteria can synthesize extracellular polymeric material, termed a capsule, which forms a well-defined structure, closely adherent to the cell wall. In the body, capsules of pathogenic bacteria interfere with phagocytosis. The tough, rigid cell walls of bacteria protect them from mechanical damage and osmotic lysis. Differences in the structure and chemical composition of the cell walls of bacterial species account for variation in their pathogenicity and influence other characteristics, including staining properties. Mycoplasmas, an important group of bacteria, lack rigid cell walls but have a flexible triple-layered outer membrane.

Table 1.1 Structural components of bacterial cells.

Structure	Chemical composition	Comments
Capsule	Usually polysaccharide; polypeptide in Bacillus anthracis	Often associated with virulence; interferes with phagocytosis; may prolong survival in the environment
Cell wall	Peptidoglycan and teichoic acid in Gram-positive bacteria. Lipopolysaccharide (LPS), protein, phospholipid and peptidoglycan in Gram-negative bacteria	Peptidoglycan is responsible for the shape of the organism. LPS is responsible for endotoxic effects. Porins – protein structures – regulate the passage of small molecules through the phospholipid layer
Cell membrane	Phospholipid bilayer	Selectively permeable membrane involved in active transport of nutrients, respiration, excretion and chemoreception
Flagellum (plural, flagella)	Protein called flagellin	Filamentous structure which confers motility
Pilus (plural, pili)	Protein called pilin	Also known as fimbria (plural, fimbriae). Thin, straight, thread-like structures present on many Gram-negative bacteria. Mediate attachment to host cells. Specialized pili are involved in conjugation
Chromosome	DNA	Single circular structure with no nuclear membrane
Ribosome	RNA and protein	Involved in protein synthesis
Storage granules or inclusions	Variable chemical composition	Present in some bacterial cells; may be composed of polyphosphate (volutin or metachromatic granules), poly-β-hydroxybutyrate (reserve energy source), glycogen

On the basis of colour when stained by the Gram method, bacteria can be divided into two major groups, Gram-positive and Gram-negative; this colour reaction is determined by the composition of the cell wall. Gram-positive bacteria, which stain blue, have a relatively thick uniform cell wall which is composed mainly of peptidoglycan and teichoic acids. In contrast, Gram-negative bacteria, which stain red, have cell walls with a more complex structure, consisting of an outer membrane and a periplasmic space containing a comparatively small amount of peptidoglycan.

The cell membranes of bacterial cells are flexible structures composed of phospholipids and proteins. Active transport of nutrients into the cell and elimination of waste metabolites are functions of the cell membrane and it is also the site of electron transport for bacterial respiration. The cytoplasm, which is enclosed by the cell membrane, is essentially an aqueous fluid containing the nuclear material, ribosomes, nutrients, enzymes and other molecules involved in synthesis, cell maintenance and metabolism.

In most bacteria, the bacterial genome is composed of a single haploid circular chromosome containing double-stranded DNA. Bacterial genomes differ in size depending on the species. Plasmids, small circular pieces of DNA which are separate from the core genome, are capable of autonomous replication. Plasmid DNA may encode characteristics such as antibiotic resistance and exotoxin production. All protein synthesis takes place on ribosomes, structures composed of ribonucleoproteins.

Motile bacteria possess flagella, attached to the cell wall, which are usually several times longer than the bacterial cell and are composed of a protein called flagellin. Fine, straight, hair-like structures called pili or fimbriae, composed of the protein pilin, are attached to the cell wall of many bacteria. In many pathogenic Gram-negative bacteria, adhesins present at the tips of pili function as attachment structures for mammalian cells.

Dormant, highly resistant structures, termed endospores, are formed by some bacteria to ensure survival during adverse environmental conditions. The only genera of pathogenic bacteria which contain endospore-forming species are Bacillus and Clostridium. The resistance of endospores is attributed to their layered structure, their dehydrated state, their negligible metabolic activity and their high content of dipicolinic acid. Because endospores are thermostable, moist heat at 121°C for 15 minutes is required for their inactivation.