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Bacterial classification is important, revealing the identity of an
organism so that its behaviour and likely response to treatment
can be predicted.
Bacterial structural components
Bacterial cell walls are rigid and protect the organism from differences
in osmotic tension between the cell and the environment.
Gram-positive cell walls have a thick peptidoglycan layer and a
cell membrane, whereas Gram-negative cell walls have three layers:
inner and outer membranes, and a thinner peptidoglycan layer.
The mycobacterial cell wall has a high proportion of lipid, including
immunoreactive antigens. Bacterial cell shape can also be used
in classification. The following cell components are important for
classification, pathogenicity and therapy.
- Capsule: a polysaccharide layer that protects the cell from
phagocytosis and desiccation.
- Lipopolysaccharide: surface antigens that strongly stimulate
inflammation and protect Gram-negative bacteria from complement-
- Fimbriae or pili: specialized thin projections that aid adhesion to
host cells. Escherichia coli that cause urinary tract infections bind
to mannose receptors on ureteric epithelial cells by their P fimbriae.
Fimbrial antigens are often immunogenic but vary between
strains so that repeated infections may occur (e.g. Neisseria
- Flagella: these allow organisms to find sources of nutrition and
penetrate host mucus. The number and position of flagella may
- Slime: a polysaccharide material secreted by some bacteria that
protects the organism against immune attack and eradication by
antibiotics when it is growing in a biofilm in a patient with bronchiectasis
or on an inserted medical device.
- Spores: metabolically inert bacterial forms adapted for longterm
survival in the environment, which are able to regrow under
Bacteria have a single chromosome and lack a nucleus (prokaryotes).
The DNA is coiled and supercoiled by the DNA gyrase
enzyme system (see Antibiotics in clinical use
). Bacterial ribosomes differ from eukaryotic ones, making them a target for antibacterial therapy
(see Antibacterial therapy
). Bacteria also contain accessory DNA in the form
of plasmids, integrons, transponsons and bacteriophages. These
may transmit antimicrobial resistance (see Resistance to antibacterial agents
) and may
also code for pathogenicity factors.
Classification of bacteria
We identify microorganisms to predict their pathogenicity: a Staphylococcus
aureus isolated from blood is more likely to be causing
disease than Staphylococcus epidermidis. It is important to identify
organisms that spread widely in the community and cause serious
disease, such as Neisseria meningitidis, so that preventative measures
can be taken (see Neisseria and Moraxella
). Bacteria are identified using
phenotypic, immunological or molecular characteristics.
- Gram reaction: Gram-positive and Gram-negative bacteria
respond to different antibiotics. Other bacteria (e.g. mycobacteria)
may require special staining techniques.
- Cell shape: Bacteria may be shaped as cocci, bacilli or spirals.
- Endospore: The presence, shape and position of the endospore
within the bacterial cell are noted.
- Fastidiousness: Certain bacteria have specific O2/CO2 requirements,
need special media or grow only intracellularly.
- Key enzymes: Some bacteria lack certain enzymes, for example,
lack of lactose fermentation helps distinguish salmonellae from E.
- Serological reactions: Interaction of antibodies with surface
structures may for example help to distinguish subtypes of salmonellae,
Haemophilus and meningococcus.
- DNA sequences: DNA sequencing of key genes (e.g. 16S ribosomal
RNA or DNA gyrase) can identify the organism precisely.
The classification systems are helpful, but strains differ in pathogenicity
and virulence within a species and there are similarities
across species. For example, some strains of E. coli
similar diseases to Shigella sonnei.
Medically important groups of bacteria
are divided into two main groups: the staphylococci
(catalase-positive), the major pathogen being S. aureus;
and the streptococci (catalase-negative), the major pathogens
being Streptococcus pyogenes, which causes sore throat and rheumatic
fever, and S. agalactiae, which causes neonatal meningitis
and pneumonia (see Streptococcal infections
and Streptococcus pneumoniae, other Gram-positive cocci and the alpha-haemolytic streptococci
include the pathogenic N. meningitidis, an
important cause of meningitis and septicaemia, and N. gonorrhoeae,
the agent of urethritis (gonorrhoea).
include the respiratory pathogens
Haemophilus and Bordetella (see Diphtheria, tetanus and pertussis
and Small Gram-negative coccobacilli: Haemophilus, Brucella, Francisella, Yersinia and Bartonella
) and zoonotic
agents, such as Brucella and Pasteurella (see Small Gram-negative coccobacilli: Haemophilus, Brucella, Francisella, Yersinia and Bartonella
Gram-positive bacilli are divided into sporing and non-sporing.
The sporing types are subdivided into those that are aerobic (Bacillus;
see Listeria, Bacillus, Corynebacterium and environmental mycobacteria
) and those that are anaerobic (Clostridium;
). Pathogens include Bacillus anthracis, which
causes anthrax, and clostridia, which cause pseudomembranous
colitis, tetanus and, more rarely, gas gangrene and botulism. The
non-sporing pathogens include Listeria and corynebacteria (see
Listeria, Bacillus, Corynebacterium and environmental mycobacteria
(including the family Enterobacteriaceae)
form part of the normal flora of humans and animals, and can be
found in the environment. They include many pathogenic genera:
Salmonella, Shigella, Escherichia, Proteus and Yersinia (see
Enterobacteriaceae clinical syndromes
). Pseudomonas and Burkholderia are environmental
saprophytes that are naturally resistant to antibiotics and are
important hospital pathogens (see Environmental pathogens: Pseudomonas, Burkholderia and Legionella
). Legionella lives in
the environment in water but can cause human infection if conditions
in the built environment allow it to gain a foothold (see
Environmental pathogens: Pseudomonas, Burkholderia and Legionella
include the small gastrointestinal pathogen Helicobacter
that colonizes the stomach, leading to gastric and duodenal
ulcers and gastric cancer, and Campylobacter spp. that cause
acute diarrhoea (see Vibrio, Campylobacter and Helicobacter
). The Borrelia may cause a chronic
disease of the skin joints and central nervous system, Lyme disease
(Borrelia burgdorferi), or rarely relapsing fever (Borrelia duttoni
and Borrelia recurrentis). The Leptospira are zoonotic agents that
cause an acute meningitis syndrome that may be accompanied by
renal failure and hepatitis. The Treponema include the causative
agent of syphilis (Treponema pallidum).
Mycoplasma and Chlamydia
are responsible for common respiratory
and sexually transmitted infections.
are the agents of typhus and rarer severe infections (see
Chlamydia, Mycoplasma and Rickettsia