Bacteria are prokaryotic (single-celled) microorganisms that are found in both human health and disease.
Bacteria range in size from approximately 0.5-5.0 micrometers in length and different in morphology. Typical shapes of bacteria include spheres (i.e. cocci) and rods (i.e. bacilli) .
The human body is colonised with trillions of bacteria in normal health. This collective is known as the human microbiota and is important for normal functioning. Bacteria are also the cause of many clinically relevant infections in humans.
The general structure of a bacterium includes the cell wall, cell membrane, capsule, flagella, fimbriae, nuclear material, cytoplasm and other intracellular components.
The cell wall forms the outer aspect of bacteria, which protects it against host immune defences and external osmotic pressures. The cell wall differs depending on whether the organisms is gram-positive or gram-negative. These are the two major groups of bacteria.
NOTE: Some bacteria do not have a cell wall (e.g. mycoplasma).
Gram staining is a technique used to identify microorganisms under a microscope.
Gram staining follows a number of steps:
In gram-positive bacteria, the thick peptidoglycan layer results in retention of the violet crystal. In gram-negative bacteria, the thin peptidoglycan layer results in loss of the crystal allowing its cell wall to be counterstained red.
Bacteria can be broadly divided into two main groups (gram-positive or gram-negative) based on gram staining of the bacterial cell well.
Gram-positive bacteria are further divided based on the morphology, which can be assessed under the microscope. The two major groups are cocci (spheres) and bacilli (rods).
Further differentiation is then based on laboratory and molecular techniques that assess the different characteristics of gram positive bacteria.
Staphylococcal infections are common and are divided into coagulase-positive (e.g. S. Aureus) and coagulase-negative (e.g. S. saprophyticus)
There are three main types of Staph to be aware of:
CoNS consists of a group of > 40 microorganisms that are generally considered together in clinical practice. They are usually detected when there is contamination during blood sampling. This is because they are commonly found on the skin. However, they can cause significant infections in patients with indwelling foreign material (i.e. lines).
Staphylococcal microorganisms form cocci in clusters.
Common locations include skin, mouth, nose and throat. They live harmlessly on the skin of 1 in 3 people.
Typical infections associated with Staphylococcus include:
Staphylococcus can release an exotoxin leading to specific infections including acute gastroenteritis, toxic shock syndrome and scalded skin syndrome.
Management depends on resistant patterns and allergy status, but options typically include flucloxacillin (MSSA) or vancomycin (CoNS, MRSA).
Streptococcal microorganisms cause a wide range of clinically relevant infections.
Streptococcal classification is more complex and bacteria are broadly divided into three groups: alpha-, beta- and gamma-haemolytic.
Streptococcal microorganisms are divided by their haemolytic properties. The two major groups are alpha- and beta-haemolytic. Beta-haemolytic bacteria are further divided based on Lancefield grouping.
Traditionally, enterococcal microorganisms were classified as beta-haemolytic group D streptococci. However, they have now been reclassified to an independent genus.
Streptococcal microorganisms typically form cocci in chains. Some Streptococci, like S. Pneumoniae, form diplococci (two bacteria together).
Streptococci are commonly found on the skin and in the throat.
Infections associated with streptococci depend on the type (i.e. alpha vs. beta-haemolytic) and include the following:
Enterococci (re-classified from streptococci) can form part of the normal bowel flora. They are commonly associated with UTIs, biliary infections, intra-abdominal infections and endocarditis.
Antibiotics used to treat streptococcal infections include beta-lactams (i.e. penicillin, ceflasporins). In severe cases, or those with high resistance, glycopeptides (i.e. vancomycin) or carbapenems (i.e meropenem) are used.
There are four major gram-positive bacilli that cause clinically significant infections in humans.
They can be divided into spore-forming (Bacillus, Clostridium) and non-sporing (Listeria, Corynebacterium). They are briefly discussed below.
Clostridium is an anaerobic gram-positive rod that causes a range of clinically important infections related to toxin release.
Listeria monocytogenes can cause meningitis, often in neonates, or patients who have consumed unpasteurised milk.
Bacillus cereus is a common cause of food poisoning (i.e. gastroenteritis). It classically occurs following consumption of re-heated rice.
Corynebacterium diphtheria releases an exotoxin that causes the potentially fatal condition diphtheria. Widespread vaccination programme now in place.
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