A pathogen is a microorganism that is able to cause disease in a host.
Define disease and explain the difference between disease and infection.
Disease is a disorder of structure or function that produces symptoms and/or signs in the host and is not a result of physical injury. Infection is the invasion of the body by pathogens, but since infection doesn't always result in the production of symptoms and/or signs, infection is not always the same as disease.
Explain what is meant by a commensal, giving examples.
Commensal bacteria are bacteria that live in or on the body. See Commensal Flora and Pathogenesis.
Explain the difference between colonised and sterile body sites and recognise which body sites belong to each category.
Not all sites of the body have commensal bacteria. Some body sites are sterile, meaning that they don't even have commensal bacteria. Therefore, if you see bacteria in these sites, there's almost certainly something bad going on. These sites include the blood, cerebrospinal fluid, bladder, peritoneal cavity, and joint cavity. Nearly everywhere else in the body will have commensal bacteria.
Give examples of exogenous and endogenous infectious diseases.
An exogenous infectious disease is one that comes from an external source. For instance, you may catch a cold or flu from another person. On the other hand, an endogenous infectious disease comes from commensal flora. This usually happens when bacteria get into a place where they normally shouldn't be (e.g. a ruptured bowel may allow gut bacteria to enter the peritoneal cavity) or when the host is immunosuppressed.
Define “carriage”, giving examples.
"Carriage," or the "carrier state," is where someone is infected but doesn't have the disease. People who are carriers can infect others or develop disease themselves. For example, many people are carriers of meningococcal disease at some point, but not everyone develops the disease.
Define “opportunistic infection”, giving examples.
An "opportunistic infection" is an infection that doesn't usually cause disease in a healthy person, but can cause disease if someone is immunosuppressed in some way. An example of an opportunistic infection is C. difficile, which is usually only a problem if our commensal flora has been wiped out by antibiotics.
Define “virulence factor”, giving an example.
A "virulence factor" is something that can allow a pathogen to cause disease. These factors might include toxins (e.g. cholera toxin) or capsules that protect from phagocytosis. More info at Commensal Flora and Pathogenesis.
Explain why, in the clinical setting of infectious diseases, knowing the genus and species name of a causative microorganism is relevant.
Knowing the causative microorganism is relevant as it helps to guide specific treatments. As mentioned in my last post, not all antibiotics target the same species of bacteria, so it is important to pick the right drug for the job. Knowing the causative microorganism might also help you to figure out the likely progression of the disease.
Outline the differences between prokaryotic and eukaryotic cells.
Outline briefly the structural components of the typical bacterial cell and the functions of these components.
Bacterial cells are simpler than eukaryotic cells, as they lack organelles. However, they still have a lot of important structures.
- Cell membrane- Since bacteria don't have organelles, the membrane performs many of the functions that organelles would otherwise do, such as protein secretion, lipid synthesis, and so on. Some bacteria only have one cell membrane, but some bacteria have two cell membranes (an outer membrane and an inner membrane). The outer membrane contains many crucial proteins, including porins, which control the influx of antibiotics and other molecules. Lipopolysaccharide (LPS), a component of the outer membrane, can also function as an endotoxin.
- Nucleoid- A single molecule of double-stranded DNA that holds most of the genetic information.
- Plasmids- Circular double-stranded DNA molecules that can be transferred between bacteria. Some plasmids may encode genes for bacterial resistance, so they are pretty crucial to know about.
- Ribosomes- Perform the same functions as in eukaryotic cells. However, bacterial ribosomes are made up of different subunits as compared to eukaryotic ribosomes (30S and 50S in bacterial ribosomes, 40S and 60S in eukaryotic ribosomes).
- Cell wall- Surrounds the cell membrane. Makes the cell rigid and protects it from bursting. The cell wall is made up of peptidoglycan, which is cross-linked by enzymes called transpeptidases. As the cell wall is found in bacteria and not human cells, the cell wall is a popular target for antibiotics.
- Capsule- Also known as the "slime layer," the capsule is a polysaccharide layer present in some bacteria. It protects from phagocytosis and helps with adhesion.
- Fimbriae- Little hair-like projections of the bacteria that help with attachment. Some fimbriae, called pili, are involved in transfer of DNA.
- Flagella- Longer hair-like structures that whip back and forth and help to move the cells.
Describe the differences between the Gram positive and Gram negative cell envelopes (= cytoplasmic membrane, cell wall, ± outer membrane) and how the Gram stain functions to distinguish these two.
Gram-positive bacteria have a single cell membrane, surrounded by a thick cell wall. Gram-negative bacteria have two cell membranes, with a thin cell wall in between them. Under the Gram stain, Gram-positive bacteria appear purple, whereas Gram-negative bacteria appear pink.
The Gram stain has four main steps:
- Addition of crystal violet, which stains the cell wall purple.
- Addition of Lugol's iodine, which helps to "fix" the crystal violet to the peptidoglycan of the cell wall.
- Washing with ethanol, which removes the crystal violet from the thin Gram-negative cell wall. (Gram-positive cell walls have much more peptidoglycan, so they hold on to the crystal violet much more strongly.)
- Addition of carbol fuchsin, which is a "counter-stain" that stains all of the Gram-negative cells that had the crystal violet washed out of them. Carbol fuchsin is pink, which is why Gram-negative cells stain pink.
Explain why it is important for the clinician to know the Gram stain and morphological appearance of the main pathogens encountered in practice.
The Gram stain is very useful, but it only distinguishes between two main groups: Gram-positive and Gram-negative. In order to narrow down the genus and/or species, you also need to have a look at other factors, such as shape and how the bacteria cluster together. Here are some examples:
- Gram-positive (stain purple)
- Cocci (round bacteria)
- Clusters- e.g. Staphylococcus
- Chains- e.g. Streptococcus
- Pairs (diplococci)- e.g. Streptococcus pneumoniae
- Bacilli (rod-shaped bacteria)
- Basic rod shape (not really sure how to describe them)- e.g. Clostridium
- Small and irregular- e.g. Corynebacterium
- Filamentous or branching- e.g. Nocardia
- Gram-negative (stain pink)
- Cocci
- Pairs- likely Neisseria meningitidis
- Bacilli
- Basic rod shape- e.g. Escherichia, Proteus, Salmonella, etc.
- Curved or comma-shaped- e.g. Vibrio
- Helical- e.g. Campylobacter
There are also many bacteria that do not take up the Gram stain and may need to be visualised in a different way, such as using a different stain or by using microscopy. Spirochaetes (e.g. Treponema, Borrelia, Leptospira) do not take up the Gram stain but can be recognised by their spiral-shaped appearance under the microscope. Mycobacteria, which have mycolic acid in their cell walls, need an acid-fast stain such as the Ziehl-Neelsen stain in order to be visualised.
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