01. Introduction
02. Gram-Positive Cocci
03. Gram-Positive Bacilli
04. Gram-Positive Branched Filaments
05. Gram-Negative Diplococci
06. Gram-Negative Bacilli
07. Gram-Negative Coccobacilli
08. Additional Bacteria
09. RNA Viruses
10. DNA Viruses
11. Fungi
12. Opportunistic Fungi
13. Protozoa (single-celled eukaryotes - small parasites)
14. Helminths (parasitic worms - large macroparasites)
15. Prions

2.01 Staphylococcus aureus + Workbook Sample

Staph Aureus – Audio and Lecture Notes – Click to Expand

Staphylococcus Aureus is a catalase-positive, coagulase-positive, beta-haemolytic, gram-positive cocci. It is also able to ferment mannitol. 

Staph Aureus occurs in grape-like clusters and can normally be found in the anterior nostrils and the vagina.

 

Staph Aureus invades the host with the help of multiple virulence factors, these include protein A, coagulase, leukocidin, haemolysin, staphylokinase, and hyaluronidase.

Their functions are as follow:

    • Protein A binds immunoglobulin G (IgG) at the F c region of this immunoglobulin; this enables the bacteria to evade destruction by phagocytes. Protein A is also important in suppressing the host’s B cell response.
    • Coagulase envelops the bacteria in a fibrin coat, thereby clotting the plasma in the vicinity of the bacteria. This extra protective layer makes it difficult for the host’s defence system to phagocytose the bacteria.Leukocidin forms pores in the cytoplasmic membrane of leukocytes, aka white blood cells, leading to their destruction due to osmotic imbalance.
    • Haemolysin causes a breakdown of Red Blood Cells.
    • Lipase breaks down fats.
    • Staphylokinase possesses pro-fibrinolytic properties, it activates plasminogen to plasmin; ultimately this process causes the breakdown of blood clots.
    • And hyaluronidase is thought to facilitate bacterial invasion by breaking down the host connective tissue
 

Staph Aureus is transmitted via direct contact. 

To cause harm to its human host, Staph aureus can directly invade the host’s tissue or it can release toxins. Both methods have distinct presentations.

 

First is direct invasion. Through this method, Staph Aureus can cause local infections of the skin and the lung; and if it is further spread through the bloodstream, it can even cause system infections.  

Local skin infections include:

    • Cellulitis aka infection of the dermis and subcutaneous tissue. 
    • Folliculitis aka hair follicle infection. 
    • Furuncles aka boils aka pus-filled deeper hair follicle infections. 
    • Carbuncles aka clusters of boils. 
    • Stye aka painful eyelid infection. 
    • And impetigo aka superficial skin infection whose appearance is often described as honey coloured crusted plaques. 

Local lung infection can manifest as cavitating pneumonia.

 

Systemic infections include:

    • Acute endocarditis aka inflammation of the inside chambers of the heart and heart valves. In intravenous drug users, endocarditis more often develops on tricuspid heart valves.  
    • Meningitis aka inflammation of the connective tissue covering the brain and spinal cord. Septic arthritis aka infectious inflammation of the joint. 
    • Osteomyelitis aka infection of the bone; 
    • And abscesses, aka painful collections of pus in organs. 
 

Second are diseases caused by the release of Staph Aureus toxins. These are food poisoning, or more severely, toxic shock syndrome and scalded skin syndrome. 

 

Food poisoning is caused by the release of heat-stable enterotoxins (Enterotoxin S E A) into foods. The common foods affected are puddings, like custard and pastries but the toxin can also be found in other food types. Following ingestion of the food, the host suffers a rapid onset, self-limiting, gastroenteritis, which would usually present with gastrointestinal symptoms like abdominal pain, nausea, vomiting and diarrhea.

 

Toxic Shock Syndrome (TSS) is caused by the release of the superantigen Toxic Shock Syndrome toxin-1 (TSS-1) released by Staph Aureus. This can occur following trauma, but it has also been linked to the use of tampons. 

Toxic Shock Syndrome presents as desquamation a.k.a peeling off of the skin at the palms of the hands and soles of the feet, in addition to hypotensive shock. It can also include additional symptoms such as fevers and rash. 

This is a potentially fatal condition as the low pressure can lead to under perfused organs failing. 

 

Scalded skin syndrome is caused by Staph Aureus’ exfoliative toxins: they include ETA aka the heat-stable toxin; and ETB aka the heat-labile toxin. 

Scalded skin syndrome typically occurs in neonates and younger children under 5, and follows the introduction of Staph Aureus through cutting of the umbilicus or from skin wounds. 

These toxins spread systemically within the child, causing epidermal separation and sloughing off of the skin. Losing their protective skin layers leaves the neonates at increased risk of secondary infections, and with an inability to retain vital fluids. 

As such this is a dangerous and potentially fatal syndrome. The more severe form of Scalded skin syndrome is known as Ritter’s disease. 

 

In the medical field, Staph Aureus is often further classified into Methicillin-sensitive Staph Aureus (MSSA) and Methicillin-Resistant staph aureus (MRSA). 

Since MSSA is sensitive to Methicillin, a type of penicillin antibiotic, MSSA is treated with penicillin antibiotics. 

Whereas, MRSA has gained resistance to methicillin, and may have also gained resistance to other beta-lactam antibiotics (more on these in the pharmacology course).

 

MRSA is therefore treated with vancomycin; though careful scrutiny is advised as more strains of MRSA are becoming resistant to vancomycin. These more resistant strains may be treated with daptomycin, but caution is advised in pneumonia cases as daptomycin’s action is inhibited by lung surfactant.

 

Lastly, remember that prevention is key when dealing with Staph Aureus, therefore it is imperative to ensure adequate hand washing techniques are followed, sterile operating fields and instruments are set up, and high wound cleaning standards are kept.

This website uses cookies to ensure you get the best experience on our website.