Question

Exercise 45

1. Basic characteristics of the bacteria found on human skin.

2. Basic characteristics of Staphylococcus aureus and Propionibacterium.

3. Types of hemolysis.

4. Plates used to test for hemolytic bacteria.

Answer 1

1) The skin is a milieu for controlled bacterial growth. Skin supports the growth of commensal bacteria, which protect the host from pathogenic bacteria. Environmental and local factors, host immunity, and organism adherence and virulence are intricately related to cutaneous infection. Resident gram- positive bacteria include Staphylococcus, Micrococcus, andCorynebacterium sp. Staphylococcus aureus andStrepto coccus pyogenes are notoriously pathogenic in the skin. In order for bacteria to be pathogenic, they must be able to adhere to, grow on, and invade the host. Bacteria possess numerous virulence genes that allow for growth in these privileged niches. Epidermal infections caused by S. aureus and S. pyogenesinclude impetigo and ecthyma. Dermal infections consist of erysipelas, cellulitis, andnecrotizing fasciitis. The pilosebaceous unit is involved in folliculitis, furunculosis, and carbunculosis. Moreover, S. aureus and S. pyogenes produce toxins that may elicit asuperantigen response, causing massive release of cytokines. Staphylococcal scalded skin syndrome, toxic shock syndrome, and scarlet fever are all superantigen-mediated. Gram-negative organisms such as Pseudo monas aeruginosa, Pasteurella multocida, Capnocytophagacanimorsus, Bartonella sp., Klebsiella rhinoscleromatis, and Vibrio vulnificus are not typical resident skin microflora but may cause cutaneous infection.

2) Staphylococcus aureus is a Gram-positive,round-shaped bacterium that is a member of the Firmicutes, and it is a usual member of the microbiota of the body, frequently found in the upper respiratory tract and on the skin. It is often positive for catalase and nitrate reduction and is a facultative anaerobe that can grow without the need for oxygen.^[1]Although S. aureus usually acts as a commensal of the human microbiota it can also become an opportunistic pathogen, being a common cause of skin infectionsincluding abscesses, respiratory infectionssuch as sinusitis, and food poisoning. Pathogenic strains often promote infectionsby producing virulence factors such as potentprotein toxins, and the expression of a cell-surface protein that binds and inactivatesantibodies. The emergence of antibiotic-resistant strains of S. aureus such asmethicillin-resistant S. aureus (MRSA) is a worldwide problem in clinical medicine.

Propionibacterium is a gram-positive, anaerobic, rod-shaped genus of bacterianamed for their unique metabolism: They are able to synthesize propionic acid by using unusual transcarboxylase enzymes.Its members are primarily facultative parasites and commensals of humans and other animals, living in and around the sweat glands, sebaceous glands, and other areas of the skin. They are virtually ubiquitous and do not cause problems for most people, but propionibacteria have been implicated in acneand other skin conditions.

3) Hemolysis is the break down of the membrane of red blood cells by a bacterial protein known as hemolysin, which causes the release of hemoglobin from the red blood cell. Many types of bacterial posses hemolytic proteins. These proteins are thought to act by integrating into the membrane of the red blood cell and either punching a hole through the membrane or disrupting the structure of the membrane in some other way. The exact molecular details of hemolysin action is still unresolved. There are three types of hemolysis, designated alpha, beta and gamma. Alpha hemolysis is a greenish discoloration that surrounds a bacterial colony growing on the agar. This type of hemolysis represents a partial decomposition of the hemoglobin of the red blood cells. Alpha hemolysis is characteristic of Streptococcus pneumonia and so can be used as a diagnostic feature in the identification of the bacterial strain. Beta hemolysis represents a complete breakdown of the hemoglobin of the red blood cells in the vicinity of a bacterial colony. There is a clearing of the agar around a colony. Beta hemolysis is characteristic of Streptococcus pyogenes and some strains ofStaphylococcus aureus. The third type of hemolysis is actually no hemolysis at all. Gamma hemolysis is a lack of hemolysis in the area around a bacterial colony. A blood agar plate displaying gamma hemolysis actually appears brownish. This is a normal reaction of the blood to the growth conditions used (37° C in the presence of carbon dioxide). Gamma hemolysis is a characteristic of Enterococcus faecalis.

4) Blood agar plates are routinely used in the clinic to test for pathenogenic bacteria in throat swabs. These throat pathogens are often Gram positive cocci that may be hemolytic, producing exotoxins called hemolysins that destroy blood cells. One such pathogen that BAP can detect is Streptococcus pyogenes which causes a number of diseases including strep throat, rheumatic fever, and necrotizing fascitis.