LG1.9 Microbiology: Intro to Musculoskeletal Infections – Dr. Pettit Flashcards
What are examples of non-specific defenses of the skin?
1) Physical barrier to infection
2) ~pH 5
3) mostly dry
4) sweat glands
5) salty
6) sebaceous glands
7) fatty acids, wax esters
8) Beta-defensins*
9) Lysozyme
10) Neutrophils, langerhans cells
11) Normal microbiota*
What is a Beta-defensin and what is its function?
β-defensins are cationic peptides and thus can interact with the membrane of invading microbes, which are negative due to lipopolysaccharides (LPS) and lipoteichoic acid (LTA) found in the cell membrane. The peptides have higher affinity to the binding site compared to Ca2+ and Mg2+ ions. The peptides will therefore exchange place with those ions, thus affecting the stability of the membrane. The peptides have a greater size compared with the ions which cause changes in the membrane structure. Due to changes in the electric potential, peptides will pass across the membrane and thus aggregate into dimers. Pore complex will be created as a result of breaking the hydrogen bonds between the amino acids in the terminal end of the strands connecting defensins monomers. Formation of pore complex will cause membrane depolarization and cell lysis.
What are the two leading agents of soft tissue and bone infection?
Staphylococcus aureus and Streptococcus pyogenes
In general which bacteria normally formed a localized abscess?
S. aureus
In general which bacteria normally tends to spread through tissues?
S. pyogenes
What is the most common cause of skin and soft tissue infection in the U.S.?
MRSA
What is cellulitis?
Acute spreading infection of deeper dermis and subcutaneous fat.
What are the most common symptoms of cellulitis?
-Pain, erythema, edema, and warmth
What are other possible (uncommon) symptoms of cellulitis?
1) Regional lymph node enlargement, malaise, chills
2) May progress to sepsis within 24-48 hours
3) Can lead to necrotizing fasciitis, osteomyelitis
What are the main points in the diagnosis of cellulitis?
- Rapid
- Clinical, history
- Can be confirmed by culture
- Specimen is taken from the advancing edge.
What is the treatment for cellulitis?
1) Oral or parenteral empiric antibiotic coverage streptococci, staphylococci until culture results in.
2) Elevate
3) Debridement
4) For more serious or rapid spreading cases, admit and IV antibiotics.
What bacteria most commonly causes necrotizing fasciitis in salt water?
Vibrio vulnificus
What bacteria most commonly causes necrotizing fasciitis in fresh water?
Aeromonas
Which Gram type bacteria is mostly on the skin and why?
Gram positive is normally more common on the skin, this is because they are handle the external environment better than gram negative.
Where does cellulitis normally originate from?
1) Skin lesions (Boils, ulcers, and tinea pedis)
2) Trauma
Where is cellulitis more common on the body?
Lower extremities
What are risk factors for cellulitis?
1) Elderly
2) Diabetes
3) Varicella
4) impaired peripheral circulation
5) Chronic steroid use
Is Staphylococcus aureus gram positive or negative?
Positive
What is Staphylococcus aureus shape?
Cocci, normally found in clusters
Is Staphylococcus aureus normal microbiota?
Yes, Found in anterior nares, skin, mucous membranes
-Sometimes throat, perineum, and vagina
What type of oxygen use does Staphylococcus aureus use?
Facultative anaerobe
Does Staphylococcus aureus have catalase and coagulase?
Yes it has both catalase and coagulase
What media is used to identify Staphylococcus aureus and why?
- MSA Mannitol Salt agar
- Because MSA selects for gram positive, and Staphylococcus aureus is a Mannitol fermenter, so it can also be differentiated by a change of color on the plate.
What are the main virulence factors of Staphylococcus aureus?
1) Protein A
2) Fibrogen and elastin binding protiens
3) Enzymes: Coagulase, Catalase, Hyaluronidase, and Fibrinolysin
4) Cytolytic Exotoxins: PVL
What is the function of Protein A in Staphylococcus aureus?
Binds to the Fc moiety of IgG, exerting an antiphagocytic effect
What is the function of Fibrogen and elastin binding proteins in Staphylococcus aureus?
Promote binding to mucosal cells and tissue matrices
What is PVL in Staphylococcus aureus?
- Paton-Valentine Leukocidin
- Forms pores in neutrophil membranes and causes lysis
What gram stain is Pseudomonas aeruginosa?
Gram negative
What is the shape of Pseudomonas aeruginosa?
Rod
Where is Pseudomonas aeruginosa normally found in the environment?
Soil, water, plants
Is Pseudomonas aeruginosa normal microbiota of the skin?
- Not normally, occasional normal microbiota of skin, but this is usually in immunocompromised/hospitalized individuals
- Opportunistic infection
What are the three main points that separates Pseudomonas aeruginosa from other gram negative bacteria?
1) Aerobe
2) Oxidase positive
3) Nonfermenter
What enzyme must a bacteria have to be oxidase positive?
Cytochrome C oxidase
What are unique identifying characteristics of Pseudomonas aeruginosa when growing on media?
Produces pigments
1) Pyocyanin (blue)
2) Pyoverdin (green)
What does Pyocyanin do for Pseudomonas aeruginosa?
Stimulates IL-8 release which is a neutrophil attractant
What does pyoverdin do for Pseudomonas aeruginosa?
It is a siderophore, binds iron
