Microbiology: skin infection agents Flashcards
what are two examples of resident skin bacteria?
Gram positive Staphylococcus epidermis and Propionibacterium acnes
what are two examples of transient skin bacteria?
Staphylococcus aureus and Streptococcus pyogenes
what is impetigo?
a bacterial infection by Group A strep and/or staph; it results in intraepidermal vesicles with exudate in them
what is erysipelas?
it is a more severe bacterial skin infection that involves dermal and lymphatic invasion and is often caused by group A streptococci - it is characterized by tender erythematous lesions
what is cellulitis?
bacterial infections that target subcutaneous fat layers and often develop into septicemia or lymphangitis
caused by Group A strep or Staphylococcus aureus
what is folliculitis?
it is an abscess caused by staphylococcus aureus around a hair follicle - a pruritic eruption
what is a boil?
enlarged folliculitis
what is a carbuncle?
a clustering of boils
what are general features of abscesses?
- neutrophil infiltration
- lysosomal enzyme release
- fibrin deposition to form capsule to sequester the abscess
what is fasciitis?
bacterial infection spreading down superficial fascia, destroying vessels and nerves, leading to patchy necrosis and cutaneous anesthesia
often caused by Group A streptococci
what is gas gangrene?
myonecrosis - caused by gas-producing bacteria like CLostridium
what are the bacteria that cause folliculitis?
staphylococci, pseudomonas
what are the bacteria that cause folliculitis?
staphylococci
what are the bacteria that cause impetigo?
strep and staph
what are the bacteria that cause impetigo?
strep
what are the bacteria that cause cellulitis?
strep, staph, haemophilus influenza type B
what are the bacteria that cause gas gangrene?
clostridia
what are the bacteria that cause necrotizing fascitis?
streptococci, enteric bacteria, anaerobes
general characteristics of streptococci?
catalase negative, GM +, PYR +, facultative anaerobes
alpha hemolytic?
greenish, partial hemolysis
beta hemolytic?
yellowish/clear, complete hemolysis
gamma hemolysis?
no hemolysis, nothing
group A streptococci characteristics?
GM+, beta-hemolytic, catalase negative, bacitracin sensitive
what are some group A strep virulence factors?
M protein, hyaluronic acid capsules, pyrogenic exotoxins, streptolysin O, DNases, Streptokinase, C5a peptidase
what is M protein
it is a coiled coil fibril that is covalently bound to cell wall
- it prevents phagocytosis
- aids in adhesion to epithelium
- binds fibrinogen and therefore prevents complement deposition
- has a hypervariable region to which antibodies will be different to
- mimics proteins on the surface of the heart - so can be dangerous
what is a hyaluronic capsule?
it has the same component as human connective tissues so it acts as camouflage
what are some pyrogenic exotoxins?
they are often encoded by phages
- superantigens that activate T cells systemically and cause release of INF-gamma and TNF-alpha causing septic shock and multi-organ failure
- scarlet fever:streptococcal pyrogenic exotoxin A, B, C
- toxic shock syndrome
what is streptolysin O?
an exotoxin that is oxygen labile and that is hemolytic - can be very toxic to some cells
what are DNases?
used to degrade/denature cell-free DNA in purulent lesions; also work against NETs
what is streptokinase?
it lyses blood clots, helping bacteria spread
what is C5a peptidase?
it degrades the C5a complement preventing neutrophil chemotaxis
what are some suppurative group A streptococcal diseases?
pharyngitis, pyoderma, streptococcal toxic shock syndrome
what is pharyngitis?
it is the inflammation of the oropharynx, accompanied by petechiae on the soft palate
what is pyoderma?
impetigo, erysipelas, fasciitis, cellulitis, may be caused by pyrogenic exotoxin A and cysteine proteases
what is streptococcal toxic shock syndrome?
pyrogenic exotoxins released that cause bacteremia and necrotizing fasciitis
what are the non-suppurative group A strep diseases?
rheumatic fever, and acute poststreptococcal glomerulonephritis
what is rheumatic fever?
occurs after strep throat and can lead to carditis, polyarthritis, and chorea
can be due to M protein anitbody cross reacting with heart tissue and is a huge risk for valvular disease
pateints require long term penicillin
acute poststreptococcal glomerulonephritis
depositoin of antibody/antigen complexes on glormeruli after throat or skin infection of strep
what is PANDAS syndrome?
“pediatric autoimmune neurpsychiatric disorder associated with group A streptococci” - OCD/tic disorder, pediatric onset, GAS infections associated, neurologic abnormalities like motor hyperactivity, choreiform movements
what are the best treatments for Group A streptococcal infections?
penicillin G, oral cephalosporins; if have a co-infection with staph, use penicillinase
what are the results of a blood agar and coagulase test for Staphylococcus epidermis?
negative coagulase test, non-hemolytic on blood agar
what are the results of a blood agar and coagulase test for Staphylococcus aureus?
positive coagulase test, beta-hemolytic on blood agar
what are some general characteristics of Staph?
GM+, catalase positive, facultative anaerobes, variable coagulase test, variable hemolysis
what test would you use to differentiate between Staph and Strep?
catalase - staph is catalase positive, strep is catalase negative
how woud you differentiate between S. aureus and S. epidermis?
use a coagulase test - aureus would be positive
or a hemolysis test, aureus would be beta hemolytic
what does the coagulase test entail?
add prothrombin to bacterial mixture, fibrinogen converted to fibrin, make clots!
what is the hallmark of Staph. aureus infections?
pus! suppurative lesions
what is hydradentis suppurative?
infection of sweat glands
what are some risk factors for skin infections by staph?
diabetes, acne, wet conditions, poor hygiene
what are some risk factors for wound infections via staph?
steroid therapy, obesity, diabetes, malnutrition, prolonged surgery, foreign body (like catheter)
what are the two major invasive infection types caused by staph aureus?
deep lesions and bacteremia
what are deep lesions and wht kind of diseases can they cause?
they are metastatic abscesses that cause deeper infections than the location of the primary infection - can cause osteomyelitis, septic arthritis, meningitis, pneumonia
what is bacteremia?
spread of bacteria into blood - a typically sterile environment; can lead to septic shock, septicemia, meningitis, pneumonia, pyelonephritis, endocarditis
what are the virulence factors for staph aureus?
protein A, catalase, leukocidin, ribotechoic and techoic acid, coagulase, capsule, hyaluronidase, cytotoxins
what is protein A?
a virulence factor in Staph aureus that binds to the Fc portion of anitbodies, pushing the Fab portion away, preventing it binding to antigen and also occupying the Fc portion preventing phagocyte binding
what is catalase?
a staph aureus virulence factor that prevents damage from free radicals secreted by neutrophils
what is leukocidin?
a staph aureus virulence factor that damages neutrophils
what are ribotechoic and techoic acids?
they are staph aureus virulence factors that bind fibronectin as well as cause septic shock (due to their similarity to G- endotoxin)
what is coagulase?
a staph aureus virulence factor - it is an enzyme that causes clotting (fibrinogen to fibrin)
what is the function of the staph aureus capsule?
preventing phagocytosis - a virulence factor
what is hyaluronidase?
a staph aureus virulence factor that degrades hyaluronic acid in allows navigation into subcutaneous layers
what is alpha-hemolysin?
a pore-forming cytotoxin
what is beta-toxin?
(aka sphingomyelinase C) it hydrolyzes membrane phospholipids
what is delta toxin?
it is cytolytic - acts like a detergent
what do gamma toxin and panton-vanlentine leukocidin do?
they are pore-forming in neutrophils and macrophages
what are some examples of staph toxin mediated diseases?
exfoliative diseases, toxic shock, food poisoning
what are staph exfoliative diseases?
they are caused by staph exfoliatin A, B which are tissue specific serine proteases that cause separation of the epidermis at the desmosomes
what are some examples of exfoliative diseases caused by staph?
bollus impetigo, staph scarlet fever, scalded skin syndrome
what is the toxin that causes staph toxic shock syndrome?
toxic shock syndrome toxin-1
what are the toxins that cause staph food poisoning?
staphylococcal enterotoxins A-E
two lab methods useful for tracking staph?
pulsed field gel electrophoresis (restriction enzymes) and phage tracking
what is another name for methicillin?
semi-synthetic penicillin
what do we call bacteria that are sensitive to methicillin?
MSSA - methicillin sensitive staph aureus
what do some S. aureus have that makes them resistant to penicillin?
beta-lactamase
what do we call baceria resistant to methicillin and what do we kill them with?
MRSA - methicillin resistant staph aureus; we treat with vancomycin
what do S. epidermidis cause?
osteomyelitis, endocarditis, UTIs, infections of indwelling foreign devices (like catheters, stents, pacemakers - biofilms can form)
