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Microbiology and Immunology 2500A/B Lecture Notes - Penicillin, Polysaccharide, Antimicrobial Resistance

Microbiology and Immunology
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Lecture 6: Staphylococci 09/19/2012
"Staphyle" Greek for Bunch of Grapes
Gram-positive, coccus shaped, facultative anaerobe (don’t need oxygen to grow)
Coagulase Positive Staphylococci
o Staphylococcus aureus
o "aureus" = Latin for gold
Coagulase Negative Staphylococci
o Staphylococcus epidermidis
o Staphylococcus saprophyticus
o many other coagulase negative species
The Pathogen: Staphylococcus aureus
An efficient colonizer of humans that doesn’t usually cause problems
Where does Staph like to colonize? Anterior nares (nostrils) and skin
Carriers of S. aureus are healthy, asymptomatic people
Colonization leads to greater risk of infection, but prognosis is also generally better
o Skin and mucous membranes, nose (about 30% of people are persistently colonized)
o Spread person-person by direct or indirect contact
o Fomites (inanimate objects capable of transmitting an infectious disease) e.g. towels, razors, bandages
o resistant to high salt (skin)
o S. aureus surface proteins bind host proteins using adhesins (e.g. fibronectin, collagen and elastin
binding proteins)
o Adhesins are also important in endocarditis
The leading cause of hospital-acquired (nosocomial) infections
In the United States:
o Causes more than 10 million skin and soft tissue infections/year
o About 94, 000 invasive infections (31.8 per 100,000)
o About 19,000 deaths
Generally an extracellular pathogen
A “pyogenic” or pus-producing infection (pus body mounting in immune response)
The “hallmark” of S. aureus infection is the abscess
o Heat, redness, swelling, and pain
o A collection of dead neutrophils (pus) due to infection
Abscesses can occur in any organ but are most frequent on the skin
Can cause major complications if the organisms spread from the abscess
Abscesses don’t typically heal on their own – require drainage and maybe antibiotics
Staphylococcus aureus Virulence Factors
Why so many diseases?
S. aureus produces many virulence factors
Expression of the virulence factors is regulated
Surface virulence factors expressed during exponential growth
Secreted virulence factors (exotoxins) expressed during stationary phase
Generally, surface virulence factors are for colonization purposes, while secreted virulence factors are for
invasion and spread
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Staphylococcus aureus Virulence Factors
Immune evasion mechanisms
Resistance to phagocytosis
o Protein A:-
o S. aureus surface protein
o Binds the Fc portion of IgG
o Antibodies are bound in the incorrect
orientation to be recognized by
neutrophil Fc-receptor
Toxins that kill leukocytes
o S. aureus can make a number of
cytolytic toxins that kill white blood
o Often called “hemolysins” because they
can lyse red blood cells
o Examples: alpha-toxin and leukocidins
o Actual targets of these toxins are likely
white blood cells
o Helps to protect S. aureus in abscesses
and for spreading
Virulence Factors are Regulated
The Pathogen: Staphylococcus aureus
Deep abscesses:
o Not superficial but still localized has a
“focus” of infection
o e.g. cellulitis, liver, lung, kidney, tooth, etc.
o Wound or surgical infections
o Symptoms may not be obvious and may be more “constitutional” (whole body) e.g. fevers, chills,
malaise, etc.
o Deep abcesses can become systemic
Systemic infections:
o Often no single “focus” of infection
o e.g. bacteremia/septicemia, pneumonia, osteomyelitis, endocarditis
o Very dangerous, often difficult to treat primarily because of its antibiotic resistance
Systemic infections: Osteomyelitis
o Infection of bone or bone marrow
o S. aureus is the most common cause
o Can come from:
hematogenous spread (blood stream) or local infection (cellulitis)
joint replacement
o Can be very difficult to treat and may require open surgery, i.v. antibiotics
Systemic infections: Infective Endocarditis
o Infection of the heart valves
o Typically occurs on damaged or prosthetic heart valves with i.v. drug users
o The lesion is called the “vegetation”
o Bacteria can grow to large numbers and “seed” causing strokes and pulmonary embolisms
o Fever, heart murmurs, splinter hemorrhages
o About 25% are due to S. aureus but are more aggressive
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