Host Defense Flashcards
Staph. Basics
Gram positive cocci that are spherical in shape and have no endospores.
Subdivided by presence or absence of catalase (breaks down hydrogen peroxide)
Grow in clusters resembling grapes
Present on skin and mucous membranes of humans
Capable of growing at high salt concentrations
Staph. sp.
Physiology and Structure
(Capsule, Peptidoglycan, and Teichoic Acids)
Covered by a polysaccaride capsule that defines different serotypes (5 and 7 most infectious) and inhibits phagocytosis
Produces a slime layer that makes it difficult to eliminate from catheters, grafts, shunts, et c.
Resistance to penicillins is given by mecA, a novel penicillin binding protein PBP2’
Peptidoglycan (50% of cell wall) has endotoxin-like activity
Teichoic acids (30-50% cell wall) are species specific and are bound to NAM residues. Non-immunogenic alone, but they are when bound to peptidoglycan. Binds fibronectin
Staph. so.
Physiology and Structure
(Unique Features)
S. aureus (but not the coagulase negative Staph. sp) is coated with protein A, which binds to the Fc receptor of IgGs.
Protein A is used as an ID for S. aureus
Bound coagulase (“clumping factor”) is a virulence factor of S. aureus that converts fibrinogen to fibrin, leading to coagulation.
Detection of coagulase is the primary ID test for S. aureus
Staph. aureus Enzymes
Coagulase leads to clotting and abscess formation
Hyaluronidase hydrolyzes hyaluronic acids
Fibrinolysin/staphylokinase dissolves fibrin clots
Lipases cleave lipids for food in sebaceous areas of the body
Nucleases hyrdrolyze viscous DNA
Staphylococcal Cytotoxins
Alpha toxin: Produced by most species that cause human disease. It is either found on a plasmid or chromosome. Disrupts smooth muscle in blood vessels. Pore former
Beta toxin: sphingomyelinase C. Specificity for cleaving sphingomyelin and lysophosphatidylcholine.
Delta toxin: Produced by almost all S. aureus strains. Acts as a surfactant that can target erythrocytes and other mammalian cells
Gamma toxin and P-V leukocidin: bicomponent toxins consisting of two polypeptide chains, a S (slow) and F (fast) component. PV is associated with community-acquired MRSA. Pore former
Staphylococcal Scalded Skin Syndrome
AKA Ritter Disease
Caused by Staphylococcal Exfoliative Toxins, primarily a disease of neonates and young children.
Two types, Exfoliative toxin A and B. Type A is a superantigen. Both are serine proteases that split desmoglein 1 (found in desmosomes)
They are not associated with cytolysis or inflammation (neither staph nor leukocytes are present in involved layer of epidermis)
Causes the skin to “melt away” after forming large cutaneous blisters called bullae, but does not cause scarring due to absence of dermal involvement
Bullous impetigo is a localized culture positive variant
Staphylococcal Food Poisoning
This is an intoxication as opposed to an infection
Enterotoxin A is most commonly associated with food poisoning, it is heat-stable.
Most commonly affected foods are processed meats, custard-filled pastries, potato salad, and ice cream.
Cause a rapid onset of severe vomiting, diarrhea, and abdominal cramping with resolution within 24 hours
Treat with rehydration, not antibiotics
Staphylococcal Toxic Shock Syndrome (TSS)
Toxic Shock Syndrome Toxin-1 (TSST-1) is a heat and proteolysis-resistant chromosomally mediated exotoxin.
Source of menstruation-assoiciated toxic shock syndrome.
Expression in vivo requires elevated oxygen and neutral pH
Superantigen, bridges MHC-II and TCR leading to cytokine storm
Staphylococcal Cutaneous Infections
Includes impetigo, folliculitis, furuncles, and carbuncles.
Impetigo: a superficial infection that mostly affects young children; occurs primarily on the face and limbs. Honey colored crusting following pustule rupture
Folliculitis: a pyogenic infection of the hair follicles (if on eyelid, called a stye)
Furuncles (boils): Extension of folliculitis; large, painful, rasied nodules that have an enderlying colletion of dead and necrotic tissue. They will drain
Carbuncles: occur when furuncles coalesce and extend to the deeper subcutaenous tissue. Presents with chills, fevers, indicating spreading bacteremia
Staphylococcal Osteomyelitis and Septic Arthritis
Can result from hematogenous spread to bone, direct implantation, or extension from an adjacent area.
Often grows in metaphyseal area of long bones in children.
A Brodie Abscess is a sequestered focus of osteomyelitis that arises in the metaphyseal area of long bones in adults
Staph is the leading cause of septic arthritis, leading to a painful, erythematous joint, with purulent material obtained on aspiration
Clinical Syndromes of Coagulase Negative Staphylococci
Often infect artificial heart valves, leading to endocarditis. This can lead to valve separation and mechanical heart failure.
More than 50% of catheter and shunt infections are by coagulase-negative staph (produce slime molds)
Artifical joints are similarly infected
S. saprophyticus causes urinary tract infections in young sexually active women.
Yersinia pestis
Gram negative, facultatively anaerobic rods. Fermenter, oxidase negative
Causes the bubonic plague (urban spread by rats, sylvatic spread by squirrels, rabbits, field rats, domestic cats) which causes an inflammatory swelling of lymph nodes called a bubo.
Treat with streptomycin, tetracyclines, chloramphenicol, or Bactrim.
Zoonotic, with reservoirs being rodents/domestic animals and the vectors being fleas/direct exposure
Type III secretion system is anti-phagocytic (Yop proteins)
Pasteurella multocida
Small, facultatively anaerobic, fermentative coccobacilli found as commensals in the oropharynx of healthy animals.
Associated with animal bites/licking wounds
Causes three general diseases: 1) localized cellulitis and lymphadenitis following a bite 2) exacerbation of chronic respiratory disease and 3) systemic infection in immunocompromised
Characteristic “musty odor” on culture
Penicillin is treatment of choice
Franciscella tularensis
Very small gram negative coccobacilli, nonmotile, fastidious and slow growth requirements, always pathogenic in humans, strictly aerobic
Causes tularemia (aka glandular fever, rabbit fever, tick fever, and deer fly fever)
Intracellular pathogen (lives in macrophages by inhibiting phagosome-lysosome fusion)
TH1 response important for host response
Brucella sp
Cause of Brucellosis. Zoonotic. Caused mostly by B. melitensis and results from consumption of unpasteurized dairy products.
B. abortus and B. canis produce mild disease with rare suppurative complications, while B. suis causes formation of destructive lesions and has a prolonged course.
Acute disease develops in 50% of patients infected; symptoms are nonspecific including malaise, chills, sweats, fatigue, weakness, myalgias, weight loss, arthralgias, et c. Almost all patients have fever (undulant fever)
Worldwide distribution, present in many herd animals (bison, sheep, goats, cattle, et c.)
Gram negative, nonmotile, non-encapsulated, small coccobabilli. Grows slowly in culture, cultured from blood.
