Staphylococcus Aureus

Question 1

general characteristics of Staphylococci?

Answer 1

-Gram + cocci, usually arranged as grape like clusters

-facultative anaerobes- can survive in high concentrations of NaCl, high lipid concentrations

• produce catalase- enzyme that converts hydrogen peroxide to water and oxygen (measure enzyme to determine it from strep)
• ubiquitous- commonly found on skin and mucous membranes of humans
• prolonged survival of inanimate objects under various condition, thick peptidoglycan layer
• causes opportunistic infections and a wide range of life threatening systemic diseases (skin, soft tissue, bones, urinary tracts)

Question 2

What is the most virulent staphylococci genus?

Answer 2

S. Aureus -methicillin resistant S. Aureus (MRSA) known for causing serious infections in hospitalized patients and in previously healthy children and adults

-methicillin sensitive strains are referred to as MSSA

Question 3

Unique lab characteristics of S. Aureus?

Answer 3

• colonies on plate turn yellow and, beta hemolytic on blood agar
• produces coagulase, an enzyme that converts fibrinogen to fibrin-> forms clot when mixed with plasma
• coagulase test- lysis of RBC, clearing colonies
• protein A inhibits antibody mediated clearance by binding Fc portion of IgG1, IgG2, IgG4 (complement cannot bind)

Question 4

Epidemiology of S. Aureus?

Answer 4

• ubiquitous- humans are a major reservoir
• health care workers are carriers
• susceptible to high temperatures and disinfectants -can survive on objects
• hospital acquired MRSA- spread from health care workers, surgery, antibiotics, weakened immunity
• community acquired MRSA- cutaneous infection, sever pneumonia, being athlete, shaving, tattoos, gym equipment

Question 5

Mr. B, a pastry chef, cuts his arm. Over the next week, noticed swelling at the site. After 4 days, initial site was larger and he developed a fever with chills. When he came to the emergency room, he had severe low back pain. He had an abscess over his arm. Pressure in lower spine. High WBC count. Gram stain showed gram + cocci in clusters. What happened?

Answer 5

• when he cut his arm, he made himself more susceptible to infection (skin infection)
• got into blood stream and affected his vertebrae, suggesting osteomyelitis
• organism is resistant to penicillin (make beta lactamase) but sensitive to nafcillin

-customers also developed severe vomiting and diarrhea (food poisoning)

• cultures show enterotoxin A
• S. Aureus causes greater variation of diseases
• skin infection, osteomyelitis, food poisoning

Question 6

Why does S. Aureus cause more varied diseases than any other pathogen?

Answer 6

• high degree of genetic diversity
• ability to cause infection can vary based on production of virulence factors and how expressed
• overlapping functions of virulence factors
• superantigens
• has a capsule with exotoxins

Question 7

How does S. Aureus cause diseases?

Answer 7

• direct invasion and destruction of tissue
• production of toxins, infection and intoxication

Question 8

General pathogenesis of S. Aureus?

Answer 8

• pyogenic (pus) disease, abscess formation at local or metastatic sites
• immune response- intense, PMN with influx of macrophages and fibroblasts
• immune response will contain infection or it can spread to adjoining tissue or blood
• patients with congenital defects in chemotactic or phagocytic response are more susceptible to staph
• toxigenic disease- may or may not involve viable bacteria

Question 9

Colonization of skin and mucus membranes mediated by ______. Encounter?

Answer 9

MSCRAMMs that allow them to bind to tissue

• live on people, fomites
• FnbpA, FnbpB
• CNAs (collagen adhesin)- mediates binding to collagen in connective tissue
• clumping factors A and B

Question 10

Spread and transmission?

Answer 10

• direct contact with carrier or aerosols associated with respiratory infections
• facilitated by people more prone to be colonized or carriers (hospital workers, diabetics, IV drug users)

Question 11

metastatic infections?

Answer 11

• usually requires implantation and previous damage to tissues, less bacteria when you stab yourself with something
• abscess in bones (osteomyelitis), joints, lungs, kidneys
• spread facilitated by elaboration of serine proteases, hyaluronidases, ligases, DNases (spread factors so they are not walled in)

Question 12

Host response to infection?

Answer 12

• primary response is recruitment of PMNs
• anti-capsular and anti-MSCRAMM facilitate opsonization and protect against infection

Question 13

Evasion of host response?

Answer 13

• anti phagocytic capsule
• protein A- binds Fc portion of IgG, prevents opsonophagocytosis of PMNs, prevents classical path of complement
• coagulase- fibrin clots protects bugs from WBC
• intracellular survival of small colony variants in endothelial cells and macrophages
• cytolytic exotoxins (panton valentine leukocidin P-V)
• P-V is linked with severe pulmonary and cutaneous infections

Question 14

Toxins that mediate disease?

Answer 14

-if purified, they cause disease by themselves

Exfoliative Toxins (ETA, ETB)

• exfoliates skin
• superantigens- cytokine storm
• mediates staph scalded skin syndrome (SSSS)
• serine protease that splits desmoglein-1

Enterotoxins (A-X)

• superantigens
• stable to heating to 100 C for 30 mins and resistant to hydrolysis by gastric juices and jejunal enzymes
• Enterotoxin A most common associated with food poisoning
• Entero B and C associated with 50% non menstruation associated TSS

Toxic Shock Syndrome Toxin (TSST-1)

• superantigen
• strains that carry gene responsible 90% of menstruation associated TSS, 50% other cases
• can penetrate mucosal barriers

Question 15

Clinical manifestations of S. Aureus?

Answer 15

-folliculitis- pyogenic infection in hair follicle, stye if in base of eyelid

-carbuncle- multiple interconnected boils that extend into deeper tissues, fever and chills point to systemic spread

-furuncle- extension of folliculitis, large, painful, underlying dead and necrotic tissue (abscess)

Question 16

Staph Scalded skin syndrome (SSSS)?

Answer 16

• Ritters disease
• caused by strains that produce exfoliative toxins A and B- tissue specific serine proteases that cause separation of the layers of the epidermis at desmosomes
• exfoliative dermatitis- characterized by extensive sloughing of the skin remote from infection site

Question 17

Toxic shock syndrome (TSS)?

Answer 17

caused by exotoxins that are super antigens:

• toxic shock syndrome toxin-1 (TSST-1)
• staph enterotoxin serotypes B and C
• IL-1 -> fever
• TNF alpha and beta -> capillary leakage (hypotension)
• INF gamma and IL-2 -> rash -characterized by fever, skin rash, hypotension, peeling of skin
• originally associated with use of highly absorbent tampons

Question 18

Staph food poisoning?

Answer 18

• an acute gastroenteritis (intoxication)- short incubation period (1-6 hours)
• caused by ingestion of pre formed staph enterotoxin serotypes A-X
• processed meats, custard filled pastries, potato salad, ice cream
• contamination by human carriers
• characterized by intensive intestinal peristalsis, diarrhea and vomiting

Question 19

Treatment of Staph?

Answer 19

• most (90%) S. Aureus is resistant to penicillin because of beta lactamase, functions to cleave beta lactam ring to make it inactive
• drain abscess and treat with beta lactamase resistant penicillins and cephalosporins (nafcillin), effective for methicillin sensitive strains (MSSA)
• use vancomycin for MRSA
• treat VRSA with linezolid, synercid, daptomycin (NRSA, VISA)
• anti toxin antibodies protect against TSS, SSSS and staph food poisoning, consider protein synthesis inhibiting antibiotics to treat toxin mediated disease

Question 20

Antibiotic resistance factors?

Answer 20

beta lactamase:

• hydrolyzes classical penicillin
• plasmid encoded
• most Penicillin binding protein 2a (PBP2a):
• transpeptidase with alteration in binding site that results in reduced affinity for penicillin and cephalosporin class antibiotics -keeps cross linking peptidoglycan
• PBP2a encoded on different cassettes on staph chromosome (SCCmec)
• S. Aureus strains that express PBP2a referred to as MRSA strains (hospital or community)

Question 21

Genetics of MRSA resistance?

Answer 21

mecA encodes for PBP2a on a pathogenicity island with other exotoxin genes:

• staph cassette chromosome (SCCmec)
• 11 different cassettes

hospital acquired (SCCmec types I and III):

• resistant to large number of antibiotics
• very large
• more genes, encodes more resistance

community acquired (Type IV SCC mecA)

• smaller, more susceptible to antibiotic
• resistant to meth and other beta lactams

Question 22

Immunity and prevention?

Answer 22

• immunity clearance is mediated by opsonizing IgG which allows for more efficient phagocytosis
• immunity short lived and incomplete so individuals can be re infected
• cutaneous infections can recur over many years
• good hand washing and hygiene is best prevention

Question 23

Future endeavors?

Answer 23

• vaccine development (toxoid for TSST-1)
• need to understand more CMI response
• elucidate host factors that lead to severe MRSA infections in certain individuals and mild infection in others
• learn more about virulence factor regulation and transmission between strains

Question 24

Virulence factors of S. Aureus?

Answer 24

• possesses a polysaccharide capsule
• has a plethora of redundant cell surface and exotoxin virulence factors

Question 25

How does methicillin kill bacteria? How does MRSA resist methicillin?

Answer 25

- methicillin (beta lactamase resistant) binds transpeptidases (PBPs) and prevents crosslinking of peptidoglycan, breaks down cell wall, bacterial cell lyses - MRSA makes new transpeptidases (PBP2a) that does not bind methicillin and bacteria continues to make peptidoglycan