Microbiology of staphylococcus aureus (SA)

• GP cocci in clusters

• rapid, aerobic growth on blood agar

  • coagulase positive

  • catalase positive

  • thermonuclease positive

Carriage & mode of transmission of SA

(encounter)

• humans--carried in anterior nares

• risk of subsequent infection

• transmitted P2P by close contact

• equipped to colonize skin b/c grow at high salt & lipid concentrations

• ability to colonize skin/mucous

  • bacteria cell surface protein

  • fibronectin-binding proteins

SA Entry

• don't usually penetrate into deep tissues unless there is opening

• viral infections predispose pts to 2^o bacterial pneumonia

• entry via aspiration

SA spread & multiplication

• survival of SA in tissues depends on:

  • number of entering organisms

  • site involved

  • speed with which the body mounts inflammatory response

  • immunological history of host

• complex--effective pathogenic strategy

SA virulence

• degradative enzymes convert host tissue into nutrients required for bacterial growth

• penton-valentine leukocydin (PVL)--cytotoxin--carried on bacteriophage causes leukocyte destruction & tissue necrosis

Phagocytosis

• toll like receptors on macrophages enable them to sense material is microbial in origin

• chemotaxis= movement toward infection

Oxygen-dependent compounds

• superoxide via NADPH oxidase

• H2O2 via superoxide dismutase

• activated halides via myeloperoxidase (from neutrophil)

• nitrous oxide

Oxygen INdependent compounds

• lysosome (degrades bacterial peptidoglycan)

• lactoferrin (chelates iron)

• defensins, proteases, elastase...

Leukocyte adhesion deficiency

widespread pyogenic bacterial infections

NADPH oxidase  deficiency

chronic granulomatous disease

intracellular & extracellular infection, granulomas

G6PD

defective respiratory burst, chronic infection

damage

• most local staph infections lead to formation of abscess (pus)

• early events= acute inflamm rxn= influx of neutrophils

• chemotactic factors

Damage 2

• most SA killed--some survive
• capable of killing & lysing many of the neutrophils that entered infection area
• lysed neutrophils pour out large amts of lysosomal enzymes--damage surr tissue

Disease caused by SA

• skin & soft tissue infections

  • furuncles, paronychia

  • cellulitis

  • impetigo

  • wound infections

Furuncle (SA)

small abscess that develops in hair follicle

curbuncle

(may start as furuncle)

multiloculated abscesses in dermis & subcutaneous tissues

Impetigo (SA)

• small vesicle develops

• ruptures w/superficial spread

• skin erosion & serous exudate

• dries to produce honey-colored crust

cellulitis

• potential to spread into blood

• acute inflamm of subcutaneous CT w/swelling & pain

diseases #2-5

• bacteremia

• endocarditis

• pulmonary infxns

• musculoskeletal infxns

MRSA (2)

• presence of MecA gene which encodes PBP2a

  • low affinity for beta lactams

  • leads to resistance to PRPs & cephalosporins

• encoded by upto 4 staphylococcal cassette chromosome methicillin resistance (mec) DNAs found on chromosome

MRSA (3)

• community acquired MRSA (CA-MRSA) isolates often maintain susceptability to tet, TMP-SMX, clindamycin

• if isolate is erythromycin resistant, must confirm clindamycin susceptability w/D test

MSSA RX

• Methicillin SENSITIVE Staph Aureus
  • (uncomplicated) 1st ceph or naficillin (po)
  • (complicated) naficillin+gentamicin

MRSA RX

• Methicillin Resistant Staph. Aureus
  • (uncomplicated) clindamycin or TMP-SMX or linezolid (po)
  • (complicated) vancomycin & rifampin (iv)

disease caused by SA

• toxic related disease (toxic shock--scalded skin)
• food poisoning