gram positive cocci - staphylococci

Cards (12)

  • staphylococci
    • staphylococci are gram positive cocci that are arranged in irregular clusters
    • they are either coagulase negative or coagulase positive
    • important distinction for clinical purposes
  • coagulase negative staphylococci
    • CoNS are part of normal human microbiota
    • there are over 30 species of CoNS
    • examples include:
    • S. epidermidis
    • S. hominis
    • S. haemolyticus
    • S. saprophyticus, etc
    • in the inpatient setting, CoNS commonly contaminate blood cultures in which case they are not treated (unless mult +bcx)
    • mostly found on the skin and, in the case of S. saprophyticus, the GU tract
    • generally considered opportunistic pathogens and less virulent than S. aureus
    • identification by culture
  • true infection associated w/ CoNS
    • S. epidermidis
    • bacteremia (when multiple +bcx)
    • prosthetic device infections such as prosthetic joint implants, VP shunts, and IV catheters
    • ability to form biofilm on the surfaces of foreign bodies which generally requires removal of the infected device
    • S. saprophyticus
    • is somewhat unique as it is largely associated w/ uncomplicated UTIs in outpatient setting
  • staphylococcus aureus
    • S. aureus found in human commensal microbiota (usually the nasal mucosa) in 20 - 40% of the general population
    • when the cutaneous and mucosal barriers are disrupted, S. aureus can gain access to the underlying tissues of bloodstream and cause infection
    • virulence and antibiotic resistance
    • extensive arsenal of virulence factors
    • different types of antibiotic resistance
  • staphylococcus aureus drug resistance and epidemics
    • beta lactamases
    • found in virtually all S. aureus strains
    • as a result, penicillin is never used for treatment
    • vancomycin resistance
    • strains w/ intermediate susceptibility (VISA) as well as full resistance (VRSA) have been documented but are (still) rare in clinical practice
  • methicillin resistance
    • MRSA = methicillin resistance S. aureus
    • presence of mec gene --> encodes the penicillin binding protein 2a (PBP2a), a peptidoglycan transpeptidase --> leads to resistance to nafcillin, oxacillin, and most cephalosporins
    • clinical micro labs define methicillin resistance as an oxacillin MIC > (or equal to) 4 mcg/mL
    • methicillin is no longer in use!
    • MSSA = methicillin susceptible S. aureus
    • no mec gene --> susceptible to nafcillin, oxacillin, and most cephalosporins
    • about 50 % of all S. aureus are methicillin-resistant!!
  • important staph aureus infections
    • HA - hospital acquired or healthcare associated (SNFs, dialysis centers, etc.)
    • pneumonia
    • sepsis, bacteremia, endocarditis
    • UTIs (catheter-related)
    • CA - community acquired
    • skin soft tissue infections (less common)
    • toxic shock syndrome
    • CA/HA
    • bone - joint infections
  • HA-MRSA vs CA-MRSA
    • HA-MRSA
    • pt characteristics: elderly, chronically ill
    • risk factors
    • dialysis
    • LTC (long term care) facilities
    • hospitalized pts
    • indwelling catheters
    • site of infection
    • pneumonia
    • urinary
    • bloodstream
    • surgical site infections
    • antibiotic susceptibility: IV vancomycin typically first line
  • HA-MRSA vs. CA-MRSA (cont.)
    • CA-MRSA
    • pt characteristics: prisoners, athletes, military, IV drug users
    • risk factors: crowding, sport teams, IV drug use, no recent hospitalizations
    • site of infection: SSTIs
    • antibiotic susceptibility: clindamycin or SMX/TMP or doxycycline if mild-mod SSTIs and if susceptible
    • if more severe --> IV vancomycin
  • inducible clindamycin resistance
    • erythromycin and clindamycin inhibit protein synthesis by binding to the 50S ribosomal subunits of bacterial cells
    • resistance to both can occur through methylation of their ribosomal target site
    • constitutive - where the rRNA methylase is always produced, or
    • inducible - where methylase is produced only in the presence of an inducing agent
    • erythromycin is a strong inducer of methylase
    • increase risk of treatment failures if clindamycin is used
    • in general, avoid clindamycin if inducible resistance present
  • inducible clindamycin resistance (cont.)
    • disk diffusion test (D test)
    • D test is performed on staphylococci resistant to erythromycin and susceptible to clindamycin by routine methods
    • D test positive: reported as erythromycin-R and clindamycin-R
    • D test negative: reported as erythromycin-R, clindamycin-S
  • diagnosis
    • typically done by culture
    • there are multiple drug classes that have good activity agains S. aureus
    • the specific drug of choice varies depending on type and severity of infection
    • in general, in the inpatient setting IV vancomycin is the drug of choice for the treatment of many MRSA infection