gram positive cocci - staphylococci

• 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