A Few More Bacteria to Know

There are some special bacteria that you should know a little bit about.

Methicillin-resistant Staphylococcus aureus (MRSA)

• A mutation in the penicillin-binding protein confers resistance to all beta-lactams (PBP 2a)
• More common in patients with previous hospitalisations and antibiotic exposures, a history of being in prison or homeless shelters, participation in group sports especially wrestling

Intravenous antibiotics

• Vancomycin is the most common
  • Dose is usually 10-20 mg/kg/day split bid to tid
  • Get a trough 0 to 60 minutes before the fourth dose after every dose change
  • Target trough is typically 10-15 for boring MRSA infections like SSTI, or 15-20 if they have a significant MRSA infection (bacteremia, osteomyelitis, pneumonia)
  • No specific target trough for non-MRSA infections, so just make sure that they're safe
• Daptomycin is the step up, but you should probably call ID if you're going to use it

Oral antibiotics

• There's a secret list of oral antibiotics that can be used to treat MRSA infections
• Clindamycin
• Septra (TMP-SMX)
• Doxycycline
• Linezolid ($$$)

Pseudomonas aeruginosa

• A very resistant Gram-negative bacterium that is associated with water
• Becomes more common when antibiotics kill off other bacteria
• Can cause pneumonia (usually HAP/VAP), UTI, and bacteremia, and is also implicated in intraabdominal infections

Antipseudomonal antibiotics

• Beta-lactams
  • Penicillins: piperacillin/tazobactam
  • Cephalosporins: ceftazidime and ceftolozane/tazobactam
  • Carbapenems: meropenem (+/- vaborbactam), not ertapenem
• Fluoroquinolones: ciprofloxacin, sometimes levofloxacin, but not moxifloxacin
• Aminoglycosides: tobramycin, gentamicin, amikacin
• Monobactam: aztreonam
• Polymixins: colistin

SPICE HAM organisms

• A group of Gram-negative organisms that have an inducible extended-spectrum beta-lactamase (ESBL)
• After 48 to 72 hours of antibiotics, they become resistant to all penicillins and cephalosporins, so usually need treatment with carbapenems or aminoglycosides
• They are:
  • S: Serratia marcescens
  • P: Providentia spp.
  • I: Indole-positive Proteus (that is, P. vulgaris)
  • C: Citrobacter spp.
  • E: Enterobacter spp.
  • H: Hafnia spp.
  • A: Acinetobacter spp. (not inducible, but very resistant)
  • M: Morganella spp.