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

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

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

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

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

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.