General Tips, Antimicrobial Resistance Mechanism, Bacteremia

Refer to the VUMC Antimicrobial Stewardship Webpage
Cross reference with IDSA guidelines
VUMC Pharmacy Antimicrobial Dosing Guidance can help with abx selection, dosing for infection type/severity and renal function
Info on precautions: Infection Prevention Website (https://www.vumc.org/infectioncontrol/12 177) or page Infection Control
For approvals of restricted antimicrobials: Page 317-4376 at VUMC (at the VA, page ID fellow)
Both lab and clinicians are required by law to report diagnosis and treatment for reportable disease

Common Antibacterial Resistance Mechanisms

(need contact precautions)

ESBL (extended spectrum beta-lactamase): Gram negative bacteria, commonly E. coli, Klebsiella pneumoniae, Klebsiella oxytoca, or Proteus mirabilis. Hydrolyze PCNs, cephalosporins, aztreonam. Recognize by resistance to CTX (CTX-M Positive). Consult ID
AmpC beta-lactamase: Enterobacter cloacae, Citrobacter freundii, Klebsiella aerogenes. Resistant to PCNs, beta-lactamase inhibitor combinations, cephalosporins (other than cefepime), and aztreonam. Resistance is inducible, so may initially appear as susceptible on testing. Recognize by pathogen, rather than susceptibility pattern. Tx: cefepime or carbapenem. Non-beta-lactams (e.g., fluoroquinolones, TMP-SMX) may be used if susceptibility is confirmed. Avoid CTX or pip-tazo regardless of susceptibility results
- S. marcescens, M. morganii, and Providencia spp. have <5% risk of clinically significant AmpC production, OK to use PCNs, cephalosporins if susceptible.
Carbapenem resistance: Gram-negative bacteria, commonly Klebsiella pneumoniae, E. coli, Enterobacter cloacae complex, PsA, Acinetobacter baumannii. Resistance from a variety of genes (IMP, KPC, NDM, OXA, VIM) or from non-enzymatic mechanisms (porin loss + AmpC/ESBL overexpression) not detected on carbapenemase PCR panels. Management: consult ID who may use newer agents like ceftazidime-avibactam, meropenem-vaborbactam, or cefiderocol for certain carbapenemases. Contact precautions.

When BCx turn positive, the lab reports Gram stain and GenMark ePlex® results to help guide empiric therapy, while awaiting further species identification and susceptibilities.
GenMark ePlex detects presence of resistance genes such as mecA (methicillin resistance), vanA/B (vancomycin resistance)
ID consult required for bacteremia due to Staph aureus/lugdunesis, E faecalis/faecium, Gram negative rods with carbapenem resistance
Need BCx clearance for Staph (MRSA or MSSA), Staph lugdunensis, Candida. Typically do not need to repeat for most strep and GNRs (unless endovascular infection or no source control)
Candida in a blood culture is NEVER considered a contaminant – see fungal infections
Refer to VUMC ASP Bacteremia Page for Detailed Guidance for Interpreting GenMark’s ePlex Results and Oral Abx for Uncomplicated Bacteremia.

MSSA / S. lugdunensis (mecA−): Cefazolin, ID consult
MRSA / S. lugdunensis (mecA+): Vancomycin, ID consult
S. epidermidis / Staph spp. (coag-neg): Often contaminant; Cefazolin (mecA-) or vancomycin (mecA+) if treating
Strep agalactiae, anginosus, pyogenes: PCN, ampicillin, or cefazolin
S. pneumoniae: CTX (add vancomycin if meningitis)
Other Strep: CTX
E. faecalis (vanA/B−): Ampicillin, ID consult
E. faecalis (VRE): Ampicillin, ID consult
E. faecium (vanA/B−): Vancomycin, ID consult
E. faecium (VRE): Daptomycin, ID consult
Micrococcus: Often contaminant, vancomycin if treating

Listeria: Ampicillin
Bacillus cereus, Bacillus subtilis, Corynebacterium: Often contaminant; vancomycin if treating
Cutibacterium acnes, Lactobacillus: Often contaminant; PCN if treating

Acinetobacter baumannii: Ampicillin-sulbactam
E. coli, K. pneumoniae, K. oxytoca, P. mirabilis, Proteus spp., Salmonella, H. influenzae, N. meningitidis: CTX
Enterobacter, Citrobacter, Serratia, Morganella, Proteus (non-mirabilis): Cefepime
PsA: Cefepime or Pip-tazo
Bacteroides fragilis, Fusobacterium: Metronidazole or ampicillin-sulbactam
Stenotrophomonas maltophilia: TMP-SMX + minocycline

(override above)