What is the bacterial spectrum of cefazolin, tobramycin, gentamicin, vancomycin, and ceftazidime?

Bacterial Spectrum of Cefazolin, Tobramycin, Gentamicin, Vancomycin, and Ceftazidime

Cefazolin (First-Generation Cephalosporin)

Cefazolin provides excellent coverage against methicillin-susceptible Staphylococcus aureus (MSSA) and streptococci, with additional activity against select Gram-negative organisms including Klebsiella, E. coli, and Proteus mirabilis. 1

• Primary spectrum: MSSA, streptococci (including Streptococcus pneumoniae and Streptococcus pyogenes) 1
• Secondary Gram-negative coverage: Klebsiella species, Escherichia coli, Proteus mirabilis 1
• Does NOT cover: Pseudomonas aeruginosa, anaerobes, MRSA, enterococci, or atypical organisms 2, 1
• Commonly combined with metronidazole for intra-abdominal infections to add anaerobic coverage 2

Tobramycin and Gentamicin (Aminoglycosides)

Both aminoglycosides provide potent bactericidal activity against aerobic Gram-negative bacilli, including Pseudomonas aeruginosa, with gentamicin showing slightly broader coverage against Enterobacteriaceae. 2

Tobramycin:

• Primary spectrum: Pseudomonas aeruginosa, Enterobacteriaceae (E. coli, Klebsiella, Proteus), Acinetobacter species 2
• Limited Gram-positive activity: Some activity against Staphylococcus aureus 2
• Does NOT cover: Anaerobes, streptococci, enterococci, or atypical organisms 2
• Resistance rates in Pseudomonas aeruginosa range from 1-3% in clinical studies 2

Gentamicin:

• Primary spectrum: Similar to tobramycin but with slightly better activity against Enterobacteriaceae 2
• Covers Pseudomonas aeruginosa, E. coli, Klebsiella, Proteus, Enterobacter, Serratia, and Acinetobacter 2
• Limited Gram-positive activity: Some staphylococcal coverage 2
• Does NOT cover: Anaerobes, streptococci (except when combined with beta-lactams for synergy in endocarditis), or enterococci as monotherapy 2

Critical limitations for both aminoglycosides:

• Aminoglycoside monotherapy is suboptimal even when organisms are susceptible in vitro 2
• Require combination with beta-lactams for synergistic activity against Gram-negative bacilli and to prevent resistance emergence 2
• Nephrotoxicity, ototoxicity, and hypokalemia are significant concerns requiring serum level monitoring 2

Vancomycin (Glycopeptide)

Vancomycin is active exclusively against Gram-positive organisms and serves as the standard treatment for MRSA and serious staphylococcal infections. 1

• Primary spectrum: MRSA, methicillin-susceptible Staphylococcus aureus (MSSA), coagulase-negative staphylococci, streptococci (including viridans streptococci and Streptococcus pneumoniae), and enterococci 2, 1
• Does NOT cover: Any Gram-negative bacteria, anaerobes, or atypical organisms 2, 1
• Requires serial monitoring of serum levels to avoid nephrotoxicity 1
• Should be added empirically when viridans streptococci or MRSA are suspected, as mortality from viridans streptococci may be higher without initial vancomycin therapy 2

Ceftazidime (Third-Generation Cephalosporin)

Ceftazidime provides extended-spectrum coverage against Gram-negative bacilli, including Pseudomonas aeruginosa, but has poor activity against Gram-positive organisms and no anaerobic coverage. 2

• Primary spectrum: Pseudomonas aeruginosa, Enterobacteriaceae (E. coli, Klebsiella, Proteus, Enterobacter, Serratia), and HACEK organisms 2
• Limited Gram-positive activity: Poor coverage of streptococci and staphylococci compared to other cephalosporins 2, 3
• Does NOT cover: MRSA, viridans streptococci (reliably), penicillin-resistant Streptococcus pneumoniae, enterococci, anaerobes, or atypical organisms 2, 3
• Resistance rates in Pseudomonas aeruginosa range from 0.8-5% depending on the setting 2
• Frequently combined with aminoglycosides for febrile neutropenia or serious Gram-negative infections to provide synergy and prevent resistance 2

Critical clinical caveat:

When treating febrile neutropenia or serious infections, ceftazidime monotherapy lacks adequate Gram-positive coverage, particularly against viridans streptococci, which can cause fulminant sepsis 2. Consider adding vancomycin if Gram-positive infection is suspected or if the patient has indwelling catheters, mucositis, or prior fluoroquinolone prophylaxis 2.