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Comprehensive Guide to Antibiotic Spectrum: Gram-Positive, Gram-Negative, and Anaerobic Coverage

Discover a comprehensive guide to antibiotic spectrum of activity, covering gram-positive, gram-negative, and anaerobic bacteria. Learn about the specific antibiotics for MRSA, MSSA, Pseudomonas, ESBL, and more. Optimize your clinical practice with insights on antibiotic classification and usage.

Introduction

Any specific antibiotic works against gram-positive, gram-negative, or anaerobes or a combination of them. This means we have three spectrums: anti-gram-positive, anti-gram-negative, and anti-anaerobic.

In clinical practice, it’s most effective to classify antibiotics from two perspectives: their spectrum of activity and the specific bacteria they target.

Spectrum classification

Single-spectrum antibiotics

  • Pure anti-anaerobes: Metronidazole is the only pure anti-anaerobes antibiotic, it has zero gram-positive or gram-negative coverage.
  • Pure anti-gram positive coverage without any clinically significant anti-gram-negative or anti-anaerobic activity: Vancomycin, Linezolid, Daptomycin, Oritavancin, Dicloxacillin, Cloxacillin, Oxacillin, and Nafcillin.
  • Pure anti-gram negative coverage: This group includes Aztreonam, polymyxin B, and colistin. It has pure anti-gram-negative activity but no clinically significant anti-gram-positive or anti-anaerobic activity.

     

Double-spectrum antibiotics

  • Anti-gram-positive & anti-anaerobes w/o any gram-negative coverage: Clindamycin is the only one in this category.
  • Anti-gram-positive & anti-gram-negative: This is the most common group of antibiotics and includes the following:
    • All cephalosporins except cefoxitin and cefotetan which are active against anaerobes as well.
    • Amoxicillin and Ampicillin from the broad-spectrum penicillins.
    • All quinolones except moxifloxacin which is active against anaerobes as well.
    • Macrolides (azithromycin, erythromycin, and clarithromycin).
    • The tetracycline family, including doxycycline.
    • The aminoglycosides (gentamycin, tobramycin, and amikacin), remember that aminoglycosides are mainly used synergistically with other antibiotics
    • PCN G & V although their anti-gram-negative activity is minimal.
      Double-spectrum antibiotics

       

Triple-spectrum coverage (active against gram-positive, gram-negative, and anaerobes)

  • The Carbapenems family.
  • The extended-spectrum penicillins family (piperacillin/tazobactam, ampicillin/sulbactam, amoxicillin/clavulanic acid).
  • Cefoxitin & Cefotetan form the cephalosporins.
  • Moxifloxacin from the quinolones.
  • Tigecycline ( The only one in this group that’s active against MRSA).

Anti-specific bacteria antibiotics

Anti-MRSA

The following antibiotics are -to a variable degree- active against MRSA:

  • IV vancomycin, oral vancomycin is solely used for the treatment of C. diff.
  • linezolid (Oral & IV).
  • daptomycin (IV only).
  • ceftaroline (IV only).
  • Tigecycline (IV only).
  • Oritavancin (IV only).
  • Active against community-acquired MRSA:
    • Clindamycin (Oral & IV).
    • Trimethoprim/sulphamethoxazole (Oral & IV).
    • Doxycycline (Oral & IV).

Anti-MSSA (Methicillin-sensitive staph aureus)

  • The most commonly used IV agents in clinical practice are Cefazoline and antistaphylococcal penicillins (Nafcillin, Oxacillin).
  • For oral use cephalexin (Keflex), amoxicillin/clavulanic acid (augmentin), Cloxacillin, and Dicloxacillin.
  • All anti-MRSA antibiotics are theoretically active here but we reserve them for PCN allergy patients, IV vancomycin, linezolid, clindamycin, and Bactrim are the ones to consider in such a case.
  • Anti-MRSA antibiotics are not as effective as first-line anti-MSSA regimens, for example, IV vancomycin is less active than cefazoline or antistaphylococcal agents.
  • Please, avoid using quinolones or ceftriaxone to treat MSSA as resistance can develop quickly.

Anti-enterococci (enterococcus fecalis and Enterococcus faecium)

  • Ampicillin, ampicillin/sulbactam, Piperacillin/tazobactam, IV vancomycin, linezolid, and daptomycin are active against enterococci.
  • Linezolid and Daptomycin can be used for VRE (Vancomycin resitant enterococci).
  • Aminoglycoside or ceftriaxone can be used synergistically with ampicillin in enterococcal bacteremia but never alone.

Antipseudomonal agents

  • Piperacillin/tazobactam from the extended-spectrum PCN.
  • The carbapenems family except for ertapenem which is not active against pseudomonas.
  • Cefepime and ceftazidime from the cephalosporin family.
  • Ciprofloxacin and levofloxacin from the quinolones, and are the only available oral agents to treat pseudomonas.
  • Aztreonam.
  • Aminoglycosides, remember Aminoglycosides are used synergistically with other antibiotics.
  • Ceftazidime/avibactam, Polymixin B and colistin are reserved for panresistant pseudomonas.

Anti-ESBL-producing bacteria

  • ESBL(extended-spectrum beta-lactamase)-producing bacteria are gram-negative bacteria specifically E.coli and Klebsiella pneumonia.
  • Carbapenems family is the antibiotic of choice.
  • If carbapenem can’t be used for any reason, high doses of cefepime can be used as a second-line agent, don’t pick piperacillin/tazobactam (zosyn) or other cephalosporins even if the culture says it’s sensitive to, they aren’t reliable!

Miscellaneous

  • Stenotrophomonas maltophilia: Trimethoprim/sulfamethoxazole (Bactrim) is the drug of choice, and quinolones are the second line.
  • PJP (Pneumocystis jirovecii): Trimethoprim/sulfamethoxazole (Bactrim) is the drug of choice.
  • C. diff: Fidaxomicin, oral Vancomycin, and Metronidazole (Oral and IV).
  • Legionella, mycoplasma PNA, and chlamydia PNA: These bacteria don’t gram stain well mainly M.PNA and C.PNA, respiratory quinolones or azithromycin are the drugs of choice ( Ciprofloxacin isn’t a respiratory quinolone and shouldn’t be used in CAP treatment).

Wrap up

  • Quinolones are the only oral agents available to treat pseudomonas.
  • Metronidazole is a pure anti-anaerobes.
  • All cephalosporins cover the gram-positive and gram-negative bacteria but lack anaerobic coverage except cefoxitin and cefotetan.
  • Vancomycin is less effective than cefazoline in treating MSSA.
  • Ertapenem is the only carbapenem that doesn’t cover pseudomonas.
  • Don’t use ceftriaxone or levofloxacin to treat staph.
  • Carbapenems are the drug of choice to treat ESBL-producing bacteria.
  • Moxifloxacin is the only quinolone that has anaerobic coverage.

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