By Michael Spinner
Choosing the right antibiotics to treat an infection can be a daunting task for medical students given the sheer number of different drugs, infections, and organisms we have to memorize. However, a strong understanding of antibiotics is critical as they are some of the most commonly prescribed medications as well as a very common source of questions on boards exams and pimping on the wards. In this post, we offer some useful tips to help guide you as you think about choosing the right antibiotics to treat infections.
When thinking about how to treat an infection, first consider the likely causative organism. Is the organism likely bacterial, viral, fungal, or parasitic? Remember that antibiotics are only indicated to treat bacterial infections (with a few exceptions like metronidazole which also has excellent anti-protozoal coverage). The vast majority of upper respiratory infections and some lower respiratory infections (e.g. acute bronchitis) are caused by viruses and thus do not require treatment with antibiotics. Remembering this simple fact will help you to avoid exposing patients to unnecessary side effects and prevent antibiotic resistance.
In the case of a likely bacterial infection, first consider which of the following broad groups of bacteria are the most likely culprits: gram-positives, gram-negatives, anaerobes, and/or atypicals. For community-acquired pneumonia, the most common causative organisms are gram-positive (e.g. Streptococcus pneumoniae) or atypical bugs like Mycoplasma, Chlamydia, and Legionella. Thus, an appropriate antibiotic would cover both gram-positive and atypical organisms, making azithromycin an excellent choice. For appendicitis or diverticulitis, you should cover empirically for colonic anaerobes and gram-negative enterics, making piperacillin/tazobactam an appropriate agent.
Grouping bacteria into these four broad groups is a great place to start when thinking about antibiotic coverage. However, memorizing the specific antibiotics that cover certain bugs (especially MRSA and Pseudomonas aeruginosa) can be high-yield for board exams and on the wards. A list of the antibiotics that cover MRSA and Pseudomonas is provided below:
MRSA coverage Pseudomonas coverage
Vancomycin Piperacillin/tazobactam
Clindamycin Cefepime, ceftazidime
Doxycycline Carbapenems (except ertapenem)
TMP/SMX Aztreonam
Linezolid Aminoglycosides
Daptomycin Fluoroquinolones
In addition to considering the likely causative organism, also consider the source of infection. If the antibiotic does not adequately reach the source then its coverage won’t matter! For example, both doxycycline and vancomycin cover MRSA, but the preferred treatment depends on the source. Oral doxycycline is a reasonable choice to treat MRSA cellulitis but would not be an appropriate choice for MRSA bacteremia or osteomyelitis. A bloodstream MRSA infection would require treatment with an intravenous agent like vancomycin. Similarly, treating meningitis requires an agent with good CSF penetration like ceftriaxone.
Last, be sure to consider the side effects. Aminoglycosides and vancomycin are nephrotoxic and should be avoided in renal failure. Numerous antibiotics have adverse effects in utero and should be avoided in pregnancy. Penicillin and sulfa allergies are extremely common and will often affect antibiotic choices.
As with reading a chest X-ray or EKG, having a systematic approach to choosing antibiotics will help you to approach this topic with confidence on the wards and board exams. Whether you’re studying for Step1 or Step 2 or just trying to survive on the wards, I hope that these pointers will serve you well!
