Intro to Antibiotics

In infectious disease, three factors impact decision making: the bug, the drug, and the patient. Specimens are taken from the patient’s lung secretions, urine, blood, tissue, fluid, etc and brought to a microbiology lab. 

  1. Bacteria stain
    • Gram positive organisms – have a thick cell wall and stain dark purple or blue
    • Gram negative organisms – have a thin cell wall and take up safranin (red) stain
    • Atypical organisms – do not have a cell wall and do not stain
  2. Begin empiric treatment with broad spectrum antibiotics based on the likely organism(s)
    • Keep in mind factors such as allergies, available cultures at the time, past medical history (i.e. seizures to avoid carbapenems), antibiotic choice can penetrate the infection site, renal function, and other contraindications 
  3. After identification of organisms based on culture and susceptibility, convert to a narrow-spectrum antibiotic as soon as possible
    • Once the organism is identified, susceptibility testing is performed to determine with which antibiotics are useful for treatment 
    • The minimum inhibitory concentration (MIC) is compared to the susceptibility breakpoint (the usual drug concentration that inhibits bacterial growth). If the MIC is < the breakpoint, the bacteria is susceptible to the antibiotic
      • Susceptible (S) – Sensitive, a good treatment option
      • Intermediate (I) – effective under specific circumstances
      • Resistant (R) – unlikely to be effective and is not recommended
    • Refer to your hospital’s antibiogram to check resistance trends over the last year occurring at your institution to avoid certain medications that are not susceptible. 
    • Streamline therapy: If more than one organism is present, attempt to cover both organisms with one drug. If the patient is able to eat normally, consider IV:PO conversion. 
  4. Assess the patient by monitoring for clinical improvement. Determine the duration of treatment and do not let antibiotics continue if unnecessary.  

Antimicrobial Stewardship Programs (ASPs) are a team of infectious disease physicians, pharmacists, and microbiology personnel that utilize all the above knowledge to establish antibiotic guidance for their hospital. Their main goals are to:

  • Improve patient safety and outcomes
  • Decrease resistance
  • Reduce adverse effects 
  • Promote cost effectiveness

There is usually a ASP team implemented in a hospital dedicated to provide guidance on antimicrobial therapy. The process of implementing an ASP is also provided by IDSA. 

Clinical Pearls

  • Aminoglycosides and beta lactams can be used together synergistically in endocarditis
  • Be mindful of collateral damage caused by antibiotic use that can lead to C. difficile infections especially clindamycin


  • Guidelines available at the Infectious Diseases Society of America website (

Leave a Reply

Your email address will not be published. Required fields are marked *