Antibiotics Menu

The Gram Stain

In 1882, Dr. Hans Christian Gram discovered a stain which separated types of bacteria based on their morphology and characteristics.  It separated bacteria into two groups:

  • Gram-positive bacteria: which keep a violet stain after using a decolorizing agent
  • Gram-negative bacteria: which lose the violet stain after the decolorizing agent

The differences between the two types of bacteria were based on their cell walls, which would be important later with the discovery of antibiotics, almost 50 years later. In 1928, a breach in laboratory protocol allowed a penicillin species of mold to contaminate a bacterial culture, demonstrating an antibiotic effect.


Bacteria are thus divided into Gram-positive and Gram-negative. Other classifications are also used to delineate them:

  • Aerobic vs anaerobic (whether or not they thrive in air)
  • Cocci (round) and rods (bacilli)
  • Spore-formers and those that don’t form spores
  • Pathologic and non-pathologic (with the understanding that a non-pathologic bacterium, if it becomes overgrown or grows in the wrong place, can become pathologic/infectious)


Antibiotics are medications that either kill bacteria directly (bactericidal) or stop them from multiplying (bacteriostatic). The biochemistry into what kills or stops the replication of bacteria makes use of the Gram stain, which identifies bacteria sensitive to different agents or those which are resistant.

Bacterial Resistance

As new antibiotics were discovered, an ingenuity of bacterial resistance arose due the sheer volume of organisms (millions are reproduced every hour), which makes mutations common. When exposure to antibiotics kills all of a particular bacterium except the ones with novel and coincidental mutations to resist, only the survivors carry on, containing the new genetics to pass on–as a new resistant species.

Types of Antibiotics

There have been many classes of bacteriostatic and bactericidal antibiotics developed over the years, including


  • Chloramphenicol
  • Clindamycin
  • Macrolides (erythromycin, including the later generations, azithromycin and clarithromycin)
  • Sulfurs (currently in Bactrim, Septra, etc.)
  • Tetracyclines


  • Beta-lactams (penicillins, including the next-generation ones: ampicillin and amoxicillin—2nd generation; carbenicillin and ticarcillin—3rd generation; and piperacillin—4th generation)
  • Aminoglycosides (streptomycin, including the later generations, gentamycin and tobramycin)
  • Cephalosporins (cephalexin, then cefoxitin)
  • Metronidazole
  • Quinolones (ciprofloxacin, levofloxacin)
  • Glycopeptides (vancomycin)

Some antibiotics can be either bactericidal or bacteriostatic, depending on the concentration and/or susceptibility of the infection organism (nitrofurans, e.g., nitrofurantoin). Some antibiotics prescribed for a mixed infection of different organisms can be bactericidal to one species, bacteriostatic to another, or be ineffective to yet another (resistant) bacterium, all simultaneously.

From the discovery from natural sources of the first-generation antibiotics, second-, third-, and later-generation antibiotics have been developed specifically to increase the range of susceptible organisms as well as counter the resistances to the previous generation antibiotics.


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This information is provided by Vascular Health Clinics and is not intended to replace the medical advice of your doctor or healthcare provider. Please consult your healthcare provider for advice about a specific medical condition.

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