Understanding Antibiotics: Gram Positive/Gram Negative
I often get asked "what's the best antibiotic?" The short answer (but the one people really don't want to hear) is "the one that works." It's obvious more complex than that, but, in reality, the best antibiotic to use for any given horse and any given disease IS the one that works, and the one that does so with minimal side effects, including both patient side effects (e.g. diarrhea) and microbial side effects (e.g. emergence of drug resistance).
Sometimes, certain drugs are referred to as "powerful" antibiotics, which really isn't a good description. What people are usually referring to is the ability of these drugs to work on bacteria that are resistant to many other antibiotics. These "powerful" antibiotics are not necessarily any better at treating an infection caused by a more susceptible bacterium, they are just able to work on bacteria resistant to other drugs.
When choosing an antibiotic, it's important to consider whether the bacteria involved in the infection are Gram-positive or Gram-negative. Gram staining is a simple technique used to divide bacteria into these two groups. Under a microscope, after staining Gram-positive bacteria appear purple and Gram-negative bacteria appear red (or pink). The differences in staining are a result of differences in the bacterial cell walls, which also play an important role in determining a bacterium's susceptibility to different antibiotics. Part of describing the function of an antibiotic is stating whether it is effective against Gram-positive bacteria, Gram-negative bacteria or both. (There are other aspects such as whether there is effect on aerobic versus anaerobic bacteria (those that can versus cannot survive in the presence of oxygen), but I'll address that some other time).
Some antibiotics have broad spectrum activity, being able to kill Gram-positive and Gram-negative bacteria. At first that might seem like a good thing, and indeed it is when you are treating an infection with an unknown bacterium. However, broad spectrum activity is not desirable when you know exactly what bacterium you are trying to eliminate. If possible, it's much better to target treatment more specifically against the offending bacterium for various reasons, such as to reduce the risk of resistance emerging in other bacterial groups.
Some antibiotics are narrow spectrum. Some are most effective against Gram-positive or Gram-negative bacteria, with little activity against the other. In general, we want to use an antibiotic of the most narrow spectrum possible. We can do this is a few ways:
Educated guess: A six-month-old foal with a lung abscesses most likely has Rhodococcus equi pneumonia. Even without a culture to confirm, we would usually treat the foal with drugs specifically targeting this Gram-positive bacterium, with little concern about Gram-negatives.
Gram stain: If the infection is in a location that can be sampled, then a Gram stain can be performed. This is very easy, quick and cheap, and can tell you whether you are dealing with a Gram-positive, Gram-negative or mixed infection.
Culture: This is the best method, but culture is not always possible and results may take several days. Culture can tell you the which bacterium is involved and exactly which antibiotics it's susceptible to in the lab.
