I must admit that I am an absolute medical layman, trying to keep myself informed at best though.

Today I had a consultation at my dermatologist about a bad healing wound I have, to talk about the results of a swab they did last week.
It turned out that the wound is infected by several bacteria, and he said

Worst is that we found some gram negative bacteria ... (some other facts) ... , pseudomonas in particular.

Back at home I looked up what gram negative actually means, and found out to my astonishment, that it mainly (merely?) means the reaction of these bacteria in a coloring test.

Of course I looked up pseudomonas as well, and it's a fact that these are well known to be highly resistant to antibiotics (and my doctor explained that as well of course), and sometimes hard to medicate.

Now I am wondering, why he mentioned gram negative in 1st place.

Is it that the gram negative reaction in general indicates such hard to medicate, antibiotics resistant strains of bacteria?

Please excuse my laymans wording, I'm also not a native english speaker. Feel free to edit my question for better wording.

2 Answers 2


Gram positivity/negativity is sort of a historical classification that comes down to an ability to separate bacteria types without knowing what that separation really meant physically.

Of course, what really matters isn't whether or not a bacterial species takes up a particular stain, but rather that the reason that they stain well or poorly is because they have a different type of outer coat. Gram-positive bacteria have a thick peptidoglycan (sugars+amino acids) wall. Gram-negative bacteria have a second membrane on the outside, and typically a thinner peptidoglycan layer in the middle.

There is some complexity in this because not all bacteria necessarily follow the same staining "rule" by their coat, but it works as a rough characterization for many species and is quick and cheap.

Some antibiotics target the peptidoglycan wall, so they are most effective against Gram-positive bacteria (penicillin is an example).

However, there is nothing that says a Gram-positive bacteria can't also have antibiotic resistance. Staphylococcus are a common Gram-positive pathogenic strain, and you may have heard of MRSA, a particularly troublesome antibiotic-resistant Staph strain.

  • Ty for the detailed explanation. Yes I've heard about Staphylococcus and these developped the antibiotics resistance mainly in clinical / hospital environments (strong survivors these lil' beasts :-P). Commented Oct 6, 2020 at 21:57

Seems I found the answer about the correlation of gram negative test and high resistance against antibiotics.

From Wikipedia Gram-negative bacteria (emphasis mine):

Gram-negative bacteria are found everywhere, in virtually all environments on Earth that support life. The gram-negative bacteria include the model organism Escherichia coli, as well as many pathogenic bacteria, such as Pseudomonas aeruginosa, Chlamydia trachomatis, and Yersinia pestis. They are an important medical challenge, as their outer membrane protects them from many antibiotics (including penicillin); detergents that would normally damage the peptidoglycans of the (inner) cell membrane; and lysozyme, an antimicrobial enzyme produced by animals that forms part of the innate immune system. Additionally, the outer leaflet of this membrane comprises a complex lipopolysaccharide (LPS) whose lipid A component can cause a toxic reaction when these bacteria are lysed by immune cells. This toxic reaction can include fever, an increased respiratory rate, and low blood pressure — a life-threatening condition known as septic shock.

Now it's clear for me, why gram-negative was mentioned by my dermatologist in 1st place.
It is a general indication of antibiotic resistance of bacteria strains.

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