Types of Bacterial Resistance
By spectrum. Compounds can be inherently ineffective against Gram negative or Gram positive bacteria, particularly Gram negative because of the outer cell membrane. Erythromycin, the reason why it's not active against Gram negatives is because it can't get through that outer membrane. That's the problem. If it could get through, it might be active. There are mutants that have a leaky outer membrane and erythromycin can get into these cells. When it can get in, it can do its job. So the ribosomes inside the cell of the Gram negative are not necessarily inherently resistant. But because of that cell wall structure, it prevents the erythromycin from getting in. That's also true of a creature called Haemophilus influenzae which is heavily involved with middle ear infections. By the way, some particularly invasive strains can be involved with meningitis.
Inducible resistance. That means you have to have the presence of some compound of that class for the genes to be turned on to exhibit the resistance. So with erythromycin, which is a macrolide, you have to have some member of the macrolide class present. Then the resistance is expressed. The resistance that you get with erythromycin usually is alteration of the target. The ribosome gets altered so that the erythromycin can't bind it. If the resistance is constitutive, you can have the same type of compound, but it doesn't need this inducer. The genes responsible for resistance are turned without having to be induced. So it's always ready to be resistant to the particular compound or that class.
Plasmid Mediated. That's the one I was talking about when I mentioned promiscuous bacteria. The genes coding for resistance are packaged into small pieces of DNA called plasmids and are easily transferred from one organism to another. For plasmid mediated resistance to continue, the organism has got to be continuously be transferred to and maintained on media containing some low level of antibiotic to which it is resistant. Otherwise the plasmid may be lost in the absence of antibiotic selective pressure. Chromosome mediated resistance is in direct contrast to plasmid mediated resistance. When you get a mutation or a piece of DNA that will be stuck in the large bacterial chromosome, it is more stable. Chromosomal mediated resistance is not easily transferred from one organism to another. Generally the organism doesn't require to be maintained in the presence of antibiotic. Whereas this one is maintained for a longer time.