A phenomenon parallel to the spread of resistant strains is the emergence of new resistance mechanisms which are difficult to control. The aim of this article is to present selected mechanisms of resistance of both Gram-positive and Gram-negative pathogens present in human and animal environment. Natural resistance of bacteria is their permanent characteristic resulting from the biology of the given micro-organism, which prevents an antibiotic from penetrating into the bacterial cell. What is more important in practice is the acquired resistance. Bacteria develop resistance by acquiring genes encoding proteins that protect them from the effects of the antibiotic. In some cases the genes arise by mutation; in others, they are acquired from other bacteria that are already resistant to the antibiotic. These genes are often found on circular DNA fragments (plasmids) which spread easily from one bacterium to another, even from one species of bacterium to another.