Microorganisms play an important role in the cycling of elements at a global scale, thus profoundly
and directly affecting the environments. In spite of most bacteria being harmless, some may be
beneficial to their host, providing nutrients or protection from pathogens and diseases by limiting the
ability of more harmful bacteria to colonize. If microorganisms affect their surrounding environment,
the environment in turn also engenders evolutionary pressures on the microorganisms. They have
a short generation time, lasting from minutes to hours, being able to rapidly respond to changes in their
environment. Therefore, as antimicrobial agents are introduced into the environment, microorganisms
respond by becoming resistant to these agents [9,123]. The antimicrobial resistance mechanisms
result from changes in the cellular physiology and structure of a microorganism due to changes
in its usual genetic makeup by acquiring genes from resistant microorganisms in the same niche
(acquired resistance) or developing novel ways to prevent the entrance of those agents (intrinsic
resistance) [17,123]. In the case of QACs, some studies have reported bacterial resistance to them, either
intrinsic or acquired. Intrinsic resistance is demonstrated by Gram-negative bacteria, bacterial spores,
mycobacteria and under certain conditions by Staphylococci. This is achieved by the reduction of the cell
membrane permeability through modifications in phospholipids’ synthesis or by membrane proteins’
linkage [26,49]. In the case of silver compounds, some studies have reported antibacterial resistance
against the agent, and the ease with which silver resistance can become selected in some bacteria
suggests that there would be a benefit in improved surveillance for silver-resistant isolates in the clinic,
along with a better control over silver use, in order to best preserve its clinical utility [7,124,125].