"Nature endows all bacteria with some low level of resistance." One in every million bacteria is naturally resistant to an antibiotic. Another vital factor that determines resistance is horizontal gene transfer. "Horizontal gene transfer in bacteria is mediated through one of the three mechanisms, transformation, conjugation, and transduction." .
Antimicrobial genes are usually transferred through transformation and conjugation rather than transduction. Conjugation is a form of microbial sex that is believed to be the most important mechanism in the acquisition and spread of antibiotic resistance. Conjugation results in the formation of a "bridge" between the two bacteria, genes are transported from one bacterium to another via the bridge. This results in two bacteria with the resistant gene, the bacteria then divide and multiply and the resistance begins spreading. The secondary method of resistance is transformation. In transformation, bacteria intake segments of DNA and incorporate it into their own DNA. Beta Lactams have been found to greatly facilitate resistance. When this type of antibiotic is introduced into a bacteria culture, it inhibits cell wall synthesis. While this is occurring bacteria devoid of cell walls are bred. These bacteria have an increased capacity to uptake DNA. This causes different bacteria that are not being targeted, to pass on the resistant gene they possess, to the bacteria lacking cell walls. This is a very likely scenario; for example in the human body there is a large variety of bacteria that exist in close proximity to each other. When pathogenic bacteria invade the environment they can uptake the resistant genes from the pre-existing bacteria. In the human body transformation is also facilitated by the presence of lysozymes in various body fluids, and through heat shock. These natural characteristics of bacteria assist them in developing a resistance to antibiotics.