Genes encoding antibiotic-resistant factors may be exogenous or endogenous. Most exogenous genes originate from antibiotic-producing organisms. Bacteria can transfer antibiotic-resistant genes among themselves using gene-exchanging systems, such as plasmids, bacteriophages or integrative and conjugative elements. One of the resistance mechanisms is inactivation of antibiotics by modification or decomposition. Methicillin-resistant Staphylococcus aureus, penicillin-resistant Streptococcus pneumoniae and beta-lactamase negative ampicillin-resistant Haemophilus influenzae possess mutations at the active site of antibiotics. Furthermore, all organisms have drug efflux systems. Overexpression of these systems can result in lower susceptibility of antibiotics. The carbapenems use a specific pathway to enter bacterial cells. Some carbapenem-resistant bacteria lose or decrease the expression of this outer membrane protein. Resistant organisms have appeared against all classes of antibiotics.