It has been hypothesized that r-determinants exist somewhere in nature as chromosomal genes and that they are "picked-up" by bacteria to form R-factors. These are the resistance transfer factor (RTF), and the r-determinants, genes that determine resistance to antibiotics. Molecular studies have shown that R-factors consist of two parts that are reversibly dissociable. Since these genes are not normal chromosomal components of the resistant strains, there has been considerable speculation as to their origin. The genetic loci coding for these enzymes are usually located on extrachromosomal elements, such as the R(antibiotic resistance)-factors in gram-negative bacteria. The β-lactamases hydrolyze the penicillins and cephalosporins, chloramphenicol acetyltransferase acetylates chloramphenicol, and nine enzymes acetylate, phosphorylate, or adenylylate the aminoglycoside antibiotics. Resistance to antibiotics in clinical isolates of gram-negative and gram-positive bacteria is usually mediated by the presence of various enzymes that modify the antibiotic so that it can no longer interact with its target in the cell. Feel free to jump into the MCAT sample questions below, then read on to learn more about what to expect from the actual exam! The good news is that these are all topics that will be incredibly useful in your future medical education. Just like medical school itself, the MCAT tests a wide variety of topics.
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