The origins and molecular basis of antibiotic resistance

We frequently refer to bacteria as being resistant to antibiotics, but rarely do we consider what that means. Even the most resistant bacterium can be inhibited or killed by a sufficiently high concentration of antibiotic; patients, however, would not be able to tolerate the high concentration required in some cases. Bacterial species vary tremendously in their susceptibility to an antibiotic — for example, most strains of Streptococcus pneumoniae in Britain are inhibited by 0.01 mg/l of benzylpenicillin (the minimum inhibitory concentration), whereas for Escherichia coli 32-64 mg/l are required to inhibit growth, a level which cannot be achieved in the human body. This introduces the concept of clinical resistance, which is dependent on outcome and is all too often ignored. Clinical resistance is a complex concept in which the type of infecting bacterium, its location in the body, the distribution of the antibiotic in the body and its concentration at the site of infection, and the immune status of the patient all interact.