CoachAn antibiotic is a molecule that kills bacteria or stops their growth, at concentrations harmless to the patient’s cells, by acting on a structure or a process that bacteria have and human cells lack: penicillin and its relatives block the building of the bacterial wall; streptomycin and tetracycline jam the bacterial ribosome, which differs from ours; others block the bacterial enzymes of DNA replication. Most were first found in moulds and soil bacteria, which make them against their competitors. Antibiotics have no effect on viruses, which have none of these targets.
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Example 18.2 (The arithmetic of a population)
A resistance mutation arises about once in divisions. A flask of bacteria has therefore had, in the divisions that produced it, some ten such mutations; an infected wound or a gut, with bacteria, some ten thousand. Every large population of bacteria contains, before any antibiotic is used, cells resistant to it — the replica plates of Chapter 13 proved it.
Example 18.7 (From one cell to a ward)
A patient treated with penicillin carries, among bacteria, a hundred resistant ones. In three days the sensitive ones are gone and the resistant clone has taken the place: resistant cells. On a nurse’s hands, on a bed rail, in the next patient, the clone continues — and if its resistance gene is on a plasmid, it hands it to other species along the way. Nothing in this sequence required the antibiotic to act on the DNA; it only removed the competition.