I’m sure you were asking yourself that very question this morning. OK, maybe not. But one of the interesting questions about antibiotic resistance is why a certain antibiotic resistance gene or allele (gene variant) is common and others are rare.
Among the beta-lactamases, the TEM-1 allele is the most common. Beta-lactamases protect bacteria against beta-lactam antibiotics, which include penicillin and its derivatives. TEM-1 was the first beta-lactamase gene to be characterized and provides resistance only to penicillin and ampicillin, while other, newer TEM alleles confer resistance to additional beta-lactams.
A recent paper examined the fitness effects of three TEM alleles, TEM-1, TEM-10, and TEM-12. TEM-1 only provides resistance to ampicillin, TEM-10 confers resistance to high concentrations of ceftazidime, and TEM-12 provides resistance to intermediate concentrations of ceftazidime. Interestingly, the pattern is reversed for ampicillin: TEM-1 protects against high concentrations of ampicillin, TEM-12 protects against intermediate concentrations of ampicillin, and TEM-10 provides resistance only to lower concentrations of ampicillin:
The authors created E. coli strains that were genetically identical except for the TEM allele that the strain carried. They then competed pairs of these strains in laboratory culture. When there was no antibiotic present, there was no effect of TEM on bacterial fitness. When ampicillin was added to the medium, the TEM-1 possessing strain was much more fit than the TEM-10 and TEM-12 strains. Since ampicillin and penicillin are used far more and in greater amounts, the authors conclude that, even though TEM-1 protects bacteria from far fewer antibiotics, the widespread use of ampicillin makes TEM-1 the most common TEM allele in bacterial populations.
Cited article: Mroczkowska JE, Barlow M. 2008. Fitness trade-offs in blaTEM evolution. Antimicrob Agents Chemother. 52(7):2340-5.