The Fitness Cost of Rifampicin Resistance inPseudomonas aeruginosaDepends on Demand for RNA Polymerase

Abstract

Bacterial resistance to antibiotics usually incurs a fitness cost in the absence of selecting drugs, and this cost of resistance plays a key role in the spread of antibiotic resistance in pathogen populations. Costs of resistance have been shown to vary with environmental conditions, but the causes of this variability remain obscure. In this article, we show that the average cost of rifampicin resistance in the pathogenic bacterium Pseudomonas aeruginosa is reduced by the addition of ribosome inhibitors (chloramphenicol or streptomycin) that indirectly constrain transcription rate and therefore reduce demand for RNA polymerase activity. This effect is consistent with predictions from metabolic control theory. We also tested the alternative hypothesis that the observed trend was due to a general effect of environmental quality on the cost of resistance. To do this we measured the fitness of resistant mutants in the presence of other antibiotics (ciprofloxacin and carbenicillin) that have similar effects on bacterial growth rate but bind to different target enzymes (DNA gyrase and penicillin-binding proteins, respectively) and in 41 singlecarbon source environments of varying quality. We find no consistent effect of environmental quality on the average cost of resistance in these treatments. These results show that the cost of rifampicin resistance varies with demand for the mutated target enzyme, rather than as a simple function of bacterial growth rate or stress.

Increased Survival of Antibiotic-Resistant Escherichia coli inside Macrophages

“…This is, however, not surprising, given that Ms not only inflict several different stresses on bacteria but can also modify the composition of the extracellular medium. This is consistent with earlier findings suggesting that the fitness costs of antibiotic-resistant mutants may vary in different environmental conditions (11,14).…”

“…For example, Trindade et al (14) showed increased variation in fitness effects of resistant mutations in E. coli with increased environmental stress. Similarly, Hall et al (11) demonstrated that the costs of 24 different rpoB mutations vary greatly among 41 environments with different carbon source. Having in mind that fitness effects of resistant mutations exhibit strong genotype-by-environment interactions, it is important to determine the effects of resistance in an environment imposed by the host.…”

“…This is not surprising, since mutations which cause antibiotic resistance often target physiologically important functions in the cell, such as transcription and protein synthesis, cell wall synthesis, or nucleic acid synthesis (6). Interestingly, the fitness effect of a resistance mutation can be detrimental in one environment and beneficial in another (10)(11)(12)(13)(14). For example, Trindade et al (14) showed increased variation in fitness effects of resistant mutations in E. coli with increased environmental stress.…”

General and inducible hypermutation facilitate parallel adaptation in Pseudomonas aeruginosa despite divergent mutation spectra

“…The average frequency of Rif R mutants never rose above 2.1 × 10 −5 in our mutator lineages, suggesting that mutations in rpoB derived from general and inducible hypermutation were deleterious. These results are consistent with reports of reduced fitness and RNA polymerase function in Rif R mutants of E. coli and P. aeruginosa grown in the absence of rifampicin (24,25). A balance between the supply of new Rif R mutations, provided by hypermutation, and the loss of mutants due to fitness costs linked to defects in rpoB, could explain the observed stable average frequency of Rif R mutants in our lineages.…”

“…General and inducible mutator lineages exhibited different Rif R mutant frequencies, but given the diverse mutational possibilities available to achieve resistance, this disparity likely did not result from a bias toward the mutation spectra of either hypermutation mechanism. Rather, not all point mutations in rpoB are equal with respect to their pleiotropic fitness costs (24,25), and general and inducible hypermutation clearly favor different nucleotide substitutions at the rpoB locus. The point mutation spectra of PW7148 may favor rpoB mutations that carry greater fitness defects and thus produced the observed lower frequency of Rif R mutants in these lineages.…”