Background Chronic infection with Psa in most CF patients is due to single unique strains, which over time accumulate mutations resulting in mucoid conversion, loss of motility, auxotrophy and increased antibiotic resistance, in turn leading to multiple phenotypes. However, changes in Psa population morphology related to short term pressures (eg, during IV antibiotic-treated exacerbations), have not previously been studied. We therefore looked at Psa population structure during exacerbations.
Methods Sputum samples from four CF patients chronically infected (for at least 4 years) with unique single Psa strains were analysed at the beginning and end of an intravenous antibiotic-treated exacerbation, where every patient had subjective and spirometric improvement. From each sample, 40 single Psa colonies were selected (with every morphological type proportionately represented), and colony morphology, susceptibility to six antibiotics (ciprofloxacin, ceftazidime, colomycin, meropenem, tobramycin, tazocin), hypermutability (rate of spontaneous mutation to rifampicin resistance) and auxotrophy (ability to grow on glucose M9 media) determined. Additionally, the density of Psa (colony forming units (CFU) per ml) in sputum samples was measured.
Results Although the predominant colony morphology changed from green, non-mucoid and smooth (mean 67%, range 43–98) to straw-coloured, non-mucoid and smooth (57%, 5–95) (p=0.001), there was no change in mean antibiotic resistance to all antibiotics (21.5% vs 20.3%, p=0.9), prevalence of hypermutable isolates (38% vs 25%, p=0.48) and auxotrophic mutants (66% vs 98%, p=0.17). However, there was an increase in Psa sputum density (mean CFU/ml 1.3 x105 vs 2.0x106, p<0.001), despite the use of relevant antimicrobial therapy.
Conclusion The changes in prevalent population composition following antibiotic pressure, associated with clinical improvement, might suggest that some morphotypes alone are responsible for the adverse clinical features. Conversely, the increase in sputum density of Psa despite objective clinical improvement implies that the exacerbation has occurred independently of the presence of the organism, supporting the observation that clinical improvement is often seen in CF patients even where the Psa seems resistant to the administered antibiotics. Further work need to be done to tease out the role of Psa in clinical exacerbations of CF patients with chronic infection.
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