Introduction Morphological type variation in Psa is thought to aid its survival in hostile environments, including the CF lung. Previous studies using PGFE have shown wide heterogeneity in LES strain, despite this, little is known about their phenotypic diversity (haplotypes). To look at this further, we compared the haplotypes of the most successful and prevalent UK transmissible Psa (the Liverpool epidemic strain, LES) with those of sporadic strains and assessed their response to antibiotic pressure.
Methods Sputum from two groups of adult CF patients chronically infected (for at least 4 years) with sporadic single Psa strains or LES was analysed at the beginning and end of an intravenous antibiotic-treated exacerbation, where every patient had subjective and spirometric improvement, and also for LES patients over a period of time (at least 4 months) when they were in a stable state. 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, piperacillin/tazobactam), hypermutability (rate of spontaneous mutation to rifampicin resistance) and auxotrophy (ability to grow on glucose M9 media) determined. Each Psa colony could potentially exhibit any of nine combinations: antibiotic susceptibility patterns, auxotrophy, hypermutability, and six colony morphologies. The resulting haplotypes were compared for LES and unique strains using ‘e-burst’ software.
Results Overall, 151 unique haplotypes were defined (96 LES, 75 sporadic, 20 shared), with LES demonstrating the greater number (X2=4.67, p<0.03). Following antibiotic pressure, there was an alteration in haplotypes which was similar in both groups (LES 54%, sporadic 64%, p=0.28): but during the stable state LES demonstrated a significantly greater change in haplotypes (89%, p=0.004).
Conclusion A change in haplotypes occurs in all CF Psa strains during treatment for clinical exacerbations, presumably as a defence mechanism against the challenge of hostile antibiotic therapy. However, the enhanced haplotype variability of transmissible strains with time when there is no antibiotic pressure may be a strategy that confers a survival advantage compared to sporadic strains: studies are under way to explore this further.
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