Background In CF patients with chronic Psa infection, the role of conventional antibiotic susceptibility testing (antibiotic disc diffusion on sputum subculture morphotypes according to BSAC protocols) is controversial, but many centres still rely on these methods when selecting treatment. However, morphotypes may exhibit up to six antibiograms, and this may contribute to the variable clinical response often noted. To look at this further, we performed additional Psa susceptibility testing on 26 sputum samples from 10 chronically infected CF patients provided over a 15-month period, and compared the results with those from the routine laboratory.
Methods For each sputum sample, 40 colonies proportionately representative of morphological subtypes (mean 2, range 1–4) on the Psa selective plates were cultured onto Columbia plates. From these, single colonies were mixed with sterile distilled water to attain a standard optical density (10 MacFarland units), and 10 μl spread onto iso-sense plates and incubated overnight with tobramycin, meropenem, colomycin, ceftazidime, ciprofloxacin, and piperacillin/tazobactam antibiotic discs. Antibiotic sensitivity (break point 50% of isolates) was determined by the zone of inhibition as per standardised BSAC protocols. In total, 6240 analyses were performed.
Results Although there was 100% concordance with the number of morphological subtypes of Psa with the routine laboratory, on multiple antibiotic susceptibility testing in 260 cases (25%) increased resistance was discovered. Overall, mean concordance between the routine diagnostic lab methodology and multiple antibiotic sensitivity testing was 70% (median 80%, IQR 60–100). However, in 15% of cases concordance was <50%, suggesting that more detailed testing may have altered the choice of antibiotics used.
Conclusion This study shows that routine microbiological methodology may under-represent antimicrobial resistance in Psa when patients are chronically infected. This may in part explain the clinical experience, and underlines the need for better microbiological techniques to aid the clinician in caring for these complex patients.
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