Phenotypic adaptation of clinical Pseudomonas aeruginosa to increasing ceftazidime exposure

Abstract

Background: Pseudomonas aeruginosa readily adapts to antibiotic stress, but the effect of increasing ceftazidime exposure on colony morphology across specimen sources is poorly defined. This study determined whether ceftazidime alters growth and colony morphology in a concentration-dependent manner and whether the response varies by specimen source.

Methods: Forty-eight non-duplicate clinical P. aeruginosa isolates from pus, wound swabs, urine, and tracheal aspirates were characterised for ceftazidime susceptibility, extended-spectrum β-lactamase (ESBL) and metallo-β-lactamase (MBL) production, and biofilm formation. Colony morphology was assessed after 24 hours on M9 agar with Congo red and Coomassie brilliant blue (RB medium), alone and with ceftazidime at one-half, one, and two times the isolate-specific minimum inhibitory concentration (MIC).

Results: Ceftazidime resistance was 70.8%, ESBL production 47.9%, MBL production 33.3%, and strong biofilm formation 45.8%. Growth fell from 97.9% on RB medium to 45.8% at twice the MIC (p<0.001). Mean colony diameter decreased from 19.59 to 5.00 mm among growing isolates, and from 19.18 to 2.29 mm when non-viable colonies were scored as 0 mm (p<0.001). Colony diameter differed by specimen source under every condition (p<0.05); tracheal aspirate isolates retained the largest median diameter at and above the MIC. Morphology was characterised by small, round, flat, dry, red colonies.

Conclusions: Ceftazidime produced dose-dependent reductions in bacterial growth and colony morphological variety, indicating phenotypic adaptation not detected by routine susceptibility testing. The response differed by specimen source, with respiratory isolates retaining the greatest growth capacity under drug pressure.