Researchers compared two selective media for isolating Pseudomonas aeruginosa from burn wound swabs. The standard medium contained 0.03 percent cetrimide in agar base, while the modified medium contained 0.02 percent cetrimide plus 15 micrograms per milliliter nalidixic acid. Testing 5,358 consecutive burn swabs inoculated on both media revealed that cetrimide-nalidixic acid agar (CNA) was superior for P. aeruginosa isolation. CNA yielded 6.1 percent presumptive P. aeruginosa on direct inoculation compared to 5.3 percent for cetrimide agar alone, while producing significantly fewer colonies of unwanted gram-negative bacilli like Klebsiella and Escherichia coli. The improved selectivity resulted from both better suppression of competing bacteria and reduced inhibition of P. aeruginosa itself. All 143 presumptive isolates tested confirmed as authentic P. aeruginosa using oxidase, glucose oxidation, pyocyanin production, nitrate reduction, and growth at 42°C. The authors concluded that reducing cetrimide concentration from 0.03 to 0.02 percent while adding nalidixic acid produced a more effective selective medium for clinical isolation of P. aeruginosa from clinical samples.

Key findings

• Cetrimide-nalidixic acid agar (0.02% cetrimide + 15 μg/ml nalidixic acid) isolated P. aeruginosa from 6.1% of burn swabs versus 5.3% for standard cetrimide agar alone
• CNA medium suppressed growth of unwanted gram-negative bacilli (Klebsiella, E. coli, Proteus) better than cetrimide agar alone
• Reducing cetrimide concentration from 0.03% to 0.02% decreased inhibition of some P. aeruginosa strains while maintaining selectivity
• All 143 presumptive P. aeruginosa isolates confirmed identity through oxidase, glucose oxidation, pyocyanin production, nitrate reduction, and 42°C growth tests
• CNA was more effective on subculture from liquid media, showing improved selectivity over direct inoculation methods