Facilitated detection of antibiotic-resistant Pseudomonas in cystic fibrosis sputum using homogenized specimens and antibiotic-containing media.

Sputa from 30 patients with cystic fibrosis (CF) were cultured on routine and selective media plus three Mueller-Hinton antibiotic resistance screening plates containing tobramycin (5 micrograms/ml), azlocillin (100 micrograms/ml), and ticarcillin (100 micrograms/ml). In addition to direct semiquantitative plating, samples were homogenized for semiquantitative and quantitative culture. Blood agar plates from direct semiquantitative and homogenized semiquantitative cultures were then replica plated onto the antibiotic screening plates. Homogenized semiquantitative and quantitative cultures both detected more Pseudomonas aeruginosa strains than direct semiquantitative plating (103 versus 85 strains), including more antibiotic-resistant strains. Antibiotic screening media facilitated isolation of resistant strains and decreased detection time by 24 hr. Of the 103 strains on homogenized semiquantitative and quantitative cultures isolated before replica plating, 13 (13%) were tobramycin-resistant, 67 (65%) were ticarcillin-resistant, and 42 (41%) were azlocillin-resistant; 13 of 30 cultures (43%) had at least one tobramycin-resistant organism before replica plating. Replica plating detected an additional seven tobramycin-resistant and nine ticarcillin- or azlocillin-resistant strains in seven patients. Homogenization, antibiotic screening media, and replica plating enhance recognition of antibiotic-resistant strains in CF sputum.